Quantum Information and Coherence (QuIC) Talks
( Email: akjha@iitk.ac.in if you would like to be on the QuIC email list )

14-Dec-2023

**Title:** Attosecond Physics: Let there be light and electron

**Speaker:** Prof. Gopal Dixit, Department of Physics, IIT Bombay

**Time:** 3:30 - 4:30 pm; Thursday, Dec 14th, 2023; tea@04:30 pm

**Venue:** FB-382

Abstract:

Inducing and probing the motion of electrons during numerous physical and chemical processes in real-space and in real-time are of immense importance as it can provide important insights into the mechanisms involved. This year Physics Nobel Prize is awarded to Attosecond Physics and its application to electron motion. In this talk, after discussing the important of attosecond physics, I will showcase few examples of our group where attosecond physics promises to bring technological radical changes, such as light-driven valleytronics and valley transistor, debate about the massless nature of Weyl fermions in Weyl semimetals, attosecond charge migration in molecules to name but a few. The detailed activities of our attosecond group can be seen at https://attosecondindia.com.

16-May-2023

**Title:** Quick Quantum Steering: Overcoming Loss and Noise with Qudits

**Speaker:** Vatshal Srivastava, Heriot-Watt University, Scotland, UK

**Time:** 4:00 - 5:00 pm; Tuesday, May 16th, 2023; tea+Samosa@03:45 pm

**Venue:** L-5 (Lecture Hall Complex)

Abstract:

The phenomenon of quantum entanglement enables the most secure form of communication possible—one where the communication devices themselves can be in the hands of an adversary, and security is still guaranteed. However, generating and distributing entanglement over long distances and through a noisy environment is no easy task. Entangled photons can be lost while propagating through fibres, and large amounts of noise can compromise detectors. As a result, tests of quantum nonlocality—the most stringent form of entanglement—have only been performed under very controlled conditions. Quantum steering relaxes the strict technical requirements of nonlocality by assuming an untrusted device only on one side of an (untrusted) channel. I will briefly introduce qudit entanglement in the seminar and its merits over qubit entanglement. Further, I will discuss our lab's techniques to tailor high-quality qudit entanglement. Then I will show how we harness the advantages of qudit entanglement to demonstrate quantum steering with the detection-loophole closed under extreme conditions of noise and loss. We develop a new test of quantum steering that requires only a single detector at each party, regardless of the dimension of the entanglement used. Using this test, we can "steer" entanglement through loss and noise conditions corresponding to 79km of telecom fibre and 36% of white noise. Remarkably, our technique also leads to a dramatic reduction in the total measurement time needed—by simply doubling our entanglement dimensionality, we can reduce our measurement time by almost two orders of magnitude. Our work demonstrates that the fundamental phenomenon of entanglement can transcend the limits imposed by a realistic environment and opens a clear pathway towards quantum communication protocols with unconditional security.

15-May-2023

**Title:** Fluid dynamics of partially coherent light

**Speaker:** Shaurya Aarav, Princeton University, USA

**Time:** 4:00 - 5:00 pm; Mon, May 15th, 2023; tea+Samosa@03:45 pm

**Venue:** FB-382

Abstract:

Fluid analogies in optics have a two-fold advantage: it allows table-top experiments for fluid phenomena that are otherwise not easily accessible. It also allows concepts from fluids to be adapted into optics for better beam control. This symbiotic relationship has led to the observation of many fluid phenomena in optics like shock waves, Riemann waves and solitons. However, this is mostly limited to coherent beams in nonlinear media. In this talk, I will discuss our recent work in connecting partially coherent beams with fluid dynamics. Partial coherence provides another degree of control in optics and it interacts with nonlinearity in novel ways. This novelty arises naturally from a fluid description and we provide theoretical and experimental evidence for that.

12-April-2023

**Title:** Phonons with angular momentum: Magnetic properties and applications

**Speaker:** Dr. Swati Chaudhary, University of Texas at Austin & Northeastern University, Boston, USA

**Time:** 4:30 pm ; Wednesday 12th April 2023

Abstract:

The idea that photons can carry both spin and orbital angular momentum is known for more than thirty years now and has been applied in diverse fields ranging from astronomy and optical trapping to quantum information processing and data storage. Recently, the angular momentum of phonons has also attracted a lot of attention. The angular momentum of phonons can arise either from circular lattice vibrations of ions or from the relative phase difference between lattice displacements on neighboring ions. These phonons which carry angular momentum are known as chiral phonons and can respond very strongly to the magnetic field. On the basis of purely circular ionic motion, these phonons are expected to carry a magnetic moment of the order of a few nuclear magnetons. However, some recent experiments have demonstrated a phonon magnetic moment of the order of a few Bohr magnetons. This kind of giant magnetic response points towards the electronic contribution to the magnetic moment of phonons. Many diverse mechanisms have been discovered for this enhanced magnetic response of chiral phonons. The orbital-lattice coupling is one such mechanism where low-energy electronic excitations on a magnetic ion hybridize with phonons and endows a large magnetic moment to phonons. In this talk, I’ll present a microscopic model for the effective magnetic moments of chiral phonons based on this mechanism. Our model is able to reproduce the experimentally measured magnitudes in some rare-earth paramagnets and our detailed analysis also predicts that similar effects also occur in other magnetic materials. Interestingly, this intricate mechanism arising from the coupling of lattice and orbital degrees of freedom not only leads to a large magnetic response of such phonons but can also be utilized for phononic control of electronic angular momentum in magnetic materials. Such phonons with giant magneto-phononic and phono-magnetic responses can possibly be used for dissipationless thermal Hall transport, ultrafast control of quantum matter, and quantum information processing.

05-April-2023

**Title:** Nonlinear Optics with Singular Light

**Speaker:** Prof. Siddharth Ramachandran, Boston University, USA

**Time:** 4:30 pm ; Wednesday 5th April 2023

Abstract:

Nonlinear optics involves the study of coherent interactions between multiple optical waves. In its most general manifestation, a nonlinear process must satisfy momentum conservation, or equivalently, phase or wavevector matching, which translates into a dispersion engineering problem in fibers and waveguides. Hence, advances in tailoring the dispersion of waveguides, especially with the advent of photonic crystal structures, have led to revolutionary advances in nonlinear optics. The situation is, however, different in fibers that support the propagation of light carrying spin as well as orbital angular momentum (OAM). The need to match angular as well as linear momentum provides selectivity that can enhance or even forbid some well-known nonlinear interactions such as Raman scattering. In this talk, we describe how one can manage as well as exploit these degrees of freedom for applications ranging from power-scalable frequency conversion and biomedical imaging, to classical and quantum communications.

29-March-2023

**Title:** Transport and Manipulation of High-Dimensional Entanglement
through a Complex Medium

**Speaker:** Prof. Mehul Malik, Heriot-Watt University, Edinburgh, Scotland

**Time:** 3:00 - 4:00 pm; Wednesday 29th March 2023

Abstract:

Quantum states of light entangled in high dimensions offer the potential for noise-robust quantum communication networks that harness the full information carrying capacity of a photon [1]. A central challenge in the realisation of such networks is the ability to precisely control and reliably transport high-dimensional entangled states of light. I will begin this talk by introducing the paradigmatic source of high-dimensional photonic entanglement—a nonlinear crystal generating a pair of photons entangled in the position-momentum degrees-of-freedom [2]. I will go on to discuss how knowledge of the continuous bi-photon wavefunction allows us to tailor discrete high-dimensional entangled states of light with record quality in a variety of spatial mode bases. As an example, I will present the generation and measurement of high-dimensional “pixel” entanglement with fidelities exceeding 94% and entanglement dimensionalities up to 55 [3]. In the second part of this talk, I will discuss an experiment where we transport high-dimensional entanglement through a complex scattering medium: a 2-metre section of commercial multi-mode fibre [4]. Interestingly, by using a property unique to high-dimensional entanglement, the medium is rendered transparent to entanglement by carefully “scrambling” the photon that did not enter it, rather than unscrambling the photon that did. I will go on to discuss how the same complex medium can be used for the precise manipulation of high-dimensional entangled states of light. I will present our “top-down” approach that allows us to program a general photonic quantum circuit within a commercial multi-mode fibre using methods of inverse-design. I will demonstrate how this programmability allows us to turn the multi-mode fibre into a generalised multi-outcome device, enabling us to both transport, manipulate, and certify entanglement within the transmission channel itself [5].

09-February-2023

**Title:** High-dimensional entanglement for quantum communication

**Speaker:** Prof. Marcus Huber, Technical university of Vienna, Austria

**Time:** 4:30 pm ; Thursday 09th February, 2023

Abstract:

Entanglement unlocks many applications in quantum communication, such as the highest possible level of security in quantum key distribution. As photons are inevitably lost or decohered over longer distances, it seems obvious that using the full spectrum of photonic degrees of freedom is desirable. In addition to more encodable bits per photon, entanglement in high dimensions also yields a surprising resistance to noise [1]. This comes at the expense of more complicated measurements that in themselves can contribute to the overall noise in the data, leading to an interesting optimisation. While random, noisy entanglement may not always be useful or need unrealistic control to be harnessed, I will also present a protocol that can be used in high-dimensional systems, even with restricted measurement possibilities [2], which has recently been successfully employed in path [3] and energy-time [4] experiments.

12-December-2022

**Title:** Cavity optomechanical sensing & manipulation of an atomic persistent current

**Speaker:** Prof. Mishkatul Bhattacharya, Rochester Institute of Technology, Rochester, NY, USA

**Time:** 4:30 pm ; Monday 12th December, 2022

Abstract:

Atomic superfluids contained in a ring provide a remarkable paradigm for quantized circulation. Very recently, a novel technique based on cavity optomechanics has been proposed [P. Kumar et. al., Phys. Rev. Lett. 127, 113601 (2021)] for sensing and manipulating the rotation of a bosonic ring condensate with minimal destruction, in situ and in real-time. Here, we theoretically investigate other coherent quantum effects that can be supported by the proposed configuration. Specifically, in the presence of a strong control beam, we analyze the influence of atomic rotation on the transmission spectrum of a weak probe laser through a cavity containing a ring condensate. We present a detailed study of the resulting narrow probe transmission profiles and group delay and show that they can be tuned by means of persistent currents. Our results explore a new facet of rotating matter waves and are relevant to applications such as atomtronics, sensing and information processing.

22-April-2022

**Title:** Rays, waves, coherence and structured light-III

**Speaker:** Prof. Miguel Alonso, Aix Marseille Univ., France and the University of Rochester, USA

**Time:** 11:00 am ; Friday 22nd April 2022

Abstract:

This series of presentations will cover several aspects of the connection between the ray and wave models of light. The first part will be discussion of the ray-wave connection of fully coherent fields, and the analogies with the classical-quantum connection for mechanics. The second part will focus on the use of these ideas in the study of structured light beams, and how the more primitive ray model often gives more direct intuition than the more accurate wave model. The third part will be devoted to partially coherent fields.

21-April-2022

**Title:** Rays, waves, coherence and structured light-II

**Speaker:** Prof. Miguel Alonso, Aix Marseille Univ., France and the University of Rochester, USA

**Time:** 11:00 am ; Thursday 21st April 2022

Abstract:

This series of presentations will cover several aspects of the connection between the ray and wave models of light. The first part will be discussion of the ray-wave connection of fully coherent fields, and the analogies with the classical-quantum connection for mechanics. The second part will focus on the use of these ideas in the study of structured light beams, and how the more primitive ray model often gives more direct intuition than the more accurate wave model. The third part will be devoted to partially coherent fields.

19-April-2022

**Title:** Rays, waves, coherence and structured light-I

**Speaker:** Prof. Miguel Alonso, Aix Marseille Univ., France and the University of Rochester, USA

**Time:** 11:00 am ; Tuesday 19th April 2022

Abstract:

This series of presentations will cover several aspects of the connection between the ray and wave models of light. The first part will be discussion of the ray-wave connection of fully coherent fields, and the analogies with the classical-quantum connection for mechanics. The second part will focus on the use of these ideas in the study of structured light beams, and how the more primitive ray model often gives more direct intuition than the more accurate wave model. The third part will be devoted to partially coherent fields.

17-November-2021

**Title:** Quantum mechanics with patterns of light

**Speaker:** Prof. Andrew Forbes, University of Witwatersrand, Johannesburg, South Africa

**Time:** 5:30 pm; Wednesday 17th Nov 2021

Abstract:

Photons can be described in terms of their spatial modes – the “patterns” of light. As there are an infinite number of spatial modes, entanglement in this degree of freedom offers the opportunity to realize high-dimensional quantum states. In this talk, I will review the recent progress in quantum entanglement of photons in their spatial degree of freedom. I will explain how to create high-dimensional quantum states in the laboratory, how to measure them, and what the present state of the art is in terms of applications. In particular, I will outline the advantages and disadvantages of using such entangled states as a means to encode information for secure quantum communication channels and will consider the preservation of entanglement through noisy channels, e.g., a turbulent atmosphere.

10-November-2021

**Title:** Boosting Entanglement with Partial Coherence

**Speaker:** Prof. Stephen Walborn, University of Concepción, Chile

**Time:** 5:30 pm; Wednesday 10th Nov 2021

Abstract:

Mixed quantum states are inherently different from pure states. This is especially apparent in their quantum correlations, as mixed states can be classified according to a hierarchy of different types of correlations that are equivalent for pure states. A similar scenario appears in optics, where partially coherent beams present phenomena that may not be present in perfectly coherent beams, such as the ``twist phase”, first discussed by Simon and Mukunda. In this regard, Spontaneous Parametric Down-conversion (SPDC) is an interesting scenario, since it is known that optical properties of the pump beam are transferred to the quantum properties of the down-converted photon pairs. SPDC has been a workhorse in quantum optics, used to produce two-photon entanglement, but almost always with a coherent pump laser beam. Here I review SPDC with coherent and partially coherent beams, and show how novel properties, such as “twist phase” are responsible for novel phenomenon that appear in the two-photon entanglement and correlations. We also present recent results concerning stimulated PDC with partial transverse coherent beams. Our work contributes to the emerging interest in spatially entangled mixed states, with possible applications in quantum communication and imaging.

04-October-2021

**Title:** Imaging at the speed of light

**Speaker:** Prof. Jonathan Leach, Heriot-Watt University, Scotland

**Time:** 5 pm; Monday, October 4th, 2021

Abstract:

Single-photon detector array technologies have advanced significantly in recent years. Cameras now exist that are not only sensitive to single photons but the individual pixels in the sensor provide photon time-of-arrival information the picosecond regime. Such unprecedented sensitivity and temporal resolution opens up a number of exiting new applications, such as light-in-flight imaging, looking around corners with laser echoes, seeing through dense scattering media, and ultra-fast three-dimensional imaging. I will discuss the recent developments of the camera technology and discuss our latest results. I will give details of our latest field trials, where we have been using single-photon detector array sensors to see through fog and smoke. I will also highlight my group’s activities in applied and fundamental quantum science, specifically focussing on optical neural networks for quantum meteorology.

24-September-2021

**Title:** Structured Quantum Waves: From Fundamentals to Quantum Information Processing and Beyond - III

**Speaker:** Prof. Ebrahim Karimi, University of Ottawa, Canada

**Time:** 5 pm; Friday, September 24th, 2021

Abstract:

Photons, the quanta of light, possess several different degrees of freedom, e.g., frequency, polarization, spatial and temporal modes, which can be used as platforms for quantum information applications. Polarization, corresponding to the vectorial nature of light, is bi-dimensional, and thus can represent ‘0’ and ‘1’ in the digital world. Unlike, polarization, transverse and temporal modes would provide anunbonded vector space and could be used to extend the alphabet beyond the ‘0’ and ‘1’s to any arbitrary integer numbers. Photons in superposition states of these different of degrees of freedom is known as Structured Photons. In the classical regime, structured light has found tremendous applications; e.g. overcoming the diffraction limit (STED microscopy), for optical spanners, communication multiplexing, and generating non-trivial 3D topologies such as Möbius and Knots. In the quantum domain, structured photons may be used to realise higher- dimensional states, and thus are used for quantum communication, computation and simulations. Moreover, the quantum mechanical principle of wave-particle duality is not limited to photons. Indeed, matter can also be made to exhibit wavelike behaviour, as dramatically demonstrated by numerous experiments in diffractive electron optics. Remarkably, the wavelike properties of matter suggest that the very same techniques used to structure beams of light can be applied to shaping matter waves. We have been particularly interested in investigating the unique properties of structured electron waves. Apart from showing tremendous potential for a host of applications in electron microscopy and nano-fabrication, structured electron waves can allow us to observe otherwise inaccessible physical effects, putting a range of exciting and fundamental phenomena within our reach, to be studied as never before. In these three lectures, I will provide the basics of shaping optical and electron beams, and review the recent progress, challenges and applications of structured photons and electrons in modern. In particular, stability and dynamics of two- and three-dimensional complex structured photons, at the classical domain, will be presented. Finally, I will present their applications in high-dimensional quantum key distribution, quantum hacking, simulating complex quantum systems as well as in quantum microscopy.

22-September-2021

**Title:** Structured Quantum Waves: From Fundamentals to Quantum Information Processing and Beyond - II

**Speaker:** Prof. Ebrahim Karimi, University of Ottawa, Canada

**Time:** 6 pm; Wednesday, September 22nd, 2021

Abstract:

Photons, the quanta of light, possess several different degrees of freedom, e.g., frequency, polarization, spatial and temporal modes, which can be used as platforms for quantum information applications. Polarization, corresponding to the vectorial nature of light, is bi-dimensional, and thus can represent ‘0’ and ‘1’ in the digital world. Unlike, polarization, transverse and temporal modes would provide anunbonded vector space and could be used to extend the alphabet beyond the ‘0’ and ‘1’s to any arbitrary integer numbers. Photons in superposition states of these different of degrees of freedom is known as Structured Photons. In the classical regime, structured light has found tremendous applications; e.g. overcoming the diffraction limit (STED microscopy), for optical spanners, communication multiplexing, and generating non-trivial 3D topologies such as Möbius and Knots. In the quantum domain, structured photons may be used to realise higher- dimensional states, and thus are used for quantum communication, computation and simulations. Moreover, the quantum mechanical principle of wave-particle duality is not limited to photons. Indeed, matter can also be made to exhibit wavelike behaviour, as dramatically demonstrated by numerous experiments in diffractive electron optics. Remarkably, the wavelike properties of matter suggest that the very same techniques used to structure beams of light can be applied to shaping matter waves. We have been particularly interested in investigating the unique properties of structured electron waves. Apart from showing tremendous potential for a host of applications in electron microscopy and nano-fabrication, structured electron waves can allow us to observe otherwise inaccessible physical effects, putting a range of exciting and fundamental phenomena within our reach, to be studied as never before. In these three lectures, I will provide the basics of shaping optical and electron beams, and review the recent progress, challenges and applications of structured photons and electrons in modern. In particular, stability and dynamics of two- and three-dimensional complex structured photons, at the classical domain, will be presented. Finally, I will present their applications in high-dimensional quantum key distribution, quantum hacking, simulating complex quantum systems as well as in quantum microscopy.

20-September-2021

**Title:** Structured Quantum Waves: From Fundamentals to Quantum Information Processing and Beyond - I

**Speaker:** Prof. Ebrahim Karimi, University of Ottawa, Canada

**Time:** 5 pm; Monday, September 20th, 2021

Abstract:

Photons, the quanta of light, possess several different degrees of freedom, e.g., frequency, polarization, spatial and temporal modes, which can be used as platforms for quantum information applications. Polarization, corresponding to the vectorial nature of light, is bi-dimensional, and thus can represent ‘0’ and ‘1’ in the digital world. Unlike, polarization, transverse and temporal modes would provide anunbonded vector space and could be used to extend the alphabet beyond the ‘0’ and ‘1’s to any arbitrary integer numbers. Photons in superposition states of these different of degrees of freedom is known as Structured Photons. In the classical regime, structured light has found tremendous applications; e.g. overcoming the diffraction limit (STED microscopy), for optical spanners, communication multiplexing, and generating non-trivial 3D topologies such as Möbius and Knots. In the quantum domain, structured photons may be used to realise higher- dimensional states, and thus are used for quantum communication, computation and simulations. Moreover, the quantum mechanical principle of wave-particle duality is not limited to photons. Indeed, matter can also be made to exhibit wavelike behaviour, as dramatically demonstrated by numerous experiments in diffractive electron optics. Remarkably, the wavelike properties of matter suggest that the very same techniques used to structure beams of light can be applied to shaping matter waves. We have been particularly interested in investigating the unique properties of structured electron waves. Apart from showing tremendous potential for a host of applications in electron microscopy and nano-fabrication, structured electron waves can allow us to observe otherwise inaccessible physical effects, putting a range of exciting and fundamental phenomena within our reach, to be studied as never before. In these three lectures, I will provide the basics of shaping optical and electron beams, and review the recent progress, challenges and applications of structured photons and electrons in modern. In particular, stability and dynamics of two- and three-dimensional complex structured photons, at the classical domain, will be presented. Finally, I will present their applications in high-dimensional quantum key distribution, quantum hacking, simulating complex quantum systems as well as in quantum microscopy.

5-July-2021

**Title:** Adventures in Hilbert Space: Quantum Information Using the Shape of Light

**Speaker:** Prof. Mary Jacquiline Romero, University of Queensland, Australia

**Time:** 11:00 am - 12:00 pm; Monday, July 5th, 2021

Abstract:

Information is physical, it is encoded in physical systems, for example in the electrons that flow through our digital devices. In this way, the rules that information follow is dictated by physics: when the information is encoded in a quantum system, quantum physics rules! The shape of light has emerged in recent years as a promising platform for encoding quantum information, for the multiple levels that it affords and the ease with which shape can be controlled. I will first start by highlighting the differences between classical and quantum information. Then I will proceed to discuss two recent experiments. The first is on ignorance—one might ask if ignorance of a whole system implies ignorance of its parts. Our classical intuition tells us yes, however quantum theory tells us no: it is possible to encode information in a quantum system so that despite some ignorance of the whole, it is impossible to identify the unknown part. I will give an experimental evidence that supports this counterintuitive fact. The second is on learning, we implemented a self-learning tomographic technique wherein the experiment guides itself to estimate an unknown quantum state. This is especially important for systems that are of high dimensionality, where making tomographically complete measurements become impractical.

14-June-2021

**Title:** From classical to quantum computation

**Speaker:** Prof. Jaikumar Radhakrishnan, TIFR, Mumbai

**Time:** 4:00 - 5:00 pm; Monday, June 14th, 2021

Abstract:

Quantum computing attempts to exploit the power of quantum mechanics, especially its counter-intuitive notions of superposition and interference, to perform some computational tasks faster than we can currently on classical computers. We will review the basics of classical deterministic and randomized computing and describe how they relate to quantum computing. Then we will describe the main theoretical proposals for fast algorithms.

29-Jan-2021

**Title:** Theory of Optical Rotation of Few-Ion Crystals

**Speaker:** Mishkatul Bhattacharya, Rochester Institute of Technology, Rochester, NY, USA

**Time:** 11:00 am - 12:00 noon; Friday, Jan 29th, 2021

Abstract:

Techniques for manipulating the linear vibration of trapped ions have been developed to a high degree of sophistication over several decades, resulting in quantum simulators, commercial ventures for making quantum computers, and recognition with a Nobel prize. Control over of the center-of-mass rotation of ions has received comparatively much less attention. Such control is desirable for rotational interferometry, Aharonov-Bohm type experiments, tests of mesoscopic quantization of angular momentum, investigation of time crystals and detection of Hawking radiation in acoustic analogs of black holes. Recently Haffner et al. at Berkeley have demonstrated coherent control (Rabi and Ramsey spectroscopy) of a two-ion crystal in circular motion. In this talk I will describe our theoretical modeling of few-ion crystal rotation in circular, elliptical, planar, nonplanar, and multi-loop configurations. Our theory shows excellent agreement with the data of our experimental collaborators Dr. Akira Ozawa and Dr. Thomas Udem at the Max Planck Institute of Quantum Optics in Garching, Germany.

03-Jan-2020

**Title:** Levitated nanoparticle phonon laser

**Speaker:** Mishkatul Bhattacharya, Rochester Institute of Technology, Rochester, NY, USA

**Time:** 3:00 - 4:00 pm; Friday, Jan 3rd, 2020; tea@02:45 pm

**Venue:** FB-382

Abstract:

I will describe our recent theoretical work and its experimental confirmation on realizing a mechanical laser using the center-of-mass oscillations of an optically levitated nanoparticle. The discussion will include threshold behavior, coherence, subthermal number squeezing, time dynamics, phase space characterization, higher order correlations and the role of stimulated emission in our single mode phonon laser. Based on this discussion, I will conclude that our device provides a pathway for engineering a coherent source of phonons on the mesoscale that can be applied to both fundamental problems in quantum mechanics as well as tasks of precision metrology. This work was recently published in Nature Photonics 13, 402 (2019) and was included among the top 30 optics breakthroughs in 2019 by Optics and Photonics News.

14-March-2019

**Title:** Quantifying atomic coherence

**Speaker:** Arif Warsi Laskar, Physics Dept., IIT Kanpur

**Time:** 4:00 - 5:00 pm; Thu, Mar 14th, 2019; tea@03:45 pm

**Venue:** FB-382

Abstract:

Precise control over superposed states gathered significant interest in recent years due to its potential application in quantum technologies. Accordingly, there has been a major thrust in creating and engineering stable superposed states in room temperature system. A widely used technique to generate such stable superposed state in atomic media is electromagnetically induced transparency, where a strong control field drives the atoms in ground state superposition in presence of a weak probe field. Here, two excitation pathways of atoms to the excited state destructively interfere, and leads to a transparency window in the probe absorption profile. For long time, such ground state coherence has been characterized by the narrowness of the transparency window. In contrast to that we provide a phenomenological quantifier which directly measures the effective quantum coherence in such system. The measurement is based on a single shot time-domain measurement. Time response of probe also shows how this ground state superposition builds in a thermal ensemble of atoms in presence of classical processes and how they can be separated out from quantum dynamics. Using this quantifier, we show that two different phenomena Autler-Townes splitting and electromagnetically induced transparency, producing similar effect in probe absorption profile, can be separated unambiguously. Furthermore, we introduce a decay compensation channel composed of highly red detuned Raman field to freeze the coherence against decoherence present in the system.

07-Mar-2019

**Title:** Local dynamical processes in spin chains

**Speaker:** Saikat Sur, Physics Dept., IIT Kanpur

**Time:** 4:00 - 5:00 pm; Thu, Mar 07th, 2019; tea@03:45 pm

**Venue:** FB-382

Abstract:

We will focus on the effect of a local instantaneous quantum dynamical process (QDP) on a quantum many body dynamics through a unitary evolution. A QDP (ex. Measurement) at a given epoch of time on a single spin will cause decoherence in a quantum system. The signal of the QDP intervening the background dynamics will propagate in the chain. From the dynamical unitary evolution of the decohered state from the epoch time, it is possible to detect the occurrence of the dynamical process. The propagation of the signal for the dynamical process, and the speed of the signal are investigated for various spin models. The interference effects of the QDP (both unitary and non unitary) signal with the unitary dynamics can change the dynamical quantities substantially. The effect of QDP on state transfer fidelity, quantum correlations, the Loschmidt Echo etc. will be discussed.

12-Feb-2019

**Title:** Tripartite mutual information, entanglement, and scrambling in permutation symmetric systems

**Speaker:** Arul Lakshminarayan, IIT Madras, Channai

**Time:** 4:00 - 5:00 pm; Tuesday, Feb 12th, 2019; tea @03:45 pm

**Venue:** FB-382

Abstract:

I will discuss tripartite mutual information (TMI), a measure that is being studied currently as a proxy for information scrambling, a negative sign of this quantity indicating scrambling and that the "whole is more than the sum of its parts". In particular we concentrate on completely symmetric states of many qubits and show how the TMI is typically positive and is related to the marginal entanglement of these states. Yet, contrary to expectations systems with symmetric states can scramble in certain ways.

24-Jan-2019

**Title:** Allowed region of the mean values of angular momentum observables and their uncertainty relations

**Speaker:** Arun Sehrawat, HRI, Allahabad

**Time:** 4:00 - 5:00 pm; Thursday, Jan 24th, 2019; tea @03:45 pm

**Venue:** FB-382

Abstract:

The expectation values of operators drawn from a single quantum state cannot be outside of a particular region, called their allowed region or the joint numerical range of the operators. Basically, the allowed region is an image of the state space under the Born rule. The maximum-eigenvalue-states---of every linear combination of the operators of interest---are sufficient to generate boundary of the allowed region. In this way, we obtain the numerical range of certain Hermitian operators (observables) that are functions of the angular momentum operators. Especially, we consider here three kinds of functions---combinations of powers of the ladder operators, powers of the angular momentum operators and their anticommutators---and discover the allowed regions of different shapes. By defining some specific concave (and convex) functions on the joint numerical range, we also achieve tight uncertainty (and certainty) relations for the observables. Overall, we demonstrate how the numerical range and uncertainty relations change as the angular momentum quantum number grows. Finally, we apply the quantum de Finetti theorem by taking a multi-qubit system and attain the allowed regions and tight uncertainty relations in the limit where the quantum number goes to infinity.

10-Jan-2019

**Title:** Duality and quantum state transfer in cavity array

**Speaker:** Nilakantha Meher, Dept. of Physics, IIT Kanpur

**Time:** 4:00 - 5:00 pm; Thu, Jan 10th, 2019; tea@03:45 pm

**Venue:** FB-382

Abstract:

Controlled generation and transmission of quantum states are vital to quantum information processing and quantum communication. Novel methods and gadgets to perform these tasks are required. Coupled cavity arrays are considered suitable for this purpose. In this talk, I present a scheme for a perfect transfer of photon in an array. The condition for realizing such a transfer is obtained by a duality argument. Extension to nonlinear cavities is explored to achieve controlled transfer.

02-Jan-2019

**Title:** Understanding elementary processes in surface chemistry using quantum state resolved molecular beam surface-scattering experiments

**Speaker:** Pranav R. Shirhatti, TIFR Hyderabad

**Time:** 4:00 - 5:00 pm; Wed, Jan 02nd, 2019; tea@03:45 pm

**Venue:** FB-382

Abstract:

Chemical reactions occurring on metal surfaces are a subject of wide interest, primarily because of its direct connections to heterogeneous catalyst technology. These processes are often complex and are composed of several elementary steps such as adsorption, diffusion, bond breaking/making and desorption. Further, each of these steps is accompanied by energy exchange with the underlying catalyst surface (often a metal) via different pathways and spanning different time scales. Understanding the energetics and dynamics of these elementary steps can have profound implications on our understanding of surface chemistry. In this talk, I will give an overview of some recent efforts in this direction, especially focusing on studies involving quantum state resolved molecular beam surface–scattering experiments. I will also discuss the initiatives currently being undertaken in our laboratory (at TIFR Hyderabad) for studying energy transfer processes and the dynamics of chemical reactions at surfaces.

21-Dec-2018

**Title:** Optomechanical detection of atomic rotation

**Speaker:** Mishkatul Bhattacharya, Rochester Institute of Technology, Rochester, NY, USA

**Time:** 4:00 - 5:00 pm; Friday, Dec 21st, 2018; tea@03:45 pm

**Venue:** FB-382

Abstract:

We theoretically propose a new cavity optomechanical system, namely an annular rotating Bose-Einstein condensate (BEC) coupled to light fields carrying orbital angular momentum inside an optical resonator. We demonstrate that this configuration offers novel possibilities for the sensing and engineering of the atomic current. First we show that detection of the light transmitted by the cavity allows for a measurement of the angular momentum of the persistent current. In contrast to existing techniques, our method is minimally destructive, works in situ and in real time. Second, we show how counter-rotating states of atomic motion and entanglement between them can be optically engineered and detected. For realistic parameters, we present the linear and nonlinear response of the system, the sensitivity of the measurement including its quantum limit, and studies of bi- and tri-partite entanglement. We find that our work can improve rotation sensing of ring BECs by more than an order of magnitude and opens up new possibilities for the manipulation of multiply-connected rotating degenerate gases and characterization of atomtronic circuits.

06-Sep-2018

**Title:**Simulating light using Maxwell's equations for EM and Photonic Applications

**Speaker:** Rishabh Sahu, Dept. of Physics, IITK

**Time:** 3:00 pm - 4:00 pm, Thu, Sep 6th, 2018; tea@2:45 pm

**Venue:** FB-382

Abstract:

In post world-war era, many research efforts were made to improve radar technology and develop stealth aircrafts. These efforts led to development of various techniques that can be used to numerically simulate light in various kinds of media. The most successful of these techniques was the Finite Difference Time Domain (FDTD) method proposed by Yee et al. in mid 1960s which is relevant till today. Nowadays these FDTD simulations of light are used in various electromagnetic applications ranging from making faster optical switches to even detecting cancer. In this talk, I will present an FDTD solver, Luxum, which implements this already efficient technique and parallelizes it, so that it can be run on multiple computers simultaneously for huge simulations that require high precision. I will present the features this software offers followed by various application examples that go on to show its usefulness.

05-Sep-2018

**Title:** Quantum-optical tests of Planck-scale physics

**Speaker:** Shreya P Kumar, Ulm University, Ulm, Germany

**Time:** 11:00 am-12:00 noon, Wed, Sep 5th, 2018; tea@10:45 am

**Venue:** FB-382

Abstract:

Recently, it was proposed to use cavity-optomechanical systems to test for quantum gravity corrections to quantum canonical commutation relations [I. Pikovski et al., Nat. Phys. 8, 393 (2012)]. Improving the achievable precision of such devices represents a major challenge that we address with this work. More specifically, we develop sophisticated paths in phase space of such optomechanical systems to obtain significantly improved accuracy and precision under contributions from higher-order corrections to the optomechanical Hamiltonian. An accurate estimate of the required number of experimental runs is presented based on a rigorous error analysis that accounts for mean photon-number uncertainty, which can arise from classical fluctuations or from quantum shot noise in measurement. Furthermore, we propose a method to increase precision by using squeezed states of light. Finally, we demonstrate the robustness of our scheme to experimental imperfection, thereby improving the prospects of carrying out tests of quantum gravity with near-future optomechanical technology. Reference: Shreya P. Kumar and Martin B. Plenio. Phys. Rev. A 97, 063855 https://doi.org/10.1103/PhysRevA.97.063855

24-Aug-2018

**Title:** Microscopy and Light Sheet Microscopy

**Speaker:** Manish Kumar, Northwestern University, Evanston IL, USA

**Time:** 9:30 am-10:30 am, Fri, Aug 24th, 2018; tea @ 09:15 am

**Venue:** FB-382

Abstract:

I shall mainly give an overview of optical microscopy and talk about where light sheet microscopy fits in all this. Light sheet microscopy is changing the way we image biological samples for developmental biology and neuroscience. I would talk about some beautiful optics hacks which have taken light sheet microscopy to the next level.

02-Jan-2018

**Title:** Levitated optomechanical phonon laser

**Speaker:** Mishkatul Bhattacharya, Rochester Institute of Technology, Rochester, NY, USA

**Time:** 2:30 pm-3:30 pm, Tue, Jan 2nd, 2018; tea @ 02:15 pm

**Venue:** FB-382

Abstract:

I will describe the optomechanics and coherent control of phonons in an optically levitated system, including recent theory from our group and experiments by our collaborators.

13-Nov-2017

**Title:** Orbital Angular Momentum of Photons - III

**Speaker:** Anand Kumar Jha, Physics Dept., IITK

**Time:** 3:30 pm-4:30 pm, Mon, Nov 13th, 2017; tea @ 03:15 pm

**Venue:** FB-382

Abstract:

In a pioneering work published in 1992, L Allen and coworkers showed that a photon in a light beam can carry orbital angular momentum (OAM) in the integer multiples of hbar. This result has made OAM a very important degree of freedom for both classical and quantum information protocols. This is because an information protocol requires a discrete basis, and the OAM degree of freedom provides a basis that is not only discrete but can also be high-dimensional and thus providing several unique advantages in terms of security, supersensitive measurements and fundamental tests of quantum mechanics. In this talk series, we will first describe what it means to have a photon carrying OAM and then discuss effects, such as angular interference and diffraction etc., that arise due to the fact that photons can carry OAM. We will then describe the existing techniques by which OAM carrying photons are generated and detected and also discuss the limitations of these technique. Finally, we will discuss a new experimental technique that we have developed in order to measure the OAM spectrum of photons through just two intensity measurements.

06-Nov-2017

**Title:** Orbital Angular Momentum of Photons - II

**Speaker:** Anand Kumar Jha, Physics Dept., IITK

**Time:** 3:30 pm-4:30 pm, Mon, Nov 06th, 2017; tea @ 03:15 pm

**Venue:** FB-382

Abstract:

In a pioneering work published in 1992, L Allen and coworkers showed that a photon in a light beam can carry orbital angular momentum (OAM) in the integer multiples of hbar. This result has made OAM a very important degree of freedom for both classical and quantum information protocols. This is because an information protocol requires a discrete basis, and the OAM degree of freedom provides a basis that is not only discrete but can also be high-dimensional and thus providing several unique advantages in terms of security, supersensitive measurements and fundamental tests of quantum mechanics. In this talk series, we will first describe what it means to have a photon carrying OAM and then discuss effects, such as angular interference and diffraction etc., that arise due to the fact that photons can carry OAM. We will then describe the existing techniques by which OAM carrying photons are generated and detected and also discuss the limitations of these technique. Finally, we will discuss a new experimental technique that we have developed in order to measure the OAM spectrum of photons through just two intensity measurements.

30-Oct-2017

**Title:** Orbital Angular Momentum of Photons - I

**Speaker:** Anand Kumar Jha, Physics Dept., IITK

**Time:** 4:00 pm-5:00 pm, Mon, Oct 30th, 2017; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

In a pioneering work published in 1992, L Allen and coworkers showed that a photon in a light beam can carry orbital angular momentum (OAM) in the integer multiples of hbar. This result has made OAM a very important degree of freedom for both classical and quantum information protocols. This is because an information protocol requires a discrete basis, and the OAM degree of freedom provides a basis that is not only discrete but can also be high-dimensional and thus providing several unique advantages in terms of security, supersensitive measurements and fundamental tests of quantum mechanics. In this talk series, we will first describe what it means to have a photon carrying OAM and then discuss effects, such as angular interference and diffraction etc., that arise due to the fact that photons can carry OAM. We will then describe the existing techniques by which OAM carrying photons are generated and detected and also discuss the limitations of these technique. Finally, we will discuss a new experimental technique that we have developed in order to measure the OAM spectrum of photons through just two intensity measurements.

23-Oct-2017

**Title:** Controlling Spatial Coherence of Light - II

**Speaker:** Anand Kumar Jha, Physics Dept., IITK

**Time:** 4:00 pm-5:00 pm, Mon, Oct 23th, 2017; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

This talk will first discuss the basics of spatial coherence. We will then discuss some of the applications that exploit the spatial coherence properties of light and thereby motivate why it is important to be able to control the spatial coherence properties of light. Finally, we will discuss a recent technique that we have developed in order to produce light fields with arbitrary spatial coherence function.

16-Oct-2017

**Title:** Controlling Spatial Coherence of Light - I

**Speaker:** Anand Kumar Jha, Physics Dept., IITK

**Time:** 4:00 pm-5:00 pm, Mon, Oct 16th, 2017; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

This talk will first discuss the basics of spatial coherence. We will then discuss some of the applications that exploit the spatial coherence properties of light and thereby motivate why it is important to be able to control the spatial coherence properties of light. Finally, we will discuss a recent technique that we have developed in order to produce light fields with arbitrary spatial coherence function.

18-Sep-2017

**Title:** Quantum Error Correction Codes

**Speaker:** Tejas Gandhi, CSE Dept., IITK

**Time:** 4:00 pm-5:00 pm, Mon, Sep 18th, 2017; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

Deutsch's problem is a very simple and contrived problem to show that a quantum computer can perform better than the classical computer. We are given a function, f:{0,1} --> {0,1}. The task is to find whether the function is balanced or constant. We will present Deutsch's algorithm, given by David Deutsch, which will only query the function once. It is easy to show that any classical algorithm will take at least two queries.

11-Sep-2017

**Title:** Deutsch's algorithm- II

**Speaker:** Rajat Mittal, CSE Dept., IITK

**Time:** 4:00 pm-5:00 pm, Mon, Sep 11th, 2017; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

Deutsch's problem is a very simple and contrived problem to show that a quantum computer can perform better than the classical computer. We are given a function, f:{0,1} --> {0,1}. The task is to find whether the function is balanced or constant. We will present Deutsch's algorithm, given by David Deutsch, which will only query the function once. It is easy to show that any classical algorithm will take at least two queries.

04-Sep-2017

**Title:** Deutsch's algorithm- I

**Speaker:** Rajat Mittal, CSE Dept., IITK

**Time:** 4:00 pm-5:00 pm, Mon, Sep 04th, 2017; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

Deutsch's problem is a very simple and contrived problem to show that a quantum computer can perform better than the classical computer. We are given a function, f:{0,1} --> {0,1}. The task is to find whether the function is balanced or constant. We will present Deutsch's algorithm, given by David Deutsch, which will only query the function once. It is easy to show that any classical algorithm will take at least two queries.

18-Apr-2017

**Title:** Cutting Prescription for Amplitude computation involving off-shell propagators

**Speaker:** Joydeep Chakraborty, Dept. Of Physics, IITK

**Time:** 4:30 pm-5:30 pm; Tue; Apr 18th, 2017; tea @ 4:15 pm

**Venue:** FB-382

Abstract:

I will talk about our proposed prescription that simplifies the computation of a cascade decay of any length. Cascades are schematic representations of interaction among particles of different quantum numbers, say spin. This interaction chain may contain one or more particles as propagators. When these propagators do not satisfy the criteria p^2=m^2 (Here, p and m are the four momentum and mass of the propagators respectively), we call the off-shell. And for off-shell propagators the usual technique is a bit cumbersome. Our prescription is intended to ease the computation.

11-Apr-2017

**Title:** How Solid-state Environment affects Luminescence of Quantum Dots? - III

**Speaker:** Shilpi Gupta, Elec. Engg. Dept., IITK

**Time:** 4:30 pm-5:30 pm; Tue, Apr 11th, 2017; tea @ 4:15 pm

**Venue:** FB-382

Abstract:

Quantum dots are known as artificial atoms because tight spatial-confinement of charge carriers results in discretized energy states. These artificial atoms also interact with their solid-state environment (phonons) that plays a significant role in their luminescence properties. In this talk, we will discuss both coherent and incoherent interactions of a band-edge exciton with phonon modes in colloidal quantum dots. We will discuss a theoretical model that captures these interactions and explains experimentally observed features in luminescence spectrum and in decay dynamics of colloidal quantum dots.

28-Mar-2017

**Title:** How Solid-state Environment affects Luminescence of Quantum Dots? - II

**Speaker:** Shilpi Gupta, Elec. Engg. Dept., IITK

**Time:** 4:30 pm-5:30 pm; Tue, Mar 28th, 2017; tea @ 4:15 pm

**Venue:** FB-382

Abstract:

Quantum dots are known as artificial atoms because tight spatial-confinement of charge carriers results in discretized energy states. These artificial atoms also interact with their solid-state environment (phonons) that plays a significant role in their luminescence properties. In this talk, we will discuss both coherent and incoherent interactions of a band-edge exciton with phonon modes in colloidal quantum dots. We will discuss a theoretical model that captures these interactions and explains experimentally observed features in luminescence spectrum and in decay dynamics of colloidal quantum dots.

21-Mar-2017

**Title:** How Solid-state Environment affects Luminescence of Quantum Dots? - I

**Speaker:** Shilpi Gupta, Elec. Engg. Dept., IITK

**Time:** 4:30 pm-5:30 pm; Tue, Mar 21th, 2017; tea @ 4:15 pm

**Venue:** FB-382

Abstract:

Quantum dots are known as artificial atoms because tight spatial-confinement of charge carriers results in discretized energy states. These artificial atoms also interact with their solid-state environment (phonons) that plays a significant role in their luminescence properties. In this talk, we will discuss both coherent and incoherent interactions of a band-edge exciton with phonon modes in colloidal quantum dots. We will discuss a theoretical model that captures these interactions and explains experimentally observed features in luminescence spectrum and in decay dynamics of colloidal quantum dots.

07-Mar-2017

**Title:** Multipartite entanglement distribution and generation in Spin Systems-III

**Speaker:** V. Subrahmanyam, Dept. Of Physics, IITK

**Time:** 4:30 pm-5:30 pm; Tue, Mar 07th, 2017; tea @ 4:15 pm

**Venue:** FB-382

Abstract:

Quantum entanglement is recognised, in recent years, as a resource in information processing and communication protocols. I will introduce and discuss some measures of entanglement in the context of well-known quantum spin model ground states. Results for pair quantum correlations the ground states, random states will be discussed for Heisenberg-XY model. We will see how multipartite entanglement structure can evolve through Hamiltonian dynamics. Results for the dynamics of a kicked transverse field Ising model will be presented from the view point of generating multi-party entanglement structure, long-ranged Bell pairs.

28-Feb-2017

**Title:** Multipartite entanglement distribution and generation in Spin Systems-II

**Speaker:** V. Subrahmanyam, Dept. Of Physics, IITK

**Time:** 4:30 pm-5:30 pm; Tue, Feb 28th, 2017; tea @ 4:15 pm

**Venue:** FB-382

Abstract:

Quantum entanglement is recognised, in recent years, as a resource in information processing and communication protocols. I will introduce and discuss some measures of entanglement in the context of well-known quantum spin model ground states. Results for pair quantum correlations the ground states, random states will be discussed for Heisenberg-XY model. We will see how multipartite entanglement structure can evolve through Hamiltonian dynamics. Results for the dynamics of a kicked transverse field Ising model will be presented from the view point of generating multi-party entanglement structure, long-ranged Bell pairs.

21-Feb-2017

**Title:** Multipartite entanglement distribution and generation in Spin Systems-I

**Speaker:** V. Subrahmanyam, Dept. Of Physics, IITK

**Time:** 4:30 pm-5:30 pm; Tue, Feb 21th, 2017; tea @ 4:15 pm

**Venue:** FB-382

Abstract:

Quantum entanglement is recognised, in recent years, as a resource in information processing and communication protocols. I will introduce and discuss some measures of entanglement in the context of well-known quantum spin model ground states. Results for pair quantum correlations the ground states, random states will be discussed for Heisenberg-XY model. We will see how multipartite entanglement structure can evolve through Hamiltonian dynamics. Results for the dynamics of a kicked transverse field Ising model will be presented from the view point of generating multi-party entanglement structure, long-ranged Bell pairs.

14-Feb-2017

**Title:** Rethinking Nonlinear Optics (NLO): Quantum Interference based NLO-IV

**Speaker:** Harshawardhan Wanare, Dept. of Physics, IITK

**Time:** 4:30 pm-5:30 pm; Tue; Feb 14th, 2017; tea @ 4:15 pm

**Venue:** FB-382

Abstract:

Conventional perturbation methods underlying nonlinear optical phenomena are quite insufficient in realistic multilevel systems. A family of phenomena including large anisotropy to wave-mixing can arise due to quantum interference. Such coherent contributions cannot be explained within the conventional perturbative framework. We provide a systematic treatment in the light of quantum interference.

07-Feb-2017

**Title:** Rethinking Nonlinear Optics (NLO): Quantum Interference based NLO-III

**Speaker:** Harshawardhan Wanare, Dept. of Physics, IITK

**Time:** 4:30 pm-5:30 pm; Tue; Feb 07th, 2017; tea @ 4:15 pm

**Venue:** FB-382

Abstract:

Conventional perturbation methods underlying nonlinear optical phenomena are quite insufficient in realistic multilevel systems. A family of phenomena including large anisotropy to wave-mixing can arise due to quantum interference. Such coherent contributions cannot be explained within the conventional perturbative framework. We provide a systematic treatment in the light of quantum interference.

31-Jan-2017

**Title:** Rethinking Nonlinear Optics (NLO): Quantum Interference based NLO-II

**Speaker:** Harshawardhan Wanare, Dept. of Physics, IITK

**Time:** 4:30 pm-5:30 pm; Tue; Jan 31st, 2017; tea @ 4:15 pm

**Venue:** FB-382

Abstract:

Conventional perturbation methods underlying nonlinear optical phenomena are quite insufficient in realistic multilevel systems. A family of phenomena including large anisotropy to wave-mixing can arise due to quantum interference. Such coherent contributions cannot be explained within the conventional perturbative framework. We provide a systematic treatment in the light of quantum interference.

24-Jan-2017

**Title:** Rethinking Nonlinear Optics (NLO): Quantum Interference based NLO-I

**Speaker:** Harshawardhan Wanare, Dept. of Physics, IITK

**Time:** 4:00 pm-5:00 pm; Tue; Jan 24th, 2017; tea @ 3:45 pm

**Venue:** FB-382

Abstract:

28-Dec-2016

**Title:** Rotational Optomechanics

**Speaker:** Mishkatul Bhattacharya, Rochester Institute of Technology, Rochester, NY, USA

**Time:** 11:30 am-12:30 pm; Wed; Dec 28th, 2016; tea @ 11:15 am

**Venue:** FB-382

Abstract:

In this talk I will address the coupling of confined as well as unconfined electromagnetic modes with mechanical rotation.

27-Dec-2016

**Title:** Cavityless Optomechanics

**Speaker:** Mishkatul Bhattacharya, Rochester Institute of Technology, Rochester, NY, USA

**Time:** 11:30 am-12:30 pm; Tue; Dec 27th, 2016; tea @ 11:15 am

**Venue:** FB-382

Abstract:

In this talk I will address the coupling of unconfined or propagating electromagnetic modes with linear mechanical motion. The discussion will center around theoretical work carried out in our group and experiments in the group of our collaborator A. N. Vamivakas at the University of Rochester.

26-Dec-2016

**Title:** Optomechanics: Cavities Coupled to Linear Motion

**Speaker:** Mishkatul Bhattacharya, Rochester Institute of Technology, Rochester, NY, USA

**Time:** 11:30 am-12:30 pm; Mon; Dec 26th, 2016; tea @ 11:15 am

**Venue:** FB-382

Abstract:

In this talk I will address the basic paradigm of optomechanics, which couples confined electromagnetic modes to mechanical oscillation. Features such as cooling, displacement sensing and regenerative motion will be addressed with reference to experimental platforms.

22-July-2015

**Title:** Entanglement Measures - V

**Speaker:** V Subrahmanyam, Dept. of Physics, IITK

**Time:** 11:30 am-12:30 pm; Wed; July 22nd, 2015; tea @ 11:15 am

**Venue:** FB-382

Abstract:

We will discuss various aspects of entanglement of quantum states: bipartite states, non-completely positive operations, mixed state entanglement, entanglement of formation, concurrence, localizable entanglement, bound entanglement, squashed entanglement etc.

15-July-2015

**Title:** Entanglement Measures - IV

**Speaker:** V Subrahmanyam, Dept. of Physics, IITK

**Time:** 11:30 am-12:30 pm; Wed; July 15th, 2015; tea @ 11:15 am

**Venue:** FB-382

Abstract:

We will discuss various aspects of entanglement of quantum states: bipartite states, non-completely positive operations, mixed state entanglement, entanglement of formation, concurrence, localizable entanglement, bound entanglement, squashed entanglement etc.

08-July-2015

**Title:** Entanglement Measures - III

**Speaker:** V Subrahmanyam, Dept. of Physics, IITK

**Time:** 11:30 am-12:30 pm; Wed; July 08th, 2015; tea @ 11:15 am

**Venue:** FB-382

Abstract:

We will discuss various aspects of entanglement of quantum states: bipartite states, non-completely positive operations, mixed state entanglement, entanglement of formation, concurrence, localizable entanglement, bound entanglement, squashed entanglement etc.

01-July-2015

**Title:** Entanglement Measures - II

**Speaker:** V Subrahmanyam, Dept. of Physics, IITK

**Time:** 11:30 am-12:30 pm; Wed; July 1st, 2015; tea @ 11:15 am

**Venue:** FB-382

Abstract:

24-Jun-2015

**Title:** Entanglement Measures - I

**Speaker:** V Subrahmanyam, Dept. of Physics, IITK

**Time:** 11:30 am-12:30 pm; Wed; June 24th, 2015; tea @ 11:15 am

**Venue:** FB-382

Abstract:

17-Jun-2015

**Title:** Non-radiative recombination versus spontaneous emission in cavity-coupled quantum dots - II

**Speaker:** Shilpi Gupta, Dept. of Electrical Engineering, IITK

**Time:** 11:30 am-12:30 pm; Wed; June 17th, 2015; tea @ 11:15 am

**Venue:** FB-382

Abstract:

Quantum dots are interesting material system for a variety of applications in the field of photonics and electronics. A specific category of quantum dots that are synthesized using colloidal chemistry (known as colloidal quantum dots) are particularly interesting for developing on-chip-devices such as single photon sources, LEDs, and lasers, operating at room-temperature. In this talk, I will discuss what (a non-radiative recombination process) hinders low-threshold lasing with these quantum dots. Then I will talk about a potential solution to this problem (cavity-enhanced spontaneous emission). We will discuss a theoretical model, based on master equation formalism, for such a laser and analyze how lasing-threshold can be lowered.

10-Jun-2015

**Title:** Non-radiative recombination versus spontaneous emission in cavity-coupled quantum dots - I

**Speaker:** Shilpi Gupta, Dept. of Electrical Engineering, IITK

**Time:** 11:30 am-12:30 pm; Wed; June 10th, 2015; tea @ 11:15 am

**Venue:** FB-382

Abstract:

Quantum dots are interesting material system for a variety of applications in the field of photonics and electronics. A specific category of quantum dots that are synthesized using colloidal chemistry (known as colloidal quantum dots) are particularly interesting for developing on-chip-devices such as single photon sources, LEDs, and lasers, operating at room-temperature. In this talk, I will discuss what (a non-radiative recombination process) hinders low-threshold lasing with these quantum dots. Then I will talk about a potential solution to this problem (cavity-enhanced spontaneous emission). We will discuss a theoretical model, based on master equation formalism, for such a laser and analyze how lasing-threshold can be lowered.

27-May-2015

**Title:** Optimal copying of light beams - II

**Speaker:** Harshawardhan Wanare, Dept. of Physics, IITK

**Time:** 11:30 am-12:30 pm; Wed; May 27th, 2015; tea @ 11:15 am

**Venue:** FB-382

Abstract:

I will present the work of Romero and Dickey [JOSA, Vol. 24, 2280 (2007)] on the theory of optimally splitting a beam by phase gratings. This technique is proven and has been used in a variety of contexts including separating Angular momentum of light. We will understand the technique and hopefully generalize it and ask questions related to such optimal copying in the quantum context!

20-May-2015

**Title:** Optimal copying of light beams - I

**Speaker:** Harshawardhan Wanare, Dept. of Physics, IITK

**Time:** 4:00-5:00 pm; Wed; May 20th, 2015; tea @ 3:45 pm

**Venue:** FB-382

Abstract:

I will present the work of Romero and Dickey [JOSA, Vol. 24, 2280 (2007)] on the theory of optimally splitting a beam by phase gratings. This technique is proven and has been used in a variety of contexts including separating Angular momentum of light. We will understand the technique and hopefully generalize it and ask questions related to such optimal copying in the quantum context!

14-Oct-2014

**Title:** A universal lower bound on precision achievable in any general measurement

**Speaker:** Saikat Ghosh, Dept. of Physics, IITK

**Time:** 11:00 am-12:00 pm; Tue; Oct 14th, 2014; tea @ 10:45 am

**Venue:** FB-382

Abstract:

Any general measurement of a parameter (discrete or continuous) with a finite resource necessarily yields a set of distinct outcomes (say M) or estimates of the parameter. We observe that the corresponding precision is bounded below by 1/M and the mutual information between the parameter and the estimated value is bounded above by log(M). The number M, defined here as the precision capacity for the estimation, is independent of the dynamics of the model, setting an absolute and universal bound on the achievable precision. The corresponding probability distribution that saturates the bound is constructed explicitly, both for biased and unbiased estimators. Analysing few standard Heisenberg-limited estimation strategies (Kitaev's phase estimation strategy and quantum metrology with GHZ-like states), we demonstrate that saturating the Heisenberg limit is not a sufficient condition for exhausting the precision capacity. In effect, this work demonstrates that exhausting the precision capacity and thereby attaining the absolute precision limit of 1/M is equivalent to optimizing the overall measurement scheme, including optimal probe states along with judiciously chosen operators to measure an estimate of the parameter. Accordingly, a question of interest is what is the best one can do in terms of precision, using N probe particles and any amount of classical or quantum correlations? We find that the answer depends on particle symmetry. For distinguishable particles, it is found that Kitaev's phase estimation algorithm saturates the bound proposed here with identical distribution. With N indistinguishable particles, the upper bound for classical estimation strategies on precision capacity is N, in conformity with Holevo Bound. However, it scales as N^2 for a quantum estimation strategy. Finally, we construct the optimal set of operators that achieve this precision limit of 1/N^2 explicitly.

07-Oct-2014

**Title:** Quantum non-local games from Computer Science perspective - III

**Speaker:** Rajat Mittal, CSE Dept., IITK

**Time:** 11:00 am-12:00 pm; Tue; Oct 07th, 2014; tea @ 10:45 am

**Venue:** FB-382

Abstract:

Bell's inequality and experiments were instrumental in providing distinction between quantum correlations and classical correlations. This method can be generalized to the framework of quantum non-local games where these differences can be studied. It turns out that quantum non-local games are natural quantum analogs of multi prover interactive proof system, studied by computer scientists for last 30 years. This provides significant interest to study non-local games. We look at the question, how hard is it to calculate the value of these non-local games in the quantum framework? In this talk series, we will define these systems and look at preliminary results which answer the above question partially.

30-Sept-2014

**Title:** Quantum non-local games from Computer Science perspective - II

**Speaker:** Rajat Mittal, CSE Dept., IITK

**Time:** 11:00 am-12:00 pm; Tue; Sept 30th, 2014; tea @ 10:45 am

**Venue:** FB-382

Abstract:

Bell's inequality and experiments were instrumental in providing distinction between quantum correlations and classical correlations. This method can be generalized to the framework of quantum non-local games where these differences can be studied. It turns out that quantum non-local games are natural quantum analogs of multi prover interactive proof system, studied by computer scientists for last 30 years. This provides significant interest to study non-local games. We look at the question, how hard is it to calculate the value of these non-local games in the quantum framework? In this talk series, we will define these systems and look at preliminary results which answer the above question partially.

23-Sept-2014

**Title:** Quantum non-local games from Computer Science perspective - I

**Speaker:** Rajat Mittal, CSE Dept., IITK

**Time:** 11:00 am-12:00 pm; Tue; Sept 23rd, 2014; tea @ 10:45 am

**Venue:** FB-382

Abstract:

Bell's inequality and experiments were instrumental in providing distinction between quantum correlations and classical correlations. This method can be generalized to the framework of quantum non-local games where these differences can be studied. It turns out that quantum non-local games are natural quantum analogs of multi prover interactive proof system, studied by computer scientists for last 30 years. This provides significant interest to study non-local games. We look at the question, how hard is it to calculate the value of these non-local games in the quantum framework? In this talk series, we will define these systems and look at preliminary results which answer the above question partially.

09-Sept-2014

**Title:** Semiclassical theory of quantum quenches

**Speaker:** Uma Divakaran, Dept. of Physics, IITK

**Time:** 11:00 am-12:00 pm; Tue; Sept 9th, 2014; tea @ 10:45 am

**Venue:** FB-382

Abstract:

Equilibrium physics of a zero temperature quantum phase transition has been the focus of interest for many years. Recently, the non-equilibrium dynamics in systems undergoing such phase transitions has received a lot of attention due to the discovery of ultra-cold atoms trapped in optical lattices. I shall discuss the various theoretical aspects of the non-equilibrium dynamics studied till now, focusing mainly on sudden quenches and the corresponding semiclassical theory developed to explain the results. Such a semiclassical theory not only explains the evolution qualitatively but also quantitatively under certain limits. Taking various examples, I shall explain the success of this simple yet elegant semiclassical theory of sudden quenches in quantum Hamiltonians.

26-Aug-2014

**Title:** Information Geometry and Quantum Mechanics

**Speaker:** Tapobrata Sarkar, Dept of Physics, IITK

**Time:**11:00 - 12:00 pm; Tue; Aug 26th, 2014; tea @ 10:45 am

**Venue:** FB-382

Abstract:

We will review the notion of geometry in quantum mechanics. We will illustrate this by examples of atomic coherent states and the harmonic oscillator. We will then understand the information geometry of many body systems (especially the transverse field Ising model), and review some proposals to experimentally measure the information metric. These are closely related to correlations of the system.

21-Aug-2014

**Title:** Can gravity cause collapse of the wave function?

**Speaker:** Prof. T P Singh, TIFR, Mumbai

**Time:** 4:00 - 5:00 pm; Thu; Aug 21st, 2014; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

During a quantum measurement, the quantum system goes from being in a superposition of the eigenstates of the measured observable, to being in just one of the eigenstates. The principle of linear superposition is apparently violated. If this violation is caused by a dynamical mechanism, one possible origin for such a mechanism could be the gravitational field. In the present talk we discuss how gravity could bring about such a violation, and the current status of our understanding of the role of gravity in this context.

19-Aug-2014

**Title:** Information geometry and applications to Physical Systems - II

**Speaker:**Tapobrata Sarkar, Dept of Physics, IITK

**Time:** 11:00 - 12:00 pm; Tue; Aug 19th, 2014; tea @ 10:45 am

**Venue:** FB-382

Abstract:

In this talk, I will discuss the information geometry of some specific models in classical and quantum phase transitions. This will include the mean field Van der Waals and Weiss model, the Ising model, the Dicke model and the quantum XY spin chain. The meaning of geometric quantities will be elaborated upon, and I will try to focus on what new we can obtain by the geometric method, by taking inputs from the theory of the renormalization group.

12-Aug-2014

**Title:** Information geometry and applications to Physical Systems - I

**Speaker:** Tapobrata Sarkar, Dept of Physics, IITK

**Time:** 11:00 - 12:00 pm; Tue; Aug 12th, 2014; tea @ 10:45 am

**Venue:** FB-382

Abstract:

The use of elementary geometric methods in Statistics was initiated by P. C. Mahalanobis, in the 1930s. The applications of his work in the Physics of statistical systems were realized much later. Geometric methods in classical thermodynamics were developed by Ruppeiner and that in quantum systems by Provost and Vallee, in the late 1970s. Only a few years back, the topic started attracting a lot of attention following its relevance in the physics of classical and quantum phase transitions. In the first talk, I will give a historical and pedagogical introduction to the basics of information geometry and its applications to classical and quantum phase transitions. I will highlight its importance in a universal description of systems at widely different length scales.

05-Aug-2014

**Title:** Quantum Error Correcting Codes -III

**Speaker:** Piyush Kurur, CSE Dept., IITK

**Time:** 11:00 am - 12:00 pm; Tue; Aug 5th, 2014; tea @ 10:45 am

**Venue:** FB-382

Abstract:

My aim is to give a self-contained exposure to the theory of quantum error correcting codes. We start with the basics of classical error correcting codes and then generalize it to the theory of quantum codes. In the classical setting, codes that are often used in practice are the linear codes. The appropriate generalization of this class is the class of stabilizer codes (also known as additive codes), which we will discuss in the talk. I hope to make the talk self-contained with very little background required. In fact, the final construction of quantum stabilizer code can be seen as an exercise in combinatorics much like the classical case with hardly any quantum theory required to understand it.

29-July-2014

**Title:**Quantum Error Correcting Codes -II

**Speaker:** Piyush Kurur, CSE Dept., IITK

**Time:** 11:00 am - 12:00 pm; Tue; July 29th, 2014; tea @ 10:45 am

**Venue:** FB-382

Abstract:

My aim is to give a self-contained exposure to the theory of quantum error correcting codes. We start with the basics of classical error correcting codes and then generalize it to the theory of quantum codes. In the classical setting, codes that are often used in practice are the linear codes. The appropriate generalization of this class is the class of stabilizer codes (also known as additive codes), which we will discuss in the talk. I hope to make the talk self-contained with very little background required. In fact, the final construction of quantum stabilizer code can be seen as an exercise in combinatorics much like the classical case with hardly any quantum theory required to understand it.

22-July-2014

**Title:** Quantum Error Correcting Codes -I

**Speaker:** Piyush Kurur, CSE Dept., IITK

**Time:**11:00 am - 12:00 pm; Tue; July 22nd, 2014; tea @ 10:45 am

**Venue:** FB-382

Abstract:

My aim is to give a self-contained exposure to the theory of quantum error correcting codes. We start with the basics of classical error correcting codes and then generalize it to the theory of quantum codes. In the classical setting, codes that are often used in practice are the linear codes. The appropriate generalization of this class is the class of stabilizer codes (also known as additive codes), which we will discuss in the talk. I hope to make the talk self-contained with very little background required. In fact, the final construction of quantum stabilizer code can be seen as an exercise in combinatorics much like the classical case with hardly any quantum theory required to understand it.

01-July-2014

**Title:** What is so "Quantum" in Quantum Measurements-IV

**Speaker:** Saikat Ghosh, Dept. Of Physics, IITK

**Time:**11:30 - 12:30 pm, Tue, July 1st, 2014; tea @ 11:15 am

**Venue:** FB-382

Abstract:

Once a friend commented "Come on! Anything you can see in an oscilloscope (i.e. an everyday electronic device) can be explained classically." In this series of three talks, we will try to understand this statement and how far it might be true or false. In particular, we will explore issues of understanding and interpreting what is really measured in an experiment and what is it in those measurements that defies simple classical logic and mathematical intuition. Accordingly, we will explore specific quantum measurement techniques like homodyning, heterodyning, state tomography and possibly, measuring entanglement. We will use examples from real physical systems like atoms, photons and quantum mechanical oscillators. These talks will therefore, be an exercise to understand continuous variable quantum measurements and entanglement. All through, we will try to develop basic ideas centering around a simple physical measurement model of one or two cantilevers (harmonic oscillators close to their ground states) coupled to a classical (Von-Neumann) measurement device consisting of N harmonic oscillators.

24-June-2014

**Title:** What is so "Quantum" in Quantum Measurements-III

**Speaker:**Saikat Ghosh, Dept. Of Physics, IITK

**Time:** 11:30 - 12:30 pm, Tue, June 24th, 2014; tea @ 11:15 am

**Venue:** FB-382

Abstract:

Once a friend commented "Come on! Anything you can see in an oscilloscope (i.e. an everyday electronic device) can be explained classically." In this series of three talks, we will try to understand this statement and how far it might be true or false. In particular, we will explore issues of understanding and interpreting what is really measured in an experiment and what is it in those measurements that defies simple classical logic and mathematical intuition. Accordingly, we will explore specific quantum measurement techniques like homodyning, heterodyning, state tomography and possibly, measuring entanglement. We will use examples from real physical systems like atoms, photons and quantum mechanical oscillators. These talks will therefore, be an exercise to understand continuous variable quantum measurements and entanglement. All through, we will try to develop basic ideas centering around a simple physical measurement model of one or two cantilevers (harmonic oscillators close to their ground states) coupled to a classical (Von-Neumann) measurement device consisting of N harmonic oscillators.

23-June-2014

**Title:** CSL as a plausible mechanism for quantum to classical transition of primordial perturbations

**Speaker:** Suratna Das, Dept. Of Physics, IITK

**Time:** 11:30 - 12:30 pm; Mon; June 23rd, 2014; tea @ 11:15 am

**Venue:** FB-382

Abstract:

Once a friend commented "Come on! Anything you can see in an oscilloscope (i.e. an everyday electronic device) can be explained classically." In this series of three talks, we will try to understand this statement and how far it might be true or false. In particular, we will explore issues of understanding and interpreting what is really measured in an experiment and what is it in those measurements that defies simple classical logic and mathematical intuition. Accordingly, we will explore specific quantum measurement techniques like homodyning, heterodyning, state tomography and possibly, measuring entanglement. We will use examples from real physical systems like atoms, photons and quantum mechanical oscillators. These talks will therefore, be an exercise to understand continuous variable quantum measurements and entanglement. All through, we will try to develop basic ideas centering around a simple physical measurement model of one or two cantilevers (harmonic oscillators close to their ground states) coupled to a classical (Von-Neumann) measurement device consisting of N harmonic oscillators.

16-June-2014

**Title:** What is so "Quantum" in Quantum Measurements-II

**Speaker:** Saikat Ghosh, Dept. Of Physics, IITK

**Time:** 11:30 - 12:30 pm; Mon; June 16th, 2014; tea @ 11:15 am

**Venue:** FB-382

Abstract:

10-June-2014

**Title:** What is so "Quantum" in Quantum Measurements-I

**Speaker:**Saikat Ghosh, Dept. Of Physics, IITK

**Time:** 11:30 - 12:30 pm; Mon; June 10th, 2014; tea @ 11:15 am

**Venue:** FB-382

Abstract:

03-June-2014

**Title:** Photonic crystals: enablers of coherent control of light -III

**Speaker:** R. Vijaya, Dept. Of Physics, IITK

**Time:** 11:30 - 12:30 pm; Tue; June 3rd, 2014; tea @ 11:15 am

**Venue:** FB-382

Abstract:

After a brief discussion on the motivation for coherent control of light, I will introduce the concepts in the study of photonic crystals. Spontaneous emission is a fundamental optical process due to interaction between light and matter. While it is dependent on the properties of the excited atomic system, it depends also on the nature of the environment to which the atomic system is coupled. It is possible to tailor the accessible modes into which an excited atom can radiate through an appropriate design of photonic structures. In 2-3 lectures, these aspects will be elaborated through a discussion on the coherent control of light through photonic crystals.

26-May-2014

**Title:** Photonic crystals: enablers of coherent control of light -II

**Speaker:** R. Vijaya, Dept. Of Physics, IITK

**Time:** 04:00 - 05:00 pm; Mon; May 26th, 2014; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

After a brief discussion on the motivation for coherent control of light, I will introduce the concepts in the study of photonic crystals. Spontaneous emission is a fundamental optical process due to interaction between light and matter. While it is dependent on the properties of the excited atomic system, it depends also on the nature of the environment to which the atomic system is coupled. It is possible to tailor the accessible modes into which an excited atom can radiate through an appropriate design of photonic structures. In 2-3 lectures, these aspects will be elaborated through a discussion on the coherent control of light through photonic crystals.

19-May-2014

**Title:** Weak measurement and sub-Planck structure

**Speaker:** Prof. Prashant Panigrahi, IISER Kolkata

**Time:** 04:00 - 05:00 pm; Mon; May 19th, 2014; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

15-May-2014

**Title:** Photonic crystals: enablers of coherent control of light -I

**Speaker:** R. Vijaya, Dept. Of Physics, IITK

**Time:** 11:30 - 12:30 pm; Thu; May 15th, 2014; tea @ 11:15 am

**Venue:** FB-382

Abstract:

After a brief discussion on the motivation for coherent control of light, I will introduce the concepts in the study of photonic crystals. Spontaneous emission is a fundamental optical process due to interaction between light and matter. While it is dependent on the properties of the excited atomic system, it depends also on the nature of the environment to which the atomic system is coupled. It is possible to tailor the accessible modes into which an excited atom can radiate through an appropriate design of photonic structures. In 2-3 lectures, these aspects will be elaborated through a discussion on the coherent control of light through photonic crystals.

11-March-2014

**Title:** Second law of thermodynamics in the context of relativistic fluids - II

**Speaker:** Sayantani Bhattacharyya, Dept. Of Physics, IITK

**Time:** 4:00 - 5:00 pm; Tue; Mar 11th, 2014; tea @ 3:45 pm

**Venue:** FB-382

Abstract:

As a system evolves from one equilibrium to another, its total entropy always increases. Further if we know that the dynamics is governed by local equations, we expect this increase to be local as well. In this talk we shall explore the constraints, the dynamics must satisfy in order to ensure this local increase of entropy. Our final goal would be an attempt to prove that the existence of an equilibrium and its dynamical stability is sufficient for a local version of second law to be true, at least in the context of relativistic fluids.

04-March-2014

**Title:** Second law of thermodynamics in the context of relativistic fluids - I

**Speaker:** Sayantani Bhattacharyya, Dept. Of Physics, IITK

**Time:** 4:00 - 5:00 pm; Tue; Mar 04th, 2014; tea @ 3:45 pm

**Venue:** FB-382

Abstract:

As a system evolves from one equilibrium to another, its total entropy always increases. Further if we know that the dynamics is governed by local equations, we expect this increase to be local as well. In this talk we shall explore the constraints, the dynamics must satisfy in order to ensure this local increase of entropy. Our final goal would be an attempt to prove that the existence of an equilibrium and its dynamical stability is sufficient for a local version of second law to be true, at least in the context of relativistic fluids.

25-Feb-2014

**Title:** Elementary notions of Quantum Measurement (QM) - II

**Speaker:** Harshawardhan Wanare, Dept. of Physics, IIT Kanpur

**Time:** 04:00 - 05:00 pm; Tue; Feb 18th, 2014; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

I will quickly summarize POVM and describe an experiment in detail. The classic Young's double slit experiment where the path taken by the photon is "tracked" using "weak" measurement without destroying the interference pattern. "Kocsis, S. et al. Science 332, 1170-1173 (2011)."

11-Feb-2014

**Title:** Elementary notions of Quantum Measurement (QM)

**Speaker:**Harshawardhan Wanare, Dept. of Physics, IIT Kanpur

**Time:** 04:00 - 05:00 pm; Tue; Feb 11th, 2014; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

I hope to illustrate each of the following topics with a simple example and build-up towards weak measurement: (1) QM in orthonormal basis (2) Projective or von Neumann Measurement (3) Positive-Operator-Valued-Measure (POVM) (4) QM of Joint states Weak measurement.

24-Jan-2014

**Title:** Monogamy of shared quantum quantities

**Speaker:** Ujjwal Sen, HRI, Allahabad

**Time:** 12:00 - 01:00 pm; Fri; Jan 24th, 2014; tea @ 11:45 pm

**Venue:** FB-382

Abstract:

Monogamy is an important aspect of shared quantum quantities, including entanglement. We will discuss about the key features of this area. We will argue that while the structure remains a complex one with large deviations in behavior from one shared quantity to another, several connecting themes have begun to emerge in recent years. In particular, we will show that all quantum correlations can be made monogamous, and that almost all quantum states of moderately large systems are monogamous for all quantum correlation measures.

15-Jan-2014

**Title:** Quantum Shannon Theory-4: Measurement entropy, Bipartite systems, Entanglement, Purification, Secure Key Distribution

**Speaker:** V Subrahmanyam, Dept. Of Physics, IITK

**Time:** 4:00 - 5:00 pm; Wed; Jan 15th, 2014; tea @ 3:45 pm

**Venue:** FB-382

Abstract:

We will discuss the measurement basis and entropy continuing from the last talk. Then, we go on to discuss bipartite systems, entnaglement and Schmidt decomposition. We will briefly discuss key distribution protocols using qubits and their security.

07-Jan-2014

**Title:** Quantum Shannon Theory-3: Measurement entropy, Bipartite systems, Entanglement, Purification, Secure Key Distribution

**Speaker:** V Subrahmanyam, Dept. Of Physics, IITK

**Time:** 4:00 - 5:00 pm; Wed; Jan 7th, 2014; tea @ 3:45 pm

**Venue:** FB-382

Abstract:

We will discuss the measurement basis and entropy continuing from the last talk. Then, we go on to discuss bipartite systems, entnaglement and Schmidt decomposition. We will briefly discuss key distribution protocols using qubits and their security.

31-Dec-2013

**Title:** Quantum Shannon 2: Qubit Evolution, Stern-Gerlach apparatus, Shannon / von Neumann entropy, Bipartite composite systems

**Speaker:** V Subrahmanyam, Dept. Of Physics, IITK

**Time:** 4:00 - 5:00 pm; Tue; Dec 31st, 2013; tea @ 3:45 pm

**Venue:** FB-382

Abstract:

We will discuss how to prepare a pure/mixed qubit state using Stern-Gerlach appartus, discuss entropies of preparation and measurement, Shannon entropy of a random process (and its practical use in source coding theorem) and von Neumann entropy of a quantum system. We will describe the most general qubit evolution. We will then go over to composite quantum system, in particular bipartite system, the use of tensor product states, local and nonlocal operations, and the sub-system dynamics.

24-Dec-2013

**Title:** Quantum Shannon Theory: 1. Single-Qubit States and Evolution

**Speaker:** V Subrahmanyam, Dept. Of Physics, IITK

**Time:** 4:00 - 5:00 pm; Tue; Dec 24th, 2013; tea @ 3:45 pm

**Venue:** FB-382

Abstract:

We will discuss how to prepare a pure/mixed qubit state using Stern-Gerlach appartus, discuss entropies of preparation and measurement, Shannon entropy of a random process (and its practical use in source coding theorem) and von Neumann entropy of a quantum system. We will describe the most general qubit evolution. We will then go over to composite quantum system, in particular bipartite system, the use of tensor product states, local and nonlocal operations, and the sub-system dynamics.

05-Nov-2013

**Title:** Measuring the wavefunction with quantum weak values

**Speaker:** Dr. Mehul Malik, University of Vienna, Austria

**Time:** 12:00 - 1:00 pm; Tue; Nov 05, 2013 ; tea @ 11:45 am

**Venue:** FB-382

Abstract:

First introduced in 1988, quantum weak values have long dwelled in the realm of abstract theoretical physics. Over the last decade, however, there has been an explosion of laboratory experiments that use weak values for a variety of applications. For example, weak values have been used to amplify a detector signal and enable the detection of extremely small evolution parameters such as an angular mirror rotation of 400frad due to linear piezo motion of 14fm. More recently, weak values have been used to "directly" measure the quantum state itself. In this method, sequential weak and strong measurements are performed on specific observables of a quantum state, resulting in a weak value that is directly proportional to the complex probability amplitudes of the quantum state. In this talk, I will describe quantum weak values in terms of real, measurable quantities in the lab. Then, through the use of an experimental example, I will go on to explain how they are measured. Finally, I will describe some recent results from an experiment where we used weak values to measure a quantum state in the discrete, high-dimensional basis of orbital angular momentum.

21-Oct-2013

**Title:** Field-theoretic treatment of fluid turbulence-II

**Speaker:** Dr. Mahendra K. Verma, Dept. Of Physics, IITK

**Time:** 4:00 pm - 5:00 pm; Mon; Oct 21st, 2013; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

Exact calculations by Kolmogorov, experiments and numerical simulations reveal that the physics of intermediate length scales of fluid turbulence follow a universal power law.We will show that the above power law can be derived (under certain assumptions) using the field theory techniques. The procedure and its assumptions will be highlighted. I will try to relate to the interesting field-theoretic treatment of photonics that Anand gave sometime back.

14-Oct-2013

**Title:** Field-theoretic treatment of fluid turbulence

**Speaker:** Dr. Mahendra K. Verma, Dept. Of Physics, IITK

**Time:** 4:00 pm - 5:00 pm; Mon; Oct 14th, 2013; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

Exact calculations by Kolmogorov, experiments and numerical simulations reveal that the physics of intermediate length scales of fluid turbulence follow a universal power law.We will show that the above power law can be derived (under certain assumptions) using the field theory techniques. The procedure and its assumptions will be highlighted. I will try to relate to the interesting field-theoretic treatment of photonics that Anand gave sometime back.

07-Oct-2013

**Title:** Harnessing quantum interference in atomic systems

**Speaker:** A. Kani, Dept. Of Physics, IITK

**Time:** 4:00 pm - 5:00 pm; Mon; Oct 7th, 2013; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

Quantum interference between the excitation pathways modifies the optical response of the atomic system and leads to Coherent Population Trapping (CPT) and/or Electromagnetic Induced Transparency (EIT), or Electromagnetic Induced Absorption (EIA), or Amplification without Inversion (AWI), or Enhanced Refractive Index with vanishing absorption. We will present a simple four-level atomic system interacting with a bi-chromatic light field (double V-system) and discuss all these quantum interference effects and discuss the way to control over these effects.

30-Sept-2013

**Title:** Application of Information Theory to Chaos Theory: A Pedagogical Overview (Part III)

**Speaker:** Dr. Sagar Chakraborty, Dept. Of Physics, IITK

**Time:** 4:00 pm - 5:00 pm; Mon; Sept 30th, 2013; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

Chaos theory is one of the biggest and youngest highlights of 20th century physics/mathematics. It arguably spans a much wider range of disciplines than pervaded by other two highlights of 20th century: relativity and quantum mechanics. Still, for debatable reasons, conventional physics curriculum sidelines it. In two pedagogical talks, we shall see some of the beautiful ways of looking at chaos from the view-point of information theory. Our main aim is to introduce ourselves to the enigmatic topic of state-space reconstruction. We can then ask if any quantum mechanical extension of the idea is possible at all.

23-Sept-2013

**Title:** Application of Information Theory to Chaos Theory: A Pedagogical Overview (Part II)

**Speaker:** Dr. Sagar Chakraborty, Dept. Of Physics, IITK

**Time:** 4:00 pm - 5:00 pm; Mon; Sept 23rd, 2013; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

Chaos theory is one of the biggest and youngest highlights of 20th century physics/mathematics. It arguably spans a much wider range of disciplines than pervaded by other two highlights of 20th century: relativity and quantum mechanics. Still, for debatable reasons, conventional physics curriculum sidelines it. In two pedagogical talks, we shall see some of the beautiful ways of looking at chaos from the view-point of information theory. Our main aim is to introduce ourselves to the enigmatic topic of state-space reconstruction. We can then ask if any quantum mechanical extension of the idea is possible at all.

16-Sept-2013

**Title:** Application of Information Theory to Chaos Theory: A Pedagogical Overview (Part I)

**Speaker:** Dr. Sagar Chakraborty, Dept. Of Physics, IITK

**Time:** 4:00 pm - 5:00 pm; Mon; Sept 16th, 2013; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

Chaos theory is one of the biggest and youngest highlights of 20th century physics/mathematics. It arguably spans a much wider range of disciplines than pervaded by other two highlights of 20th century: relativity and quantum mechanics. Still, for debatable reasons, conventional physics curriculum sidelines it. In two pedagogical talks, we shall see some of the beautiful ways of looking at chaos from the view-point of information theory. Our main aim is to introduce ourselves to the enigmatic topic of state-space reconstruction. We can then ask if any quantum mechanical extension of the idea is possible at all.

19-Aug-2013

**Title:** Limits on Precision Measurement - II

**Speaker:** Dr. Saikat Ghosh, Dept. Of Physics, IIT Kanpur

**Time:** 4:00 pm - 5:00 pm; Mon; Aug 19th, 2013; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

Is there a fundamental limit to how precisely can one measure the value of a physical parameter? This question will be discussed here and in the next few lectures.

18-July-2013

**Title:** Temporal Coherence in Two-Photon Interference-IV

**Speaker:** Dr. Anand K. Jha, Dept. Of Physics, IIT Kanpur

**Time:** 11:00 am - 12:00 noon; Thursday; July 18th, 2013; tea @ 10:45 am

**Venue:** FB-382

Abstract:

The aim of this talk is to get into in-depth discussions about two-photon interference and related phenomena. In this talk we will conclude our discussions on the role of temporal coherence and indistinguishability in one- and two-photon interference experiments. We will keep our discussions of two-photon temporal coherence centered on parametric down-conversion, a non-linear optical phenomenon that produces entangled two-photon field.

11-July-2013

**Title:** Temporal Coherence in Two-Photon Interference-III

**Speaker:** Dr. Anand K. Jha, Dept. Of Physics, IIT Kanpur

**Time:** 11:00 am - 12:00 noon; Thursday; July 11th, 2013; tea @ 10:45 am

**Venue:** FB-382

Abstract:

The aim of this talk (and the next few) is to get into in-depth discussions about two-photon interference and related phenomena. In this talk we will conclude our discussions on the role of temporal coherence and indistinguishability in one- and two-photon interference experiments. We will keep our discussions of two-photon temporal coherence centered on parametric down-conversion, a non-linear optical phenomenon that produces entangled two-photon field.

04-July-2013

**Title:** Temporal Coherence in Two-Photon Interference-II

**Speaker:** Dr. Anand K. Jha, Dept. Of Physics, IIT Kanpur

**Time:** 4:00 pm - 5:00 pm; Thursday; July 04th, 2013; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

The aim of this talk (and the next few) is to get into in-depth discussions about two-photon interference and related phenomena.In this talk we will continue our discussions on the role of temporal coherence and indistinguishability in one- and two-photon interference experiments. We will keep our discussions of two-photon temporal coherence centered on parametric down-conversion, a non-linear optical phenomenon that produces entangled two-photon field.

28-June-2013

**Title:** Temporal Coherence in Two-Photon Interference

**Speaker:** Dr. Anand K. Jha, Dept. Of Physics, IIT Kanpur

**Time:** 11:00 am - 12:00 noon; Friday; June 28th, 2013; tea @ 10:45 am

**Venue:** FB-382

Abstract:

In the earlier two talks, we had discussed two-photon interference and entanglement from a very broad perspective. The aim of this talk (and the next few) is to get into in-depth discussions about two-photon interference and related phenomena. In this talk we will be discussing the role of temporal coherence and indistinguishability in two-photon interference. We will keep our discussions of temporal coherence centered around parametric down-conversion, a non-linear optical phenomenon that produces entangled two-photon field.

10-June-2013

**Title:** How many measurements are needed to generate an N-pixel image?

**Speaker:** Dr. Kedar B. Khare, Department of Physics, IIT Delhi

**Time:** 11:00 am - 12:00 noon; Monday; June 10th, 2013; tea @ 10:45 am

**Venue:** FB-382

Abstract:

In Scientific and Technological research ranging from sub-nano to astronomical length scales, imaging systems are playing a key role as tools that allow us to visualize natural phenomena or objects of interest, thereby, directly influencing new discoveries. With ever increasing demands on imaging performance (e.g. resolution, speed, sensitivity to noise, etc.), it is becoming clear that imaging systems of tomorrow need to acquire, process and utilize imaging data in an efficient non-redundant manner. In this seminar I will discuss some current ideas on image recovery from "incomplete" or sub-Nyquist sampled data with examples from my own research work in diagnostic healthcare and optical imaging. It will be shown that a hybrid approach - combining imaging hardware and new image recovery algorithm ideas - allows us to overcome conventionally perceived limits on imaging systems.

05-June-2013

**Title:** Coherent Pump-Probe Spectroscopy of Atomic and Molecular Systems

**Speaker:** Dr. Niharika Singh (BARC)

**Time:** 11:00 am - 12:00 noon; Wednesday; June 5th, 2013; tea @ 10:45 am

**Venue:** FB-382

Abstract:

Quantum coherence and interference provide an interesting outlook for designing strategies for the control of optical response of atomic/molecular medium. This theme is the main focus of work presented in this talk. The specific issues addressed are electromagnetically induced transparency (EIT), electromagnetically induced absorption (EIA), amplification without inversion (AWI), spontaneously generated coherence (SGC), Kerr nonlinearity and the effect of laser phase fluctuations. Also examined are the issues related to the effect of permanent dipole moments on the coherent dynamics of molecular systems, subluminal and superluminal light propagation and realization of negative refractive index in coherently prepared atomic medium. These phenomena help to understand the subtle quantum effects in laser-atom interactions, they on the other hand provide useful platform for development of quantum technologies.

31-May-2013

**Title:** Open Quantum Systems II: Spontaneous emission and decay

**Speaker:** Dr. Saikat Ghosh, Dept. Of Physics, IIT Kanpur

**Time:** 4:00 pm - 5:00 pm; Friday; May 31st, 2013; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

Continuing our discussions on Master Equations and possible alternatives, in this lecture we will discuss few examples of system-reservoir coupling (spontaneous emission, multi-level coupling, decay out of a cavity) by constructing explicit microscopic models, in the usual spirit of master equations. Physical implications of such couplings for a few-level quantum system will be discussed as a special case. We will end with a brief introductory sketch of the alternative (and more intuitive) (stochastic) wave-function approach of Carmichael and Gardiner, for easy comparison.

24-May-2013

**Title:** Open Quantum Systems I: Atom-Light Interactions

**Speaker:** Dr. Saikat Ghosh, Dept. Of Physics, IIT Kanpur

**Time:** 5:00 pm - 6:00 pm; Monday; May 24th, 2013; tea @ 04:45 pm

**Venue:** FB-382

Abstract:

Text book examples of isolated quantum systems are infact never observed in practice. In reality, every imaginable quantum system is "open" to the rest of the universe with the system eventually decaying to trivial "classical particle" like behavior. Though there are several insights and well established techniques for understanding such interactions in specific cases, there is no general consensus for an universal language to decribe or understand this transition. For example, untill recently, there was no method to describe even the simplest scenario of emission of a photon by a single quantum emitter like an atom or a quantum dot, with the photon eventually absorbed by a second atom(or an elementary photo-detector) located at a distant location! In next few lectures, we will try to discuss these issues with an atempt to develop the problem through a set of examples. This first lecture of the series will be an elementary introduction to atom-light interaction that will introduce the general notiions of decay and decoherence. The discussion will eventually be carried over to atom-photon interactions. In particular, we will take the case of generation of correlated photons(following Prof. Anand Jha's lectures) and discuss how the underlying atomic ineractions generate such exotic photonic states.

17-May-2013

**Title:** Exploring Energy-Time Entanglement using Geometric Phase

**Speaker:** Dr. Anand Kumar Jha, Dept. Of Physics, IIT Kanpur

**Time:** 4:00 pm - 5:00 pm; Friday; May 17th, 2013; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

This talk will begin with a brief introduction to the Pancharatnam phase, which is a geometric phase in polarization optics. We will then briefly discuss the Bell's inequality for two-particle entanglement and will illustrate how geometric phase could be used to explore two-particle, time-energy entanglement. Finally, some experimental results demonstrating the violation of Bell's Inequality for time-energy will be presented.

07-May-2013

**Title:** Two-Photon Interference and Entanglement-II

**Speaker:** Dr. Anand Kumar Jha, Dept. Of Physics, IIT Kanpur

**Time:** 4:00 pm - 5:00 pm; Friday; May 7th, 2013; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

This is the second talk that will be given on this topic. The talk will start with a brief introduction to parametric down-conversion, a nonlinear optical process that produces entangled photons. We will then review some of the very famous and intriguing two-photon interference effects that have been observed with these entangled photons. Although all of these effects have been explained using the mathematical framework worked out by Glauber, different conceptual pictures have often been used to describe them. In this talk, we will show that all of these effects have a unified description in terms of the two length parameters that are constructed using the six different length parameters that a general two-photon interference experiment involves. One of these length parameters has an exact analog in classical interferometry while the other length parameter is unique to two-photon interference and is central to the related quantum effects that are observed. The talk will conclude by reporting some new experimental results.

03-May-2013

**Title:** Two-Photon Interference and Entanglement-I

**Speaker:** Dr. Anand Kumar Jha, Dept. Of Physics, IIT Kanpur

**Time:** 4:00 pm - 5:00 pm; Friday; May 17th, 2013; tea @ 03:45 pm

**Venue:** FB-382

Abstract:

This will be the first of the two talks that will be given on this topic. The talk will start with a brief introduction to parametric down-conversion, a nonlinear optical process that produces entangled photons. We will then review some of the very famous and intriguing two-photon interference effects that have been observed with these entangled photons. Although all of these effects have been explained using the mathematical framework worked out by Glauber, different conceptual pictures have often been used to describe them. In this talk, we will show that all of these effects have a unified description in terms of the two length parameters that are constructed using the six different length parameters that a general two-photon interference experiment involves. One of these length parameters has an exact analog in classical interferometry while the other length parameter is unique to two-photon interference and is central to the related quantum effects that are observed. The talk will conclude by reporting some new experimental results.