Abstracts

Abstracts

Trade-off relations between the relative uncertainty of currents and the associated entropy production have been of enormous interest in the last few years in the field of non-equilibrium stochastic thermodynamics. It is now realized, for example, that the optimization of heat engines should balance power, efficiency as well as power fluctuations. In this talk, I will first present our recent results for steady-state thermodynamic uncertainty relation (TUR) for quantum charge and energy transport. In the second part of my talk, I will present our experimental results for heat exchange within the NMR setup. Our measurements of heat cumulants demonstrate the validity of generalized versions of the TUR, which are based on the fluctuation symmetry, but violations of the 'specialized' TUR, which holds only under certain dynamics. Theoretical calculations excellently trace experiments, providing deep insights about the violation of this specialized TUR.

We investigate weak coin flipping, a fundamental cryptographic primitive where two distrustful parties need to remotely establish a shared random bit. A cheating party can try to bias the output bit towards a preferred value. For weak coin flipping the parties have known opposite preferred values. By a weak coin flipping protocol with bias ϵ we mean that neither player can force the outcome towards their preferred value with probability more than 1/2 + ϵ. While it is known that classically, ϵ = 1/2 (the worst possible), Mochon showed in 2007 that quantumly, weak coin flipping can be performed with arbitrarily small bias (near perfect). His non-constructive proof used the so-called point game formalism—a series of equivalent reductions which were introduced by Kitaev to study coin flipping. He constructed point games with bias ϵ_M(k) = 1/(4k + 2) to prove the existence. The best, known explicit protocol, however, had bias approaching ϵ_M(1) = 1/6 (also due to Mochon, 2005). In the present work, we try to make the non-constructive part of the proof constructive, to wit, we make three main contributions towards the conversion of point-games into explicit protocols. First, we propose a framework—TIPG-to-Explicit-protocol Framework (TEF)—which simplifies the task of constructing explicit protocols. We use this framework to construct a protocol with bias ϵ_M(2) = 1/10. We then give the exact formulae for the unitaries corresponding to the point-games due to Mochon, allowing us to describe (almost) perfect coin flipping protocols analytically, i.e. with bias ϵ_M(k) for arbitrarily large k. Finally, we introduce an algorithm we call the Elliptic Monotone Align (EMA) algorithm. This algorithm, together with TEF, lets us convert any point-game into an explicit protocol numerically. We conclude by giving another analytic construction of unitaries for Mochon’s games using the ellipsoid picture introduced for the EMA algorithm.

I will discuss using NV centers spins in diamond to study ferromagnetic dynamics. I will present a broad overview of an approach using these spins' sensitivity to gigahertz-scale magnetic noise generated by the ferromagnets. The noise can either be thermally excited or due to the relaxation of a driven magnetization or spinwave mode. This relaxometry approach, where the resonant spin noise modifies the lifetime of the NV spins, allows for spectroscopy of ferromagnetic dynamics at sub-micron length scales and is robust to the NV spin orientation. I will discuss our observation of this phenomenon in various materials and our experiments to understand the coupling mechanism between NV centers and the ferromagnets. I will also discuss our recent results for the high driving field regime where multi-magnon processes can enhance the applicable field-frequency range for this kind of spectroscopy. This work was done in collaboration with groups of Profs. Chris Hammel (OSU) and Greg Fuchs (Cornell U.), and Dr. Michael Page (AFRL).

The canonical NISQ era algorithm for approximating the ground state and ground state energy of a Hamiltonian is variational quantum eigensolver (VQE). The training parameter landscape corresponding to VQE, however, can be highly non-convex, and, does not correspond to any well-characterized optimization program in general. Furthermore, recent works on the appearance of the vanishing gradients problem, as the circuit size or hardware noise increases, has led to valid concerns about the fate of VQE. At such a juncture, the quest for alternatives to VQE becomes pertinent. I will discuss systematic alternatives to VQE i.e. quantum assisted eigensolver (QAE) and iterative quantum assisted eigensolver (IQAE). We applied our algorithm to deep variational circuits, which were used by Google to show the barren plateau problem. As it turns out, Google's example problem was solved trivially by our algorithm for any random choice of the initial quantum state.

We address the question of existence of private quantum channel for qubits encoded in polarization degrees of freedom of a photon, that remains secure even if multi-photon (instead of single-photon) pulse is emitted. We show that random unitary channel distributed according to SU(2) Haar measure has this property. Further we analyze the qubit unitary k-designs. We show they ensure security if the photons' parity of the source is guaranteed. Otherwise, the qubit unitary k-designs do not guarantee perfect security.

Contintous variable measurement device independent quantum key distribution (CV MDI QKD) has been implemented over large transmission distances using non-Gaussian states. However, tech- nological difficulties and low keyrates withhold its progress. In this letter we address these issues by proposing a protocol in which in each run we stochastically subtract different number of photons on a two mode squeezed coherent state. This way we fully utilize the initial resource without discarding any runs unlike the current protocols using non-Gaussian states. We analyse the security of our protocol and find that it offers a higher keyrate than prevalent CV MDI QKD protocols over a longer transmission distance. The above factors make our protocol a viable option for a commercial application of the same.

Any quantum violation of a Bell inequality can be rephrased as a nonlocal game with quantum advantage. This provides an intuitive understanding of where the quantum advantage lies. Contextuality is known to be a crucial resource for quantum computation. However, although some forms of contextuality can be converted into violations of Bell inequalities (and nonlocal games), there is no general method which connects quantum contextuality, including both state-dependent and state-independent contextuality, to nonlocal games. Here we address this problem and introduce a general method that achieves this task. We apply it to the arguably most fundamental form of quantum state-dependent contextuality on single systems, the violation of the Klyachko-Can-Binicioglu-Shumovsky inequality, and discuss the virtues and limitations of the method.

The long-time behavior of classical dynamical systems, and the degree of randomness they exhibit (ranging from mere ergodicity to fully-developed chaos), have been investigated extensively in the literature. They continue to be subjects of abiding interest and current research. While the evolution equations for the dynamical variables describing the behavior of a classically chaotic system are necessarily nonlinear, the corresponding quantum dynamics is governed by the Schrodinger equation (or, in the more general case of mixed states, by the Liouville equation), which is inherently linear. It is not straightforward, therefore, to assess the extent of randomness in a quantum system, or the precise manner in which the information content of a quantum system changes with time. In this talk I will be discussing nonlinear dynamics of quantum systems using the expectation values of observables.

Quantum effects such as the environment assisted quantum transport (ENAQT) displayed in photosynthetic Fenne-Mathews-Olson (FMO) complex has been simulated on analog quantum simulators. Digital quantum simulations offer greater universality and flexibility over analog simulations. However, digital quantum simulations of open quantum systems face a theoretical challenge; one does not know the solutions of the continuous time master equation for developing quantum gate operators. I will present a new framework we have propose for digital quantum simulation of ENAQT by introducing new quantum evolution operators. As an example, using the dynamical equations we have proposed, we will present the simulation of the FMO complex in the digital setting, reproducing theoretical and experimental evidence of the dynamics. Will also discuss the optimal method for quantum circuit implementation that can be extrapolated to study other open quantum systems..Reference : Pragati Gupta and C. M. Chandrashekar.https://doi.org/10.1088/1367-2630/abcdc9

Microring resonator (MRR) is an important component, especially for silicon based integrated photonic circuits. In this talk, we'll discuss some fundamental limitations of silicon MRR and techniques to overcome for its applications in the area of nonlinear optics and/or quantum photonics applications.

A common-sense perception of a physical system is that it is inseparable from its physical properties. The notion of quantum Cheshire cat challenges this, as far as quantum systems are concerned. It shows that a quantum system can be decoupled from its physical property under suitable pre and postselections. However, in the quantum Cheshire cat setup, the decoupling is not permanent. The photon, for example, and its circular polarization is separated and then recombined. I will present a thought experiment where two photons (cats) are decoupled from their respective polarizations (grins) and then interchanged during recombination. The proposal hints that that the belongingness of a property for a physical system is very volatile in the quantum world.

Spin defects in diamond have revolutionized room temperature Quantum technologies including sensing, computing and communications. Whilst this huge success unraveling their full potential for a wide variety of Quantum protocols is still scanty at best. In this talk, I will give a brief overview of our recent experiments in this direction: (i) Realizing nanoscale heat engines with spin-spin and spin-mechanical systems, (ii) Steering of strong and weak Quantum measurements to generate many-body entangled states and new dynamical phase transitions in a spin bath, (iii) modern applications of Quantum Fourier transform in sensing (iv) the ability to solve NP-hard problems in classical graph theory and (v) prospects for generation of high-fidelity photon cluster states using spin defects.

Discriminating between unknown objects in a given set is a fundamental task in experimental science. Suppose you are given a quantum system which is in one of two given states with equal probability. Determining the actual state of the system amounts to doing a measurement on it which would allow you to discriminate between the two possible states. It is known that unless the two states are mutually orthogonal, perfect discrimination is possible only if you are given arbitrarily many identical copies of the state. In this talk we consider the task of discriminating between quantum channels, instead of quantum states. In particular, we discriminate between a pair of unitary channels acting on a quantum system whose underlying Hilbert space is infinite-dimensional. We prove that in contrast to state discrimination, one only needs a finite number of uses of these channels in order to discriminate perfectly between them. Furthermore, no entanglement is needed in the discrimination task. The measure of discrimination is given in terms of the energy-constrained diamond norm, and a key ingredient of the proofs of these results is a generalization of the Toeplitz-Hausdorff Theorem of convex analysis. This work was done jointly with Simon Becker (Cambridge), Ludovico Lami (Ulm) and Cambyse Rouze (Munich).

We study the tomography of unknown propagators for the spin system in the context of finite-dimensional Wigner representations, which completely characterize and visualize operators using shapes assembled from linear combinations of spherical harmonics[1]. These shapes can be experimentally recovered by measuring the expectation values of the rotated axial tensor operator. Recent works show the general methodology to experimentally recover the shapes for density matrices [2] and known quantum propagators (U) [3]. This work extends the scanning based tomography approach for the unknown propagators. We also devised a novel algorithm for the reconstruction of these unknown propagators. This approach is experimentally demonstrated on an IBM quantum device.

Photonics is the platform of choice to build a modular, easy-to-network quantum computer operating at room temperature. However, no concrete architecture has been presented so far that exploits both the advantages of qubits encoded into states of light and the modern tools for their generation. Here we propose such a design for a scalable and fault-tolerant photonic quantum computer informed by the latest developments in theory and technology. Central to our architecture is the generation and manipulation of three-dimensional hybrid resource states comprising both bosonic qubits and squeezed vacuum states. The proposal enables exploiting state-of-the-art procedures for the non-deterministic generation of bosonic qubits combined with the strengths of continuous-variable quantum computation, namely the implementation of Clifford gates using easy-to-generate squeezed states. Moreover, the architecture is based on two-dimensional integrated photonic chips used to produce a qubit cluster state in one temporal and two spatial dimensions. By reducing the experimental challenges as compared to existing architectures and by enabling room-temperature quantum computation, our design opens the door to scalable fabrication and operation, which may allow photonics to leap-frog other platforms on the path to a quantum computer with millions of qubits.

In 1984, Schrödinger-Newton equation opened the door to a non-relativistic regime of nano/micro-quantum-mechanics, instead of quantum cosmology, where quantum and gravity are equally important. The fundamental difficulties, well-known and less known ones, of this non-linear equation are summarized. Some concepts to relax or merely cope with them are interpreted.

This work is focused on the robustness of STIRAP (STImulated Raman Adiabatic Passage) and sa(superadiabatic)-STIRAP pulses, which facilitate the population transfer between the ground state and the second excited state in a three-level system. As per the quantum adiabatic theorem the system, which is initialized in an eigenstate, is always found in eigenstate of the instantaneous Hamiltonian. However, the experimental feasibility of the adiabatic processes imposes certain limitations that can be overcome by the use of superadiabatic processes such as sa-STIRAP. Sa-STIRAP speeds up the supposedly adiabatic evolution and precisely returns the expected final state of the system. We compare the theoretical findings with circuit QED based experiments using superconducting Josephson junctions.

As the miniaturization of electronic devices, which are sensitive to temperature, grows apace, sensing of temperature with ever smaller probes is more important than ever. Genuinely quantum mechanical schemes of thermometry are thus expected to be crucial to future technological progress. We propose an alternative method to measure the temperature of a bath using the weak measurement scheme with a finite-dimensional probe. The precision offered by the present scheme not only shows similar qualitative features as the usual quantum-Fisher-information-based thermometric protocols, but also allows for flexibility over setting the optimal thermometric window through the judicious choice of post-selection measurements.

Discussions of the infamous measurement problem of quantum foundations tend to focus on how the output of a measurement, the pointer position, can be thought of in consistent quantum mechanical terms, while ignoring the equally important issue of what this outcome says about the earlier microscopic situation the apparatus was designed to measure. An experimental physicist is typically much more interested in the path followed by a particle before it triggered his detector than in what happened later, and if quantum mechanics cannot provide a clear explanation, how can one claim that this theory has been confirmed by experiment? The talk will use Wheeler's delayed choice paradox to identify the fundamental conceptual issues underlying this second measurement problem, and then sketch the resolution provided by the consistent histories interpretation, using a modification of Birkhoff and von Neumann's quantum logic.

"Stabiliser operations occupy a prominent role in the theory of fault-tolerant quantum computing. They are defined operationally: by the use of Clifford gates, Pauli measurements and classical control. Within the stabiliser formalism, these operations can be efficiently simulated on a classical computer. However, an additional supply of ""magic states"" is enough to promote them to a universal, fault-tolerant model for quantum computing. To quantify the needed resources in terms of magic states, a resource theory of magic has been developed during the last years. I'll give an introduction to this line of thinking, and present a recent result which shows that two natural approaches to defining a resource theory of magic do not coincide. [Work with Markus Heinrich and Arne Heimendahl]"

Gaussian spatial-polarization entanglement in a coherent vectorial paraxial light field is studied. Detection of spatial-polarization entanglement through fringe movement on rotation of a linear polarizer, with the light field passing through the polarizer, is outlined. The fringe movement is shown to be a sufficient condition for the detection of spatial-polarization entanglement in coherent paraxial vector light fields. Two Gaussian light fields with a small relative tilt but with significant spatial overlap, and with orthogonal polarizations is shown to possess close to 1 ebit of spatial-polarization entanglement. Tunable Gaussian spatial-polarization entanglement is experimentally demonstrated.

A field with quantum correlations is seen as a resource to quantum-information applications as it could be exploited for performing tasks that would otherwise be impossible. One of the major challenges faced in the implementation of many quantum-information protocols is the efficient measurement of quantum states and quantum correlations, especially the high-dimensional quantum states. In this talk, I will present how partial coherence properties could be utilised for efficient measurements of high-dimensional quantum states and correlations. I will also present some of our works on the applications of partially coherent light fields for imaging and communication.

As a complementary analysis tool to conventional high-field NMR, zero- to ultralow-field (ZULF) NMR detects nuclear magnetization signals in the sub-microtesla regime. Ultrasensitive atomic magnetometers provide a new generation of quantum sensors for ZULF NMR. Due to the features such as low-cost, high-resolution and potability, ZULF NMR has recently attracted considerable attention in chemistry, biology, medicine, and tests of fundamental physics. In this talk, I will describe the basic principles, methodology, and our recent experimental and theoretical development of ZULF NMR with atomic magnetometers, including its applications in spectroscopy, quantum control, NMR-based quantum devices. The future prospects of ZULF NMR are also discussed.

The quantum capacity of two quantum channels with zero quantum capacity used consecutively would again be zero. But surprisingly, if one uses those two channels in a superposition of two alternative orders, one gets a perfect channel. This reveals a different kind of power that quantum switch exhibits in various situations. Interestingly, generalization of this technique gives rise to a more powerful task known as ‘random-receiver quantum communication’.

I present a coherent feedback protocol to prepare a quantum system with arbitrary initial state in a target state or drive it into target dynamics based on sequential interactions with controlling quantum systems. I discuss the mechanism, determine the conditions for the control channel to achieve convergence to the target and demonstrate resilience against noise. The control scheme does not require knowledge of the state of the system and allows for an unknown target, which is encoded in the controllers, to be the result of a quantum computation. It thus provides a mechanism for autonomous, purely quantum closed-loop control.

Enhancing the sensitivity of sensors by using quantum properties of light is one of the longstanding goals of the quantum optics. Such an enhancement will revolutionize the field of metrology through the development of quantum-enhanced sensors. In this talk, I will discuss how we have interfaced quantum states of light, known as twin beams, with the plasmonic sensors, and then show a quantum-based sensitivity enhancement is possible. These sensors are widely used in biological and chemical sensing applications, and offer a unique opportunity to bring such an enhancement to real-life devices.

Dual unitary circuits are toy solvable models of interacting many-body systems and that can range from being noninteracting to complete mixing. The talk will detail some quantum information-theoretic features of such models that provide a link between the mixing rates and the entangling power of the building blocks of these circuits. We also define Bernoulli circuits which are the most mixing of such many-body systems and provide several explicit constructions of such systems.

We study the storage capacity of quantum neural networks (QNNs) describing them by completely positive trace preserving (CPTP) maps acting on an N-dimensional Hilbert space. We demonstrate that QNNs can store up to N linearly independent pure states. For n qubits, QNN can reach an exponential storage capacity, O(2n), in contrast to classical neural networks whose storage capacity scales linearly with the number of neurons n. We estimate, employing the Gardner program, the relative volume of CPTP maps with M N stationary states and show that the volume decreases exponentially with M and shrinks to zero for M N + 1. We generalize hese results to QNNs storing mixed states as well as input-output relations for feed-forward QNNs. Our approach opens the path to relate storage properties of QNNs to the quantum features of the input-output states. This paper is dedicated to the memory of Peter Wittek.

The interpretation of the wave function in quantum mechanics has been a subject for debate ever since quantum mechanics was established. There are many interpretations of quantum mechanics, and one dominant interpretation is the Copenhagen interpretation where the wave function is a mere mathematical description. After many years of research in quantum information and teaching of quantum mechanics, I gradually formulated my own interpretation, a realistic interpretation of quantum mechanics： wavefunction is the system entity itself. In this keynote talk, I will present in details the main points of WISE. In particular, an explanation of the measurement is given. An encounter-delayed-choice experiment is described. Comparisons with other interpretations will also be discussed.

Principal component analysis (PCA) is a widely applied but rather time-consuming tool in machine learning techniques. In 2014, Lloyd, Mohseni, and Rebentrost showed that a quantum computer can speed up this process using the proposed a quantum PCA (qPCA) algorithm [Lloyd, Mohseni, and Rebentrost, Nat. Phys. 10, 631 (2014)], which still lacks experimental demonstration due to the experimental challenges in preparing multiple quantum state copies and implementing quantum phase estimations. Here, we propose a new qPCA algorithm using the hybrid classical-quantum control, where parameterized quantum circuits are optimized with simple measurement observables, which significantly reduces the experimental complexity. As one important PCA application, we implement a human face recognition process using the images from the Yale Face Dataset. By training our quantum processor, the eigenface information in the training dataset is encoded into the parameterized quantum circuit, and the quantum processor learns to recognize new face images from the test dataset with high fidelities. Our work paves a new avenue towards the study of qPCA applications in both theory and experiment.

"We consider a setting where a stream of qubits is processed sequentially. We derive fundamental limits on the rate at which classical information can be transmitted using qubits that decohere as they wait to be processed. Specifically, we model the sequential processing of qubits using a single server queue, and derive expressions for the classical capacity of such a quantum `queue-channel.' Focusing on two important noise models, namely the erasure channel and the depolarizing channel, we obtain explicit single-letter capacity formulas in terms of the stationary waiting time of qubits in the queue. Our capacity proof also implies that a `classical' coding/decoding strategy is optimal for these noise models. Our proof technique for the converse theorem generalizes readily - in particular, whenever the underlying quantum noise channel is additive, we can obtain a single-letter upper bound on the classical capacity of the corresponding quantum queue-channel. More broadly, our work begins to quantitatively address the impact of decoherence on the performance limits of quantum information processing systems..Reference: P. Mandayam, K. Jagannathan and A. Chatterjee, ""The Classical Capacity of Additive Quantum Queue-Channels,"" in IEEE Journal on Selected Areas in Information Theory, 1(2), 432-444 (2020)."

Are the global properties of complex quantum systems simply a consequence of the properties of their individual components? Or are there features that emerge, as result of self-organisation, uniquely characterising the complex object as a whole? The answer to these questions might unveil up-to-now unknown properties of molecular complexes, proteins, even living organisms, on the one hand, and engineered complex quantum communication networks on the other. In this talk I will show how our holistic approach combining quantum physics and complex network theory brings to light the existence of emergent spatial structures of quantum entanglement in a paradigmatic many-particle quantum system. I will also describe an efficient algorithm for quantum networks tomography, showing exponential improvement with respect to existing approaches, and hence allowing for the experimental observation of the new entanglement structures that we have discovered.

Modelling quantum systems as black boxes allows us to perform information-theoretic tasks without the need to trust the devices or even quantum mechanics. In many experimentally relevant situations, however, the boxes’ inputs or outputs correspond to spatiotemporal quantities like angles, time durations, or field directions, and the question arises how black box statistics can in principle “fit into space and time”. Here, I present a general framework [1] that admits to study this question with or without assuming the validity of quantum mechanics (QM). I present some preliminary results showing that assumptions on the devices’ response to spatiotemporal symmetries can significantly constrain the possible correlations. On the one hand, this leads to an exact classification of the (2,2,2) quantum Bell correlations, and on the other hand, it leads to novel protocols for witnessing Bell nonlocality. Finally, I speculate that such approaches may lead to new types of experimental tests of QM.

Thermodynamical laws strictly apply to macroscopic systems. This simple statement is, however, far from stretching the practical boundaries of thermodynamics. There is an unshakable faith among scientists that there is no escape from the thermodynamical limits, even in microscopical systems. On the other hand, revisions of how to apply the general laws to finite size, non-macroscopic objects have been discussed since the early 40s, which established the field of nanothermodynamics. In this talk, we will briefly present the basic principles of Hill’s nanothermodynamics and relate it to more modern approaches, known as Hamiltonian of mean force and temperature-dependent energy levels that allow us to extend nanothermodynamics to quantum thermodynamics of finite size quantum systems. Following the short introduction, we will present the application of these methods to the one and two-dimensional typical topological insulator models. Specifically, we will consider a Kitaev chain [1] and a Stanene thin film [2] models and discuss their quantum thermodynamical properties. Different thermodynamical behaviors, non-trivial geometric friction effects due to the presence of edge states will be pointed out. Running quantum heat engines that can exploit topological phase transitions, probing order of a topological transition using quantum thermodynamical transformations will be described. We will end by presenting other examples of surprising heat transfer effects in topological systems that can have apparent violations of thermodynamical laws at the boundaries of topological materials.

Recently, the quantum emitters such as color centers in nanodiamonds have attracted a huge research interest due to their excellent optical and spin properties at room temperature [1]. The most widely studied color center is the nitrogen vacancy (NV) center. It is an optically active center and has a unique emission spectrum with characteristic features like the zero phonon line (ZPL) which corresponds to pure electronic transitions and the phonon sidebands (PSB) which corresponds to non-resonant electronic transitions which are mediated by phonons. The PSB emission being incoherent in nature needs to be suppressed. At the same time, the ZPL which is very weak at room temperature needs to be enhanced so that the NV center can be used efficiently in various applications. Therefore, to enhance the utility of NV centers in these applications it is required to devise such methods which can be used to control the spontaneous emission and charge state conversion of NV centers. .Here we discuss the suppression in PSB emission intensity at room temperature by using engineered photonic structures such as photonic crystals which show high reflectivity at certain wavelengths called their stop gap [2]. The emission enhancement at the ZPL using the blue band-edge of the photonic stop gap would also be discussed [3]. The emission dynamics of these systems are also studied which complement the emission intensity results and directly describe the changes in density of photon states through the emission lifetime modifications of NV centers using photonic crystals [4]. The results acclaims importance in charge-state conversion processes induced by the excitation power of the laser...References.[1] I. Aharonovich, D. Englund, and M. Toth, Nature Photonics 10, 631 (2016)..[2] S. Sharma, Priya, S. K. Saini, and R. V. Nair, Review of Scientific Instruments 90, 023103 (2019)..[3] S. Sharma and R. V. Nair, Optics letters 43, 3989 (2018)..[4] S. Sharma and R. V. Nair, Physical Review A 101, 043420 (2020).

Quantum secret sharing (QSS) is a quantum cryptographic protocol that aims to distribute secret encryption keys among numerous parties. The technique harnesses quantum correlations and the uncertainty principle to ensure enhanced security against eavesdropping. Initially, the protocol was designed for quantum entangled ( Greenberger–Horne–Zeilinger) GHZ multi-photon states. The difficulty in generating multi-photon states is a main limiting factor that restricts the scalability (number of participants) and the quality of the encoded photon signals. Interestingly, an equivalent protocol was introduced that only uses single photon signals where each party applies simple unitaries consecutively on the same photon. Here we show that structured photons encoded in the polarisation, radial and azimuthal spatial degrees of freedom can be exploited to increase the information capacity, key generation rates and also improve the scalability of QSS for practical implementations.

With the rapid advances in quantum information science and technology, it is of paramount importance to efficiently characterize and develop resources that are capable of offering quantum advantages. Continuous-variable quantum computation is the most scalable implementation of quantum computation to date, but it requires non-Gaussian resources to allow for exponential speedup and fault tolerance. This can be accomplished with non-Gaussian states or non-Gaussian measurements by photon-number-resolved detection (PNRD). In this talk, we first discuss our recent results on quantum state tomography of a single-photon Fock state by PNRD using superconducting transition-edge sensor[1]. We directly probe the negativity of the Wigner function in the raw data without any inference or correction for decoherence. Next, we introduce and experimentally demonstrate a state tomography scheme by measuring the state overlap between the unknown state and a set of calibrated coherent state probes[2].The scheme is computationally efficient, loss tolerant, and robust against experimental noise. Towards the end, we discuss quantum state engineering for generating non-Gaussian states using a process known as photon catalysis, which involves coherent states, single-photon states, linear optics, and PNR measurements[3].

Cat states have played a pivotal role in illuminating the foundational and measurement aspects of quantum mechanics. Being composed of two classical coherent states, these are in a physical sense `minimally' quantum. They are robust against photon loss, retaining their original form after losing two photons. Therefore, the change in cat state after gaining a photon is then a natural question. It is of deep interest for foundational studies and detection of single photon, as also for quantum information processing through protocols like cat-code. ..Here, we investigate the effect of photon gain by the cat state, through a phase space description, involving Wigner function. Photon addition to the even cat state leads to measurable parity change, squeezing of the constituent Gaussians in phase space and distinct changes in statistics of the general cat state. In particular, the shift in the phase space interference is found to be the clear indicator of the number of photons gained by the state. Kitten state, composed of sub-Planck scale tiles in phase space, is measurably changed by photon addition, making it distinguishable. ..These studies of observable changes in cat and kitten states in phase space distribution, after a photon addition can lead to their applications in single photon detection and precision metrology.

Quantum algorithms are usually formulated using two-level sytems (qubits). However, in recent years the idea that the use of three-level systems (qutrits) would expand the computational Hilbert space ``for free´´ - that is, without having to add more physical components - has gained a renewed interest. Here we show that stimulated Raman adiabatic passage (STIRAP) and its superadiabatic version (saSTIRAP) have a natural geometric two-star representation on the Majorana sphere. In the case of STIRAP, we find that the evolution is confined to a vertical plane. A faster evolution can be achieved in the saSTIRAP protocol, which employs a counterdiabatic Hamiltonian to nullify the non-adiabatic excitations. We observe how, under realistic experimental parameters, the counterdiabaticterm corrects the trajectory of the Majorana stars toward the dark state. We also introduce a spin-1 average vector and present its evolution during the two processes. We show that the Majorana representation can be used as a sensitive tool for the detection of process errors due to ac Stark shifts and non-adiabatic transitions. Finally, we show how to extend this representation to mixed states. We also present a few results related to quantum tomography in these systems.

In classical physics, chaos is characterised as rapid divergence of evolution trajectories that are infinitesimally separated to begin with..This definition does not directly apply to the quantum case because the overlap of two quantum states is invariant under unitary evolution..A phase space evolution scenario is described to get around this problem.and understand quantum chaos.

There are many important facets of quantum cryptography and cryptanalysis, but all the aspects are not explored with equal rigor. Some of those issues will be discussed in view of our works of the last few years. Specifically, we will discuss the relevance and beauty of semi-quantum and orthogonal state based quantum cryptography and secure multipartite computation (using continuous and discrete variable resources) with a clear focus on the challenges associated with the practical realizations of such schemes. We will also discuss some strategies which can be used for quantum hacking.

Non-commutativity between two or more observables give rise to quantum superposition--- a fundamental feature of quantum mechanics. Quantum superposition then leads to coherence which is a useful resource in quantum information. We establish an inequality involving the quantum coherence of quantum state and a non-commutativity of an arbitrary observables with its diagonal part. The relation provides a direct method of obtaining an estimate of the quantum coherence of an arbitrary quantum state, without resorting to quantum state tomography. Further, we will discuss the trade-off relation between coherence and disturbance in quantum measurement. For bipartite states we will prove a trade-off relation between the quantum coherence, entanglement and disturbance. These relations provide insights on how to preserve coherence and entanglement under noisy channels.

Spontaneous parametric down-conversion is a versatile nonlinear optical process that is widely used for the preparation of photonic quantum states. However, depending on one's perspective, the resulting states can be viewed either as a bipartite state entangled in its spatiotemporal degrees of freedom, or as a squeezed state entangled in the particle-number degrees of freedom. Combining these two views, we obtain a more versatile representation of a parametric down-converted state, but it requires the use of a formalism, such as the Wigner functional formalism, that can treat both the particle-number degrees of freedom and the spatiotemporal degrees of freedom in terms of continuous variables. We'll briefly review the Wigner functional formalism. Then we show how a parametric down-converted state is represented in terms of all these degrees of freedom. The resulting representation is used to consider the entanglement and squeezing in parametric down-converted states to show how the different degrees of freedom affect these properties of the state.

A priori, there exists no preferential temporal direction as microscopic physical laws are time-symmetric. Still, the second law of thermodynamics allows one to associate the 'forward' temporal direction to a positive variation of the total entropy produced in a thermodynamic process, and a negative variation with its 'time-reversal' counterpart. This definition of a temporal axis is normally considered to apply in both classical and quantum contexts. Yet, quantum physics admits also superpositions between forward and time-reversal processes, thereby seemingly eluding conventional definitions of time's arrow. In this talk, I will demonstrate that a quantum measurement of entropy production can distinguish the two temporal directions, effectively projecting such superpositions of thermodynamic processes onto the forward (time-reversal) time-direction when large positive (negative) values are measured. Remarkably, for small values (of the order of plus or minus one), the amplitudes of forward and time-reversal processes can interfere, giving rise to entropy-production distributions featuring a more or less reversible process than either of the two components individually, or any classical mixture thereof. Finally, I will extend these concepts to the case of a thermal machine running in a superposition of the heat engine and the refrigerator mode, illustrating how such interference effects can be employed to reduce undesirable fluctuations.

In quantum mechanics, physical states are represented by rays in Hilbert space, which is a vector space imbued by an inner product. However, current quantum theory does not formally address the consequences of a changing inner product during the interval between preparation and measurement. We establish a theoretical framework for such a changing inner product, which we show is consistent with standard quantum mechanics. Furthermore, we prove that this change is described by a quantum channel, which is tomographically observable, and we elucidate how our result is strongly related to the exploding topic of PT-symmetric quantum mechanics. Experimentally, we explain how to realize a changing inner product for a qubit in terms of a qutrit protocol with a unitary channel.

We propose a protocol in which starting from several copies of bipartite noisy entangled states, we design a global and optimal local measurement-based protocol in one- and two-dimensional lattices by which any two or more prefix sites can be connected via entanglement. Production of bipartite as well as multipartite entangled states in a network is verified in a device independent way through the violation of Bell inequalities with two settings per site and with continuous range of settings.

We study the properties of stationary G-chains in terms of their generating functions. In particular, we prove an analogue of the Szegő limit theorem for symplectic eigenvalues, derive an expression for the entropy rate of stationary quantum Gaussian processes, and study the distribution of symplectic eigenvalues of truncated block Toeplitz matrices. We also introduce a concept of symplectic numerical range, analogous to that of numerical range, and study some of its basic properties, mainly in the context of block Toeplitz operators.

The amount of nonClassical correlations in a two qubit subsystem of a network of qubits is compared against the amount of genuine multiparty entanglement in the state of the network. Quantum discord is used as the quantifier of nonClassical correlations in the subsystem while the generalised geometric measure (GGM) is used to quantify global entanglement. We argue that the presence of nonClassical correlations in mixed state quantum computation models may be indicative of the ability of the mixed state to leverage the computational resources like entanglement of the global pure state that it is part of, leading to exponential speedups for certain computational problems.

Information can be encoded in bits and sent across a noisy channel. This classical notion can be generalized using quantum theory: information can be encoded in quantum bits (qubits) and sent across a noisy quantum channel. The generalization gives a quantum theory of communication. Unlike its classical counterpart, quantum communication can be non-additive: two quantum channels used together can send information at a rate strictly larger than the sum of rates for each separate channel. This non-additivity can be harnessed to enhance quantum communication, but it can also cause difficulties. For instance non-additivity makes it hard to check if a channel's quantum capacity (a quantum generalization of the Shannon capacity) is positive. We will present results about both positivity and non-additivity of the quantum capacity. These results include (1) a simple class of channels with non-zero quantum capacity and (2) a type of non-additivity where a quantum channel with no quantum capacity boosts quantum communication across another quantum channel. While these results reveal new and surprising ways for sending quantum information, they also raise conceptual issues about channels with no quantum capacity. Our results come from an interesting technical tool, log-singularities in the von-Neumann entropy [1], which may be of independent interest.

Silicon carbide (SiC) hosts many interesting defects that can potentially serve as qubits for a range of advanced quantum technologies. Some of them have very interesting properties, making them potentially useful,e.g., as interfaces between stationary and flying qubits. I will present a detailed overview of the spins' relevant properties in silicon vacancies of the 6H-SiC polytype in the talk. This includes the temperature-dependent photoluminescence, optically detected magnetic resonance, the relaxation times of the spins' longitudinal and transverse components and optical spin alignment measurements.

Interaction of light with qubits is central to most physical realizations of quantum computers. In this talk I will present the role of optical nonlinearity in light propagation through two different waveguide QED lattices, namely a chain of qubits with direct coupling between the nearest neighbors and a chain of connected resonators to each of which a qubit is side-coupled. I will discuss the loss of coherent light transmission with increasing intensity in these lattices due to effective photon-photon interactions and related photon blockade mediated by nonlinearity in qubits. In contrast to the direct-coupled qubits, we find a revival in the coherent light transmission in the side-coupled qubits at relatively higher intensities due to saturation of qubits by photons. We further study these lattices within the quasi-classical approximation, which fails for a broad set of parameters. I will then outline a technique devise by us to modify the quasi-classical analysis to give much better results. Finally I will show some curious results on light propagation in an inhomogeneous lattice of side-coupled qubits where we observe non-monotonicity in light transmission with increasing light intensity.

Unconventional systems, in which the concept of ghost schemes is adopted, have opened new directions and opportunities in imaging science using quantum and classical sources. However, their applicability in quantitative phase imaging potential remains a challenge. In this talk, we discuss issues, challenges and opportunities of the quantitative phase imaging with correlation optics. Special emphasis is given to the ghost imaging which works on the correlation calculations of the light intensity fluctuations of two beams; an object beam which propagates through the object and is detected by a single pixel detector, and a reference beam which does not interact with object and is recorded by multi-pixel detector with spatial resolution. Finally, we discuss a new basis for the quantitative phase imaging with ghost diffraction and demonstrate a ghost diffraction holographic microscopy (GDHM) for complex-valued imaging.

The preliminary results based on Hong Ou Mandel (HOM) interferometer show that the orbital angular momentum of light manifests itself in different ways in classical and quantum domain. After introducing HOM interferometer, we will present and discuss our experimental results.

Control over the quantum states of a massive oscillator is important for several technological applications and to test the fundamental limits of quantum mechanics. Recently, hybrid electromechanical systems using superconducting qubits, based on electric-charge mediated coupling, have been quite successful in this regard. In this talk, I shall introduce a hybrid device, consisting of a superconducting transmon qubit and a mechanical resonator coupled using the magnetic-flux. Such coupling stems from the quantum-interference of the superconducting phase across the tunnel junctions. Consequently, we detect thermomechanical motion using drive corresponding to average occupancy of less than one photon..In addition, the large coupling between qubit and mechanical resonator is manifested in the observation of the Landau–Zener–Stückelberg effect.

Quantum state tomography (QST) has been the traditional method for characterization of an unknown state. Recently, many direct measurement methods have been implemented to reconstruct the state in a resource efficient way. Here we present an interferometric method, in which any qubit state, whether mixed or pure, can be inferred from the visibility, phase shift, and average intensity of an interference pattern using a single-shot measurement—hence, we call it quantum state interferography[1]. This provides us with a “black box” approach to quantum state estimation, wherein, between the incidence of the photon and extraction of state information, we are not changing any conditions within the setup, thus giving us a true single shot estimation of the quantum state. In contrast, standard QST requires at least two measurements for pure state qubit and at least three measurements for mixed state qubit reconstruction. We then go on to show that QSI is more resource efficient than QST for quantification of entanglement in pure bipartite qubits. We experimentally implement our method with high fidelity using the polarization degree of freedom of light. An extension of the scheme to pure states involving d−1 interferograms for d-dimensional systems (qudits) is also presented. Thus, the scaling gain is even more dramatic in the qudit scenario for our method, where, in contrast, standard QST, without any assumptions, scales roughly as d^2..[1] S.N.Sahoo, S.Chakraborti, A.K.Pati, U.Sinha, Phys. Rev. Lett. 125 123601, 2020

We present a theoretical model of an on-chip thermally pumped three-level maser in a superconducting circuit based on a single artificial atom. We also demonstrate a method to detect the population inversion in the artificial atom from the influx of heat power into a weakly coupled output terminal. The proposed methods of converting heat into microwave radiation and control of energy-level populations by heating provide additional useful tools for circuit quantum electrodynamics experiments and heat management in quantum circuits.

Superconducting electrical circuits operating at milli-Kelvin temperatures have emerged as a leading platform for implementing quantum information processing systems. One of the important challenges in this architecture is the implementation of high-fidelity measurements of the quantum state. While tremendous progress has been made in the past decade, there are still some remaining challenges. In this talk, I will outline the quantum measurement problem and describe the various developments in this field like dispersive measurements and parametric amplifiers. I will conclude by discussing some experiments carried out in our research group to tackle some of these challenges

We discuss three primitive algorithms to evaluate overlaps and transition matrix time-series, which are used to construct a variety of quantum assisted quantum control algorithm implementable on NISQ devices. Unlike previous approaches, our method bypasses tomographically complete measurements and instead relies solely on single qubit measurements. We discuss circuit complexity of composed control algorithms and sources of noise arising from Trotterization and measurement errors.

"We consider asymptotically many non-interacting systems with multiple conserved quantities or charges. We generalize the seminal results of Sparaciari, Oppenheim and Fritz [Phys. Rev. A 96:052112, 2017] to the case of multiple, in general non-commuting charges. To this aim we formulate a resource theory of thermodynamics of asymptotically many non-interacting systems with multiple conserved quantities or charges. To any quantum state, we associate a vector with entries of the expected charge values and entropy of that state. We call the set of all these vectors the phase diagram of the system, and show that it characterizes the equivalence classes of states under asymptotic unitary transformations that approximately conserve the charges. Using the phase diagram of a system and its bath, we analyze the first and the second laws of thermodynamics. In particular, we show that to attain the second law, an asymptotically large bath is necessary. In the case that the bath is composed of several identical copies of the same ""elementary bath"", we quantify exactly how large the bath has to be to permit a specified work transformation of a given system, in terms of the number of copies of the elementary bath system per work system (bath rate). In particular, if the bath is relatively small, we show that the quantum setting requires an extended phase diagram exhibiting negative entropies. This corresponds to the purely quantum effect that at the end of the process, system and bath are entangled, thus permitting classically impossible transformations (unless the bath is enlarged). For a large bath, or many copies of the same elementary bath, system and bath may be left uncorrelated and we show that the optimal bath rate, as a function of how tightly the second law is attained, can be expressed in terms of the heat capacity of the bath."

The question of a quantum memory storage of quantum dynamics will be investigated. In particular, I will design an optimal protocol for N -> 1 probabilistic storage-and-retrieval of unitary channels on d-dimensional quantum systems. If we may access the unknown unitary gate only N-times, the optimal success probability of perfect retrieval of its single use is N/(N−1+d^2). The derived size of the memory system exponentially improves the known upper bound on the size of the program register needed for probabilistic programmable quantum processors. These results are closely related to probabilistic perfect alignment of reference frames and probabilistic port-based teleportation.

The equivalence of mode squeezing and quantum entanglement for two-mode Gaussian states under a dissipative environment is discussed. The comparative robustness of squeezing and quantum entanglement for two-mode Gaussian states under different dissipative environments is considered. First, the case of the local bath is discussed where the individual modes interact with separate thermal baths, then the case of the global thermal bath is discussed where both the modes interact with the same environment. Depending on the nature of dissipative environments and initial squeezing of the state either squeezing or entanglement can be more robust to environmental interactions hence can be a better resource to store nonclassicality, and may enhance the performance of various quantum information processing protocols.