: We first talk about indefiniteness of causal order and how it can be exploited in quantum information processing tasks. We then introduce the task of random-receiver quantum communication, in which a sender transmits a quantum message to a receiver selected from a list of n spatially separated parties. At the moment of transmission, the choice of receiver is unknown to the sender. Later, it becomes known to the n-parties, who coordinate their actions by exchanging classical messages. In normal conditions, random-receiver quantum communication requires a noiseless quantum communication channel between the sender and each of the n receivers. In contrast, we show that random-receiver quantum communication can take place through noisy, entanglement-breaking channels if the order of such channels is coherently controlled by a quantum bit that is accessible through measurements. While this phenomenon is achieved with a single control qubit, it cannot be mimicked by adding a noiseless qubit channel from the sender to any of the receivers, or more generally, from the sender to any subset of k < n parties.

In this talk, I will try to elucidate the rapport of work and information in the context of a minimal quantum mechanical setup: A converter of heat input to work output, the input consisting of a single oscillator mode prepared in a hot thermal state along with few much colder oscillator modes. We wish to achieve heat to work conversion in the setup while avoiding the use of a working substance (medium) or macroscopic heat baths. We compare the efficiency of work extraction and the limitations of power in our minimal setup by reversible manipulations and by different, generic, measurement strategies of the hot and cold modes. I will present, by generalizing a method based on optimized homodyning that we have recently proposed, the following insight: extraction of work by observation and feedforward (WOF) that only measures a small fraction of the input, is clearly advantageous to the conceivable alternatives. However, the main drawback of work extraction by measurement is it inevitably requires feedforward and outcome dependent control steps. To circumvent this, I will briefly discuss autonomous, coherent work extraction exploiting non-linear cross-Kerr interaction. To conclude, our results may become a basis of a practical strategy of converting thermal noise to useful work in optical setups, such as coherent amplifiers of thermal light, as well as in their optomechanical and photovoltaic counterparts.

In this talk, I will be discussing two classes of Bell diagonal indecomposable entanglement witnesses in C^4 ⊗ C^4. The first class is a generalization of the well-known Choi witness from C^3 ⊗ C^3 , while the second one contains the reduction map. I will show contrary to C^3 ⊗ C^3 case, the generalized Choi witnesses are no longer optimal. Thereafter, I will talk about an optimization procedure for finding spanning vectors that eventually give rise to optimal witnesses. Operators from the second class turn out to be optimal, however, without the spanning property. Our analysis sheds a new light into the intricate structure of optimal entanglement witnesses.

Incompatibility of quantum devices is a useful resource in various quantum information theoretical tasks, and it is at the heart of some fundamental features of quantum theory. In the work [A. Mitra and M. Farkas, Phys. Rev. A 105, 052202 (2022)], we revise a notion of instrument compatibility that was already introduced in the literature, and we call this notion as traditional compatibility. Then, we discuss the notion of parallel compatibility and show that these two notions are inequivalent. We then argue that the notion of traditional compatibility has conceptual drawbacks and prove that parallel compatibility does not have such drawbacks. Hence, we propose to adopt parallel compatibility as the definition of the compatibility of quantum instruments. On other hand, in the work [A. Mitra, M. Farkas, arXiv:2209.02621 [quantph] (accepted in Phys. Rev. A)], we try to characterize and quantify parallel compatibility of quantum instruments.

Quantum dense coding (DC), first introduced by Bennett and Weisner for qubit systems, was generalized to the continuous variable (CV) paradigm by Braunstein and van Look for a single sender and a single receiver. In this talk, I will discuss the continuous variable dense coding (CV-DC) scheme for multiple senders sharing a multimode entangled state with a single receiver. I will present our framework for generating a multimode-dense coding network using quantum optical fields and demonstrate the quantum advantage of the protocol using paradigmatic classes of three- and four-mode states. The protocol is scalable to arbitrary numbers of senders. The encoding operations comprise local displacements while the decoding involves homodyne measurements of the modes. Our protocol involves simple optical elements such as beam splitters and mirrors, thereby exhibiting its potentiality for experimental realization.

Non-Hermitian rotation-time reversal (RT)-symmetric spin models possess two distinct phases, the unbroken phase in which the entire spectrum is real and the broken phase which contains complex eigenspectral, thereby indicating a transition point, referred to as an exceptional point. We report that the dynamical quantities, namely short and long time average of Lo Schmidt echo which is the overlap between the initial and the final states, and the corresponding rate function, can faithfully predict the exceptional point known in the equilibrium scenario. In particular, when the initial state is prepared in the unbroken phase and the system is either quenched to the broken or unbroken phase, we analytically demonstrate that the rate function and the average Lo Schmidt echo can distinguish between the quench occurred in the broken or the unbroken phase for the nearest-neighbour XY model with uniform and alternating magnetic fields, thereby indicating the exceptional point. Furthermore, we exhibit that such quantities are capable of identifying the exceptional point even in models like the non-Hermitian XYZ model with magnetic field which can only be solved numerically.

Generating entanglement between more parties is one of the central tasks and challenges in the backdrop of building quantum technologies. I will discuss two recently proposed protocols to create genuine multipartite entangled states in networks -- measurement-based and deterministic schemes. In the former case, we employ weak entangling measurement on two parties as the basic unit of operation to produce entanglement between more parties starting from an entangled state with a lesser number of parties and auxiliary single-qubit systems which we call “multipartite entanglement inflation”. The latter deterministic procedure involves optimization over a set of two-qubit arbitrary unitary operators and the entanglement of the initial resource state which we refer to as entanglement circulation.

We give a comprehensive characterization of the computational power of shallow quantum circuits combined with classical computation. Specifically, for classes of search problems, we show that the following statements hold, relative to a random oracle:

In the classical RAM, we have the following useful property. If we have an algorithm that uses M memory cells throughout its execution, and in addition is sparse, in the sense that, at any point in time, only m out of M cells will be non-zero, then we may “compress” it into another algorithm which uses only m log M memory and runs in almost the same time. We may do so by simulating the memory using either a hash table, or a self-balancing tree. We show an analogous result for quantum algorithms equipped with quantum random-access gates. If we have a quantum algorithm that runs in time T and uses M qubits, such that the state of the memory, at any time step, is supported on computational-basis vectors of Hamming weight at most m, then it can be simulated by another algorithm which uses only O(m log M) memory, and runs in time Õ(T ). We show how this theorem can be used, in a black-box way, to simplify the presentation in several papers. Broadly speaking, when there exists a need for a space-efficient history-independent quantum data structure, it is often possible to construct a space-inefficient, yet sparse, quantum data structure, and then appeal to our main theorem. This results in simpler and shorter arguments.

Metamaterials are artificial materials consisting of micro or nano composites which exhibit properties different from their components in sub-wavelength regime. These materials are finding applications in several areas such as sensing, sub-diffractive imaging, negative refractive index materials, perfect absorption, Huygen’s lens and quantum technologies. More recently all-dielectric metamaterials have been garnering much attention due to the reduction in the dissipative losses which their metallic counterparts suffer from. Our research involves understanding the light-matter interactions in all-dielectric materials using simulations and designing reconfigurable multi-functional dielectric devices for applications in sensing, energy harvesting and quantum technologies. In the talk, I will briefly discuss the overview of our research area and give details of some recent findings on tunable soft-metamaterials.

From quantum computation to communication, design of quantum devices and peripheral technology relies strongly on the interaction of light with matter. This requires not only modelling these interactions at the microscopic level but also seeking appropriate solutions at different operational regime. In this talk, we informally look at a few approaches that can be used to study such light-matter interactions. These range from rate equations, cluster expansion and quantum trajectories to study condensates of light to tensor-network methods to design spin-ensemble based quantum memories.

We discuss Bose's notion of indistinguishability at the combinatorial level as introduced by him in his seminal 1924 paper. We further describe its extension in a quantum mechanical setting and discuss various quantum statistics (including Bose and Fermi) describing N identical and indistinguishable particles. We also show how the theory of symmetric functions can be utilised to express the partition functions for all such statistics in terms of Schur functions.

In this work, drawing inspiration from the type of noise present in real hardware, we study the output distribution of random quantum circuits under practical non-unital noise sources with constant noise rates. We show that even in the presence of unital sources like the depolarizing channel, the distribution, under the combined noise channel, never resembles a maximally entropic distribution at any depth. To show this, we prove that the output distribution of such circuits never anticoncentrates — meaning it is never too "flat"— regardless of the depth of the circuit. This is in stark contrast to the behavior of noiseless random quantum circuits or those with only unital noise, both of which anticoncentrate at sufficiently large depths. As consequences, our results have interesting algorithmic implications on both the hardness and easiness of noisy random circuit sampling, since anticoncentration is a critical property exploited by both state-of-the-art classical hardness and easiness results.

Quantum Hamiltonian simulation (QHS) is one of the fundamental quantum sub-routines in quantum physics and quantum computing. QHS prepares a unitary approximately within some error bound for a given Hamiltonian. It has immense applications in science and engineering, from solving linear systems of equations to finding the ground state energy of an atomic configuration. We focus on the algorithmic side of the QHS problem for different practical Hamiltonian systems (such as signal processing, communication, and chemistry) and analyse their performance. Further, we look for interesting applications where these algorithms may bring some quantum advantage. One of the exciting research problems is to simulate the potential energy Hamiltonian on a practical quantum computer for some atomic configuration. Given a 1-dimensional potential energy function (of an arbitrary nature), we propose a Quantum polynomial approximate encoding algorithm that can offer a complexity improvement over the existing Hadamard basis encoding with complexity for some qubit size.

Optical photons are excellent quantum information carriers, but weak optical nonlinearity poses significant challenges to these systems' scalability and computational capabilities. Currently, only probabilistic methods can achieve nonlinear quantum operations crucial for universality and fault tolerance, restricting the clock speed and making it challenging to scale due to significant resource overhead. Ultrafast quantum nanophotonics with second-order optical nonlinearity presents a potential solution to overcome these challenges. In this talk, we will discuss recent experimental advances, including the on-chip generation and measurement of ultra-broadband squeezed states and all-optical realizations of switching and nonlinear functions in an emergent thin-film lithium niobate platform. We further delve into how enhanced optical nonlinearity can enable novel functionalities such as photon-number-resolving measurements and deterministic quantum state engineering, offering a practical path to scalable, fault-tolerant quantum information processors at room temperature.  

Introduced in 2010 by Scott Aaronson and Alex Arkhipov, Boson sampling is a specific quantum task demonstrating that a quantum device, even in the presence of noise, can outperform classical computers. In this presentation, I will introduce the Boson sampling problem and its implementation using neutral atoms instead of photons. Additionally, I will discuss an extension of this work involving on-site interactions, specifically the Bose-Hubbard model. Furthermore, I will touch upon the unexpected real-world application of boson sampling in seemingly unrelated blockchain networks. If time allows, I will also cover my work on quantum algorithms for thermal state preparation using fluctuation theorems.

We explore the interplay between symmetry and randomness in quantum information. Adopting a geometric approach, we consider states as H-equivalent if related by a symmetry transformation characterized by the group H. We then introduce the Haar measure on the homogeneous space U/H, characterizing true randomness for H-equivalent systems. While this mathematical machinery is well-studied by mathematicians, it has seen limited application in quantum information: we believe our work to be the first instance of utilizing homogeneous spaces to characterize symmetry in quantum information. This is followed by a discussion of approximations of true randomness, commencing with t-wise independent approximations and defining t-designs on U/H and H-equivalent states. Transitioning further, we explore pseudorandomness, defining pseudorandom unitaries and states within homogeneous spaces. Finally, as a practical demonstration of our findings, we study the expressibility of quantum machine learning ansatze in homogeneous spaces. Our work provides a fresh perspective on the relationship between randomness and symmetry in the quantum world.

A future quantum network will allow distributing entanglement over in principle arbitrarily long distances. This topic holds importance for applications in quantum information science as well as for fundamental investigations and currently receives significant attention around the world. The most promising approach for achieving long-distance quantum communication is to use quantum repeaters, many of which employ optical quantum memories to store quantum information. Among the candidates for building such quantum memories, rare earth ion-doped crystals exhibit noteworthy promise.  In this context, our research delves into the exploration of a thulium-doped yttrium gallium garnet crystal (Tm: YGG) at ultralow temperatures, as low as 500 mK. This crystal offers an optical coherence time exceeding one millisecond and a ground-state Zeeman-level lifetime as long as tens of seconds. We take advantage of such exceptional features to show a series of pivotal experiments. These include the storage of optical pulses for durations of up to 100 μs, frequency-multiplexed storage of multiple frequency modes, and a proof-of-principle demonstration showcasing the selective retrieval of stored frequency modes.  Furthermore, I will also shed some light on the optical coherence and relaxation dynamics of Tm: YGG, which is crucial for the development of an efficient quantum memory. Our results suggest that Tm: YGG exhibits substantial potential as a multimode optical quantum memory within the framework of a frequency-multiplexed quantum repeater architecture. 

Thanks to the rapid progress and growing complexity of quantum algorithms, correctness of quantum programs has become a major concern. Pioneering research over the past years has proposed various approaches to formally verify quantum programs using proof systems such as quantum Hoare logic. All these prior approaches are post-hoc: one first implements a complete program and only then verifies its correctness. In this work, we propose Quantum Correctness by Construction (QbC): an approach to constructing quantum programs from their specification in a way that ensures correctness. We use pre- and postconditions to specify program properties, and propose a set of refinement rules to construct correct programs in a quantum while language. We validate QbC by constructing quantum programs for two idiomatic problems, teleportation and search, from their specification. We find that the approach naturally suggests how to derive program details, highlighting key design choices along the way. As such, we believe that QbC can play an important role in supporting the design and taxonomization of quantum algorithms and software.

Entanglement detection and quantification of entanglement are the two most important problems in quantum information theory because quantum entanglement is a key resource in quantum information processing. I will divide the talk into two parts: In the first part I will talk about realignment criterion, which is considered as powerful tool for detection of entangled states in bipartite and multipartite quantum system. It detects not only negative partial transpose entangled states (NPTES) but also detect positive partial transpose entangled states (PPTES). We have approximated the realignment map to a positive map using the method of structural physical approximation (SPA) and then we have shown that the structural physical approximation of realignment map (SPA-R) is completely positive. Positivity of the constructed map is characterized using moments which can be physically measured. Next, we develop a separability criterion based on our SPA-R map in the form of an inequality and have shown that the developed criterion not only detect NPTES but also PPTES. We have provided some examples to support the results obtained. In the second part, we define a physically realizable measure of entanglement for any arbitrary dimensional bipartite system ρ, which we named as structured negativity (NS(ρ)). We have shown that the introduced measure satisfies the properties of a valid entanglement monotone. For d ⊗ d dimensional state, we conjectured that negativity coincides with the structured negativity when the number of negative eigenvalues of the partially transposed matrix is equal to d(d−1)/2. Moreover, we proved that structured negativity may be implemented in the laboratory.

Classical Bayes' rule lays the foundation for the classical causal relation between cause (input) and effect (output). This causal relation is believed to be universally true for all physical processes. Here we show, on the contrary, that it is inadequate to establish correct correspondence between cause and effect in quantum mechanics. In fact, there are instances within the framework of quantum mechanics where the use of classical Bayes' rule leads to inconsistencies in quantum measurement inferences, such as Frauchiger-Renner's paradox. Similar inconsistency also appears in the context of Hardy's setup even after assuming quantum mechanics as a non-local theory. As a remedy, we introduce an input-output causal relation based on quantum Bayes' rule. It applies to general quantum processes even when a cause (or effect) is in coherent superposition with other causes (or effects), involves nonlocal correlations as allowed by quantum mechanics, and in the cases where causes belonging to one system induce effects in some other system as it happens in quantum measurement processes. This enables us to propose a resolution to the contradictions that appear in the context of Frauchiger-Renner's and Hardy's setups. Our results thereby affirm that quantum mechanics, equipped with quantum Bayes' rule, can indeed consistently explain the use of itself.