: 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.