Quantum Information

and Quantum Physics

Research topics

Our research interests involve both theoretical understanding of fundamental physics from the point of view of quantum information theory and their possible applications in quantum technologies.

Quantum Thermodynamics, Quantum Heat Engines, and Information Theory

Coherence and Superposition in States, Non-Classicality, and Quantum Correlations

Quantum Superposition in Evolution and Quantum Causality

Relativistic Information Theory for Indistinguishable Particles and Quantum Fields

Quantum Measurements, Geometry, and Foundations of Quantum Mechanics

Philosophy of Science and Science in Philosophy

Research overview

Quantum information theory (QIT) is a relatively new field at the cross-over of two of the biggest scientific developments of the last century: quantum mechanics and information theory. The ideas of quantum information science are very promising for a technological breakthrough in case a quantum computer is ever built, or reliable long-distance quantum communication is eventually achieved. But, QIT actually represents much more than that, which in fact is our main motivation for pursuing research in this area. It is clear that we are obliged to consider quantum mechanics if we want to understand the fundamental limits for computation and information transmission, the essential ingredients of information technology, in the physical world. Quantum theory turns out to be an unavoidable part of computer science and information theory. Conversely, we are also compelled to take into consideration such computational and information-theoretical aspects of quantum mechanics if we want to understand why quantum theory is the way it is, or how we can use it in order to make useful predictions about physical phenomena.

While much of the interest in this field is spurred by the possible use of quantum computers for code-breaking using fast factoring algorithms, to the physicists interested in deeper issues, it presents an entirely new set of questions based on a radically different approach to understand the quantum world. That exactly is our goal - exploiting quantum information theory - understanding physics as the flow of information. In particular, we aim to understand the quantum world better by considering the information-theoretic formalism.

Our research interests involve both theoretical understanding of fundamental physics from the point of view of quantum information theory and their possible applications in quantum technologies. It includes exploring basic questions and their physically realizable applications, in particular, related to quantum thermodynamics, heat engines, and statistical mechanics, quantum optics and spin-systems, foundations of quantum mechanics and quantum paradoxes, quantum superposition and coherence, quantum measurement and geometry, quantum quasi-probabilities and non-classicality, quantum correlations between space-like separated systems and time-like separated events, quantum causality and causal relations, information theory for indistinguishable particles and quantum field theory (QFT), and black-hole information paradox.

Quantum Thermodynamics, Heat Engines, and Information Theory

Thermodynamics is one of the most successful physical theory ever formulated. It was initially developed to deal with macroscopic heat engines, in particular, to investigate the conversion of heat into mechanical work, long before quantum mechanics had been developed. It is therefore plausible that thermodynamics in the microscopic regime, where the quantum properties dominate, departs significantly from its macroscopic counterpart. This is indeed the case. An important feature in the microscopic regime is that the quantum particles can exhibit large quantum fluctuations, non-trivial correlations, such as entanglement and other quantum correlations. Using information-theoretic understanding and tools, we explore quantum thermodynamics in presence of quantum correlations, quantum engines operating with a finite number of quantum particles, thermodynamics with multiple conserved quantities, and quantum batteries.

Coherence and Superposition in States, Non-Classicality, and Quantum Correlations

Coherence or superposition, in quantum systems, is one of the most important features that distinguishes quantum mechanics from its classical analogue. Quantum coherence is one of "the" reasons for which there exists quantum uncertainties, randomness, non-locality, entanglement, etc. In particular, in quantum optical (infinite-dimensional) systems, coherence plays esoteric roles. There are types of coherences that make the quantum optical system to behave close to classical ones. Traditionally these states are known as “coherent”-states of light. On the other hand, there also exist type of coherences that lead the quantum optical systems to exhibit non-classical behaviors, e.g., squeezed light, one-photon state, etc. Similar phenomena also exist in finite-dimensional atomic spin systems. We aim to characterize these quantum aspects of matter as the manifestation of different kinds of superpositions using information theory and quantum quasi-probability.

Superposition in Evolution, Temporal Correlations, and Quantum Causality

In the classical domain, space and time have delicate interrelations as, for the first time, introduced in theory of relativity by Einstein. In quantum mechanics, time cannot be considered as an observable, rather only a parameter. Position in space, on the other hand, could be an observable. Therefore, a classical-domain-like inter-relation is not possible in quantum mechanics, as footings of space and time are intrinsically different. Interestingly, there could present an inter-relation (or a form of symmetry) in quantum correlations in space with the quantum correlations in time. Another interesting feature of quantum mechanics is that the causal structure of an event is not only dynamical but also may have indefinite causal order. There are quantum evolutions that are a-causal in nature and may lead to both-way signalling. We aim to understand and characterize these quantum properties present in evolutions with the help of quantum information theory and explore their possible applications in quantum technologies.

Relativistic Information Theory for Indistinguishable Particles and Quantum Fields

Much of the well-established quantum information theory relies on the separability of quantum state spaces. In other words, it considers tensor-product of local Hilbert spaces and thereby, establishes the notion of locality. However, for indistinguishable particles (or, quantum fields), the Hilbert spaces are not separable ones. Rather, they are symmetrized (for bosons) or anti-symmetrized (for fermions) spaces of the separable ones. In these situations, the traditional quantum information-theoretic frameworks break down and one has to look for a generalized framework. Our research works involve this under-developed area of quantum information theory in terms of spin-statistics, the spatial theory of relativity, quantum geometry, and information theory, and to explore the phenomena related to quantum field theories in curved space-time, like the Unruh effect and the black-hole information paradox.

Quantum Measurements, Geometry, and Foundations of Quantum Mechanics

Measurement is one of the least understood processes in quantum mechanics, leading to measurement-induced collapse of states, "intrinsic" randomness, and uncertainties. Much of quantum paradoxes are due to these measurement-induced collapses. Recent studies indicate that the causal relationship between cause and effect in quantum mechanics is strikingly different from its classical analogue, see e.g., Hardy's paradox and Frauchiger-Renner paradox. The modern formulation of (Heisenberg and Heisenberg-like) uncertainty relations use entropic (like generalized Rényi entropies) measures to quantify the uncertainties. Recently it has been shown that, in the presence of quantum memory, quantum uncertainty could be made vanishing for any arbitrary and simultaneous measurements. Further, the probabilistic nature of the quantum world has been shown to be intrinsic in nature, in presence of non-local correlation. In dynamical evolution, it has been seen that presence of quantum correlation guarantees non-stationarity of a quantum state, with a finite quantum speed of evolution. All these findings indicate that, indeed, the presence of quantum correlation is one of the reasons for the existence of quantum uncertainty relations and, also, the probabilistic nature of quantum mechanics. Our research works aim to understand these fundamental issues of quantum mechanics in a consistent manner.

Philosophy of Science and Science in Philosophy

We have interests in the philosophical interpretations of physics. In particular, we explore, based on our present understanding of physics, the philosophical questions related to quantum mechanics, relativity, and information theory. For example: (1) Does there exist objective reality in the quantum world? This is also connected to the question if there exists “intrinsic” randomness in nature. One of my recent work has partially addressed this issue. However, this question is not completely resolved yet. A complete answer to these boils down to correct understanding of quantum measurements, Born’s interpretation of probabilities, subjective (Bayesian) and objective (frequentist) approach towards probability, etc. (2) What is energy? Can energy be separable from space-time? Alternatively, can there be space-time without energy? (3) Can the development of artificial intelligence (AI) reach the level of the human brain? Addressing this question will require much understanding in quantum machine-learning, quantum AI, quantum neural-networks, and the higher for conditional reflex (the secondary signalling system) that is responsible for thought and decision-making processes in the human brain.

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