Bose Institute

A Brief Summary of the Most Significant Scientific Research Contributions During last 10 Years (2008 - 2018)

Dipankar Home is one of the early Indian researchers to have initiated investigations on Foundational issues of Quantum Mechanics since 1980s and was the first Indian researcher to have done his Ph. D. work on this topic. This line of study has gradually evolved to become amenable to a variety of fundamentally important experiments, as well as has become intimately related to currently one of the frontier areas of science, viz. Quantum Information Theory and its applications in Quantum Communication, Quantum Cryptography and Quantum Computation. Total number of Research Publications to date is 139 with total Citation Number 2230.

Apart from his Research Publications that have been cited in 20 Books, Home’s notable contribution has been Two distinctive Research-level Books: “Conceptual Foundations of Quantum Physics – An Overview from Modern Perspectives” (Plenum, 1997), with a Foreword by Nobel laureate Anthony Leggett, and (ii) “Einstein’s Struggles with Quantum Theory: A Reappraisal” (Springer, 2007), co-authored with Andrew Whitaker, having a Foreword by Roger Penrose. Both these books have been peer-appreciated in Physics Today (October, 1998 and May, 2008 respectively), apart from the former being also appreciatively reviewed in The Times (London), Higher Education Supplement (25 September, 1998), Foundations of Physics, Vol. 31, pp. 855 -857 (2001), and in Progress in Quantum Electronics, Vol. 22, pp. 41-42 (1998) by some of the leading experts in the area of Quantum Foundations. These two books have Total Citation Number around 219 (Google Scholar).

Among his 43 Research Publications (Total Citation Number 223) in last 10 years with his various collaborators, the most significant works are as follows:

A) At the core of various intriguing question raised by Einstein, Schrödinger and others about foundational aspects of Quantum Mechanics (QM) is the QM incompatibility with the everyday notion of Macrorealism (MR) which assumes that, at any instant, a system is in a definite state having definite properties, irrespective of any measurement. The latest work by Home and his collaborators, just published [Physical Review Letters 120, 210402 (2018)], opens up a novel direction for extending the tests of MR as well as of the nonclassicality of QM for massive systems much beyond those possible by other methods.

In particular, using the Leggett-Garg inequality, the work by Home and his collaborators shows the QM violation of MR for large mass systems, testable for a system like harmonic oscillator which has a well defined classical description and is initially in a state which is the most classical-like of all quantum states, viz. the harmonic oscillator coherent state. Testing of this scheme using the setup proposed in this work for optically trapped and oscillating nano-scale objects of mass about million to billion times heavier than hydrogen atoms is being implemented by Hendrik Ulbricht and his group at University of Southampton, UK. This work should have wider ramifications in future.

B) A hitherto unexplored connection was revealed between two profound features of QM, viz. Quantum Indistinguishability (QI) and Quantum Entanglement (QE) by invoking QI for formulating an arbitrarily efficient generic scheme that can entangle, using any spin-like variable, for any two identical bosons/fermions coming from independent sources [Physical Review Letters 88, 050401 (2002)].

Such an efficient generic scheme for generating QE is of considerable importance since QE lies at the core of Quantum Foundations and various applications of Quantum Information Theory. Furthermore, this work suggested a novel form of complementarity between particle distinguishability and the amount of entanglement generated, besides providing one of the ingredients of the seminal work by C. W. J. Beenakker et al. [Physical Review Letters 93, 020501 (2004)] which initiated studies on Free-Electron Quantum Computation.

C) The above-mentioned line of study blending QI and QE was enriched by uncovering an earlier unnoticed property of QE involving any two identical particles which has been called ‘Duality in Entanglement’ [Physical Review Letters 110, 140404 (2013)]. Importantly, this property enables studying the transition from QI to Classical Distinguishability, as well as can be an effective tool for empirically probing aspects of QI for mesoscopic/macroscopic molecules without requiring to bring them together, thereby avoiding the effect of interaction between them.

For photons, this predicted property of ‘Entanglement Duality’ (involving the manifestation of polarization entanglement when the photons are labelled by different momenta, or, for the same source, manifesting momentum entanglement when the photons are labelled by polarization variables) has been experimentally verified at Tsinghua University, Beijing [New Journal of Physics 16, 083011 (2014)], followed by another experimental study at INRIM, Torino [Physical Review A 91, 062117 (2015)] using Bell measurements in order to probe a few subtle aspects of our predicted ‘Entanglement Duality’.

D) In the paper [Physical Review A 98, 062320 (2018)] by Home and his collaborators it has been shown that the quantum communication task of remotely preparing a quantum state (using shared correlated particles as resource assisted by one bit of classical communication) can be efficiently implemented by using any non-product state, including states that have neither entanglement nor quantum discord (the two usually discussed measures of quantum correlation). This finding is explained by linking a suitable measure of the efficiency of such a task with a relatively less explored aspect of quantum correlation captured by the existence of the simultaneous correlations in complementary bases whose measure is nonzero for any non-product state. The insight thus gained that such a measure of quantumness of correlation can determine the usefulness of all non-product states as resource for implementing a quantum communication task opens up a range of possibilities of effective realizations of various quantum information tasks, importantly, even by using separable (non-entangled) states which are easier to produce, manipulate and protect against decohering effects than the entangled states.

E) The result obtained in the paper by Home and his collaborators [Physical Review A 98, 042306 (2018)] provides for the first time an operational significance for the measures of simultaneous correlations in mutually unbiased bases in terms of a quantum task like steering. The link between steerability and measures of simultaneous correlations in mutually unbiased bases, thus, revealed suggests possible usefulness of these measures for quantifying resource for a variety of quantum information tasks pertaining to which steerable states have already been shown to provide quantum advantage.

F) Wigner’s argument seeking to demonstrate Quantum Nonlocality that was originally formulated only for bipartite states has been successfully generalized for an arbitrary multipartite state, thereby providing a powerful method for obtaining multipartite Bell-type inequalities in order to probe Quantum Nonlocality pertaining to the states of multipartite systems [Physical Review A 91, 012102 (2015)]. The efficacy of such obtained multipartite Bell-type inequalities has been demonstrated for the quadripartite entangled states, thus opening up a novel direction in this area of study concerning multipartite Quantum Nonlocality which is of much contemporary interest.

G) For multilevel spin systems, a hitherto unexplored study of robustness of the Quantum Mechanical (QM) violation of Macrorealism (MR) with respect to coarse-grained/unsharp measurements has yielded an important insight that even in the asymptotic limit of spin, the QM violations of all the three necessary conditions of MR persist (in other words, classicality does not emerge in the asymptotic limit of spin), whatever be the unsharpness and the degree of coarse-graining of measurements [Physical Review A 94, 062117 (2016)]. This finding has fundamental implications concerning the wider issue of the emergence of classicality from quantum physics.

H) An earlier unexplored application of the temporal analogue of Bell’s inequality, known as the Leggett-Garg inequality (LGI), for weak interaction induced two-state oscillations pertaining to neutral kaons and neutrinos revealed an intriguing result that CP violation occurring in the case of kaon oscillation enhances the Quantum Mechanical violation of LGI, a result whose conceptual implications call for a deeper probing [Physical Review A 88, 022115 (2013)].

I) In the area of Quantum Cryptography a novel direction of study has been initiated [Physical Letters A 381, 2478 (2017)] by formulating a variant of the Bennett-Brassard 1984 (BB84) protocol where key sharing is done in the usual BB84 way but part of the security check is done using the Leggett-Garg inequality (LGI). It has been shown that temporal correlations violating LGI can be useful for ensuring security of the BB84 protocol against a widely discussed device attack (known as the AGM attack) for which the standard BB84 protocol is insecure. Thus the potentiality of such BB84-based Quantum key Distribution protocol augmented by LGI in providing increased security advantages calls for more comprehensive studies.

J) Theoretically predicted intriguing Quantum Cheshire Cat Effect [Y. Aharonov et al. New Journal of Physics 15, 113015 (2013)] has been recently a topic of much studies, theoretical as well as experimental, leading to controversies about its unambiguous experimental verifiability and conceptual ramifications. To this end, these issues have been comprehensively examined and clarified thereby suggesting means for loophole-free verification of this curious novel quantum effect [Annals of Physics 391, 1 (2018)].

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