Research Highlights
(I) Preamble
Dipankar Home is the earliest Indian researcher who did his doctoral work in the early 1980s on the then globally emerging new line of Research on Foundations of Quantum Mechanics, followed by his sustained standout contributions over the last four decades (whose highlights mentioned in the following Sections), thereby playing a pioneering role in fostering this line of research in India, alongside mentoring a considerable number of young researchers.
In the course of Home’s research career, this line of research has remarkably evolved, revealing a number of empirically testable profound quantum insights and effects involving Quantum Entanglement laying the basis for the frontier area of Quantum Information based on Quantum Foundations, with its state-of-the-art applications in Quantum Communication, Quantum Computation, and Quantum Sensing. In fact, the Nobel Prizes in Physics 2022 were awarded for the groundbreaking works in the 1980s and 90s which paved the way for the advent of this research area that has been recognized by the Nobel Committee as ushering in the “Second Quantum Revolution”.
Currently, this area involving fascinating symbiosis between quantum fundamentals and cutting-edge quantum technologies/experiments is on the cusp of the next major leap. At this juncture, interestingly, Home’s research enterprise with his various collaborators has also been gaining increasing momentum, as illustrated in the Sections III and IV, evidenced, for example, by 3 standout papers published in 2024, all of them in Physical Review Letters (discussed in Section IV).
(II) Publication Highlights
Apart from 152 peer-reviewed Research Publications with
various collaborators (outstanding among these briefly highlighted in the
following Sec. III and IV), Home’s 2 standout Research-level Books have also been greatly influential globally in
the Quantum Physics community: (a) “Conceptual
Foundations of Quantum Physics” (Plenum,
1997) and (b) “Einstein’s Struggles
with Quantum Theory: A Reappraisal” with Andrew Whitaker (Springer, 2007) with the respective Forewords by 2 Nobel Laureates, Anthony
Leggett and Roger Penrose. Both
these books have been appreciatively reviewed in “Physics Today”, the flagship magazine of American Institute of Physics, apart from in other international
publications.
(III) Highlights of a few selected outstanding Research Works with various collaborators until superannuation in 2018
Detailed References to my published papers mentioned below,
with the names of all the co-authors, are provided in the Complete List of my
Publications given at the end of Section VI.
● Discovering Testable Connections between two Profound Quantum Mechanical Features, Indistinguishability and Entanglement, with consequent applications
(a) An earlier unnoticed remarkable general property of Entanglement essentially arising from Indistinguishabiltiy of any two identical particles was uncovered by us, calling it “Duality in Entanglement” [Physical Review Letters 110, 140404 (2013)]. Using photons, our predicted ‘Duality in Entanglement’ was experimentally verified at Tsinghua University, Beijing [New Journal of Physics 16, 083011 (2014)], soon followed by another photonic experimental demonstration of this property at INRIM, Torino [Physical Review A 91, 0623003 (2015)], with subsequent studies exploring its various implications/applications.
(b) Another novel connection between Indistinguishability and Entanglement was unravelled by us through the formulation of an ingenious scheme for harnessing Indistinguishability towards developing an arbitrarily efficient process of entanglement generation using any two identical Bosons/Fermions originating from two independent sources [Physical Review Letters 88, 050401 (2002)]. In view of the recent notable technological developments concerning molecular two-particle interferometry which is a key ingredient of our proposed scheme, the prospect for its experimental realization has brightened and is currently being investigated. Such efficient generic methods of producing entanglement are of considerable importance, since Entanglement as a resource lies at the core of Quantum Technologies of the 21st Century, apart from its deep-seated conceptual significance.
● Introducing and developing the notion of ‘Single Particle (Intraparticle) Entanglement’ for enabling empirical demonstration of the earlier untested fundamental property of Quantum Contextuality, as well as for exploring its implications and applications
(a) Our work [Physics Letters A 102, 159 (1984)] provided the earliest formulation of the notion of intraparticle entanglement between two different dynamical variables of a single particle, and derivation of Bell-type inequality for such an entanglement by invoking the notion of Noncontextuality (assuming that any measured value of a dynamical property is independent of the experimental context). This is in contrast to the usual Bell inequality based on the notion of locality, pertaining to interparticle entanglement.
(b) Later, we developed this concept by formulating an empirically testable scheme, providing the basis for an experimental demonstration of the property of Quantum Contextuality through empirically testable violation of the single particle Bell inequality we had formulated. Our proposed experiment [Physics Letters A 279, 281 (2001)] was performed by the Neutron Interferometric Group at Atominstitut Vienna [Nature 425, 45 (2003)]. Subsequent extensive works by various Groups on single particle (intraparticle) entanglement, including our further contributions for harnessing its use as resource in Quantum Information Transfer Protocols, have been comprehensively reviewed in Advanced Quantum Technology, Vol. 3, No. 10 (2020).
● Formulating testable schemes for opening up new avenues of applications of the temporal version of Bell inequality, viz. the Leggett-Garg inequality
(a) The testable violation of Leggett-Garg inequality (LGI) provides a powerful tool for certifying an unambiguous inherent quantumness of any single system, while also refuting in the macrodomain the basic notion of macrorealism (i.e., macro-objects having definite properties, independent of measurement). Hence, the empirical testing of LGI has profound conceptual implications regarding the limits of validity of fundamental quantum concepts in the macroscopic domain, alongside its potential practical applications. In this context, our formulation of a testable scheme [Physical Review Letters 120, 210402 (2018)], based on the currently available state-of-the-art technology, is of special significance, showing the way for testing LGI using optically trapped and harmonically oscillating nano-objects having masses around 106 – 109 amu (million to billion times heavier than hydrogen atom), much more massive than the objects for which LGI or other Quantum Interference effects have been tested so far.
(b) Our investigation [Physical Review A 88, 022115 (2013)] was the first to uncover implications of the Leggett-Garg inequality (LGI) in the context of weak interaction induced two state oscillations of neutral kaons and neutrinos, pointing out a remarkable connection between the quantum violation of LGI and CP non-invariance for kaon oscillation, as well as connection with mixing parameter in the case of neutrino oscillation. Subsequently, studies led to an important experiment [Physical Review Letters 117, 050402 (2016)] demonstrating quantum mechanically predicted violation of LGI for neutrino oscillation over a length scale of about 700 km. The far-reaching ramifications of such studies continue to be explored, including the implications of various intriguing results obtained in our initiating work on this topic.
● Initiating earlier unexplored directions for detecting Multipartite Nonlocality and for appropriately quantifying the effectiveness of Quantum Entanglement as Resource for the Information Transfer/Processing tasks.
(a) In view of considerable advantages provided by the high dimensional entangled states towards efficient and robust applications in Quantum Communication, the analysis of nonlocality and entanglement pertaining to such states is gaining much topical importance. In this context, our work [Physical Review A 91, 012102 (2015)] initiated a novel direction of study by developing a powerful method for detecting Multipartite Nonlocality based on generalizing Wigner’s approach for showing Quantum Nonlocality which was originally restricted to only maximally entangled states of bipartite systems. Our proposed scheme was the first to successfully adapt Wigner’s approach in order to derive appropriate Bell-type multipartite inequalities whose violations can be used for evidencing nonlocality of any multipartite entangled state. Various applications of this work continue to be investigated in recent years.
(b) A key question underpinning various applications of Quantum Entanglement concerns the issue as to which aspect of Quantum Correlation can serve as the appropriate Quantitative Resource for various protocols in Quantum Communication so that this identification may enable judicious assessment of the usefulness of a given entangled state for its efficient application in any given context. To this end, our twin papers [Physical Review A 98, 042306 (2018); Physical Review A 98, 062320 (2018)] made significant contributions by showing that an appropriate particular measure of simultaneous correlations in mutually unbiased bases can serve as a suitable quantifier of the usefulness of an entangled state as resource for both the widely used important Quantum Information Transfer Protocols, viz. Remote State Preparation (RSP) and Quantum Steering (QS) respectively. This finding, thus, has opened up a stimulating line of studies towards exploring the potential uses of such a quantifier in optimising the efficient harnessing of RSP and QS for various information transfer/processing tasks.
● Devising DNA molecular analogue of the Schrödinger’s Cat example for probing the Quantum Measurement Problem
The much-discussed Quantum Measurement Problem (viz. how definite outcomes occur in an actual experiment, consistent with quantum mechanics) is regarded as the most intriguing puzzle concerning the Quantum Foundations (for a comprehensive review, see for example, M. Schlosshauer, Reviews of Modern Physics 76, (4), 1267 (2004)). A strikingly novel contribution towards studying this Problem was made by us by formulating the use of a DNA molecule (playing the role of Schrödinger’s Cat) as a detecting device for photons and studying in this context the vexed question as to when a Quantum Measurement is completed [Physical Review Letters 76, 2836 (1996)].
This biomolecular twist to the famous Schrödinger’s Cat paradox is particularly significant because of its potentiality to provide empirically relevant constraints on the models proposed for addressing the Measurement Problem. The futuristic significance of this work was widely commented upon, including in the “Encyclopaedia Britannica Book of the Year 1996” (pp. 242 – 243), and in the widely cited Review article by the Nobel laureate A. J. Leggett, J. Phys. Condens. Matters 14, R415 (2002). In recent years, in view of the advances in techniques harnessing biomolecular systems for investigating fundamental quantum issues, further development of our original proposal for empirically probing the Measurement Problem is acquiring more topical significance.
(IV) Highlights of a few selected outstanding Research Works with various collaborators during 2019 – 2024 (including the NASI Senior Scientist Fellowship period 2019 – 2023).
Total No. of Research Publications during this period = 14, including 3 in Physical Review Letters, 4 in Physical Review A, 1 in Communications Physics (Nature Group), 1 in Physical Review X Quantum, 1 in Physical Review Research, 1 in Journal of Optical Society of America, 2 in Quantum Information Processing, and 1 in Physica Scripta.
Among these works, a few outstanding ones are briefly
highlighted as follows.
Detailed References to my published papers mentioned below, with the names of all the co-authors, are provided in the Complete List of my Publications given at the end of Section VI.
● Proposal for Testing whether Gravity acts as a Quantum entity when measured
The question regarding Quantum Nature of Gravity is one of the major open issues in contemporary science. While theoretical models of Quantum Gravity have been much studied, empirical evidence remains elusive. No cosmological/astrophysical unambiguous signature of the quantumness of gravity has yet been obtained. Against this backdrop, investigations concerning the possibility of table-top tests of Quantum Gravity using massive objects have recently been gaining much momentum. In this respect, while the proposals so far have focused on testing essentially the quantum superposition principle for gravity, our latest work [Physical Review Letters 133, 180201 (2024)] charts out an entirely novel direction for evidencing quantumness of gravity by formulating a feasible test of the intrinsically quantum mechanical effect arising from the measurement induced disturbance which would inevitably occur when gravity as a quantum entity is measured. This is shown to be testable using a suitable multi-interferometer experimental setup which would enable detection of the intrinsic disturbance arising from quantum measurement of gravitational field. A hallmark of our proposed setup is its robustness, since as we have argued in our paper, the measurement interaction induced nonclassical disturbance in such a setup would remain non-vanishing and detectable, whatever be the rate of decoherence.
This work has attracted considerable attention globally, featuring in the Research Highlights of Nature Vol. 635, p. 261 (14 November, 2024), apart from Reports in Physics World (21 October, 2024), and in New Scientist (9 October, 2024).
● Devising a novel experimental scheme to explore the limits of Quantum Theory for arbitrarily massive objects
Since the boundary between the quantum mechanical microworld and the macroscopic classical world of everyday objects still remains unspecified, the question up to what extent the quantum mechanical principles remain valid for macroscopic objects continues to be one of the most fundamental unresolved questions of quantum physics. The state-of-the-art demonstrations of quantum features have so far reached only up to macromolecules of masses ten thousand times the hydrogen atom. Hence, breakthrough ideas, feasible to be experimentally implemented, are crucially needed to scale up the tests of macroscopic quantumness to ever more massive objects.
It is this challenging goal that has been achieved by our formulated scheme [Physical Review Letters 132, 030202 (2024)] enabling demonstration of the quantum behaviour of an arbitrarily massive harmonic oscillator prepared initially in the most classical-like state, for any mass, initial momentum, and frequency, evidenced through temporal correlations between the results of judiciously designed measurements of two appropriately chosen different observables of such a time-evolving system at successive instants. Our work, therefore, paves the way for testing the nonclassicality of a wide range of massive objects, spanning from nano-objects to oscillating mirrors of about, say, 10 kg mass used in LIGO setups for gravitational wave detection.
Already, a comprehensive experimental programme along this direction has been launched by one of our collaborators, Prof. H. Ulbricht at University of Southampton, UK using optically levitated nano-diamonds about billion times heavier than hydrogen atoms. Such experiments would provide the most emphatic demonstration to date of macroscopic quantumness, thereby imposing strong empirical constraints on possible departures from the quantum superposition principle at the macrolevel which have been predicted by the various wave function collapse models proposed for addressing the Measurement Problem. This will also open up possibilities for leveraging such macroscopic quantumness for practical applications, such as for developing high precision quantum sensors which are one of the major ingredients in the emerging new wave quantum technologies.
Articles on this work appeared in Scientific American, 27 February, 2024; Physics World, 4 January, 2024; as well as a Report on this work issued by the Press Information Bureau, Dept. of Science and Technology, Govt. of India, 1 November, 2024.
● Theory-Experiment Collaborative Work: Single system based Generation of Certified Randomness using temporal Bell inequality, i.e. Leggett-Garg inequality.
This work [Physical Review Letters 133, 020802 (2024)] has been implemented in collaboration with the photonic experimental group of Prof. Urbasi Sinha at Raman Research Institute, Bangalore by providing the required theoretical inputs.
Privacy of Communication using Genuine Random Numbers for encryption is a key requirement for any scheme of Secure Quantum Communication. However, reliability of even the best commercially available Quantum Random Number Generators (QRNGs) which harness fundamental unpredictability of quantum processes can be seriously compromised by imperfections/tampering by an adversary. Towards overcoming this limitation, our work opens up an unexplored avenue for developing compact, affordable RNGs which are immune to tampering. This has been achieved by formulating and demonstrating an ingenious single system based method for producing certified randomness, even from untrustworthy devices. For this purpose, measurement outcomes showing the loophole-free violation of a temporal analogue of Bell inequality, known as the Leggett-Garg inequality, have been used, and the amount of generated randomness has been rigorously estimated analytically. Using these theoretical ingredients and without requiring to preserve quantum correlations over large distances, the experiment performed using single photons has been able to generate a substantial number of about 9×105 certified random bits at an appreciable rate of 9×104 bits/second.
Thus, with further innovations, the devices adopting our method can potentially find powerful applications not only in cyber security and data encryption, but also in the context of varied types of randomness based application, as well as for any futuristic technology that would rely on genuinely unpredictable random numbers as a critical resource. A Report on this work was issued by Press Information Bureau, Dept. of Science and Technology, Govt. of India (11 July, 2024), apart from being reported in Nature India (30 August, 2024).
● Theory-Experiment Collaborative Work: Formulation and Loophole-free Demonstration of the Quantum Cheshire Cat effect
It has been observed in recent years that if a minimally disturbing weak interaction coupling a particular property of a measured system to a suitable property of the measuring system is implemented in each arm of an interferometer, while at the same time maintaining coherence between the states in the two arms, quantum theory predicts that certain particle properties can be associated with only one of the two possible paths in the interferometer. Interestingly, this would imply the possibility of jointly observing empirically discernible effects/signatures of two different properties of a single system in the two respective arms, along with coherence being preserved between the states in the two arms [Y. Aharonov et al. New Journal of Physics 15, 113015 (2013)]. This intriguing phenomenon has been called the Quantum Cheshire Cat effect whose testability and conceptual implications have been much debated in recent years.
In this context, through our collaborative work with the photonic experimental group of Prof. Urbasi Sinha at Raman Research Institute, Bangalore, we have made a significant breakthrough [Communications Physics (Nature Group) 6, 203, (2023)] by theoretically formulating clearly how such an effect can be unambiguously observed, as well as actually achieving in a loophole-free way the experimental verification of this striking Quantum Cheshire Cat Effect through joint observation of the spatial and polarization degrees of freedom of a single photon in the two respective arms of an interferometer. While the probing of deeper foundational implications of such an effect regarding, for example, Bohr’s principle of wave-particle complementarity is a stimulating line of study, the techniques used in this photonic experiment can be invoked for developing high precision quantum sensors and useful variants of quantum information transfer/processing protocols, such as by suitably using this setup for sharing the state of a qubit (single photon) among spatially separated parties, each coupling an ancilla to the qubit.
● Theory-Experiment Collaborative Work: Single photon based interferometric loophole-free test of the temporal counterpart of Bell inequality, viz. the Leggett-Garg inequality
This work [Physical Review X Quantum 3, 010307 (2022)] developed by providing the required theoretical inputs, in collaboration with Prof. Urbasi Sinha’s experimental Quantum Optics Group at Raman Research Institute, Bangalore, resulted in an unambiguous demonstration of the empirical violation of the Leggett-Garg inequality (LGI) for single photons by closing all the relevant loopholes for the first time. Given the growing importance of LGI, both from the foundational and applicational perspectives, such loophole-free testing of LGI for a single system complements the loophole-free tests of Bell inequality for the entangled systems.
In this work, appropriately devising suitable strategies for addressing conclusively the various possible loopholes like the detection efficiency loophole, multiphoton emission loophole, … along with precision testing of the classical invasiveness of measurement as required for plugging the clumsiness loophole involved in the LGI testing called for ingenious blending of theoretical and experimental efforts. The loophole-free architecture of the experimental platform thus developed through our collaborative enterprise provides a powerful means for certifying inherent nonclassicality of single photons using the observed violation of LGI. Importantly, this can be reliably and efficiently harnessed towards Quantum Communication based applications for which the single photon is a ubiquitous workhorse.
Notably, this setup has been used in our other work (discussed earlier in this Section) for secure and user-friendly generation of Genuine Random Numbers. Prospect for its application in the context of Quantum Cryptography is being investigated by examining whether the violation of LGI can be effectively used to enhance the security of the standard BB84 cryptographic protocol. Also, it should be worth exploring the possibility of using this setup for testing the information-theoretic temporal version of Bell inequality because of its potential relevance to Quantum Computation.