Research

We carry out experimental and theoretical research in a range of topics such as optical coherence, quantum entanglement, spontaneous parametric down-conversion, orbital angular momentum, squeezed states, vacuum fluctuations, and foundations of quantum theory. Our work lies at the intersection of quantum optics, nonlinear optics, coherence theory, and quantum information science.

Introduction: In the last few decades, numerous experiments have demonstrated that quantum systems can potentially outperform their classical counterparts in a variety of tasks such as computation [2, 3], secure communication [4–6], metrology [7, 8], and sensing [9]. Such enhanced capabilities are believed to primarily arise from two factors: (i) the intrinsic correlations of high-dimensional superposition states that enable parallelism in information processing, and (ii) the nonlocal correlations present in multiparticle quantum states that cannot be separated into states for their individual constituent particles. While the former intrinsic correlations, which enable physical systems to superpose and exhibit interference effects, constitute a property known as coherence that is found in both classical and quantum systems, the latter nonlocal correlations arise from a property known as entanglement, which is unique to quantum systems and has no known classical analog. In addition to enabling quantum-enhanced practical applications, entanglement also raises intriguing fundamental questions about the consistency and completeness of the quantum description of physical reality [10,11]. Our research focuses on theoretically characterizing the correlations of light fields and developing experimental methods for generating light fields with specifically tailored correlations and also measuring them in an efficient and accurate manner.

High-dimensional states of light and partially coherent fields: In contrast with the two-dimensional polarization degree of freedom, the orbital angular momentum (OAM) degree of freedom of light [12, 13] provides a discrete and infinite-dimensional basis for realizing high-dimensional states of light that have a number of distinct advantages such as improved security and enhanced bandwidth in optical communication protocols [14, 15], efficient gate implementations in computing protocols [16,17], and improved noise tolerance for fundamental tests of quantum theory [18, 19]. Moreover, when such high-dimensional superpositions are prepared in a partially coherent manner, they gain important advantages for applications such as long-distance communication and imaging through scattering media [20]. This is mainly because partially coherent fields are structurally more robust [21, 22], produce lesser scintillation [23], and also generate lesser speckle [24] under scattering and turbulence than their fully coherent counterparts. Our research aims to develop methods for generating high-dimensional states and partially coherent light fields with desired coherence properties in a controlled way, but also to innovate techniques for measuring such fields in an efficient and accurate manner.

Nonclassical light from spontaneous parametric down-conversion (SPDC): SPDC is a nonlinear optical process in which a single photon from an incident field known as pump interacts with a nonlinear crystal to produce a pair of entangled photons known as signal and idler [25]. This process is routinely harnessed for producing entangled states and squeezed states in fundamental quantum optics experiments [26, 27] and optical quantum technologies [28, 29]. Furthermore, by pumping two crystals in series or parallel configuration, researchers have realized nonlinear interferometers [30] and induced coherence experiments [31] that not only have important applications in phase metrology [32] and imaging [33], but have also led to major insights on indistinguishability and its role in interference [34]. Our research will work towards controlled generation of entangled states from PDC using a variety of means such as tailoring the coherence of the pump, tuning the parameters of nonlinear interferometers, and also employing partially coherent seed fields. In addition, we will work towards developing novel interferometric and mode projection-based techniques to measure such states in an accurate and efficient manner. Furthermore, we will also work towards scaling up these entangled states to span higher number of dimensions and include more than two photons with the two-fold aim of probing the fundamental physics of high-dimensional multipartite entanglement and investigating novel ways in which such entanglement can be harnessed for more powerful information processing capabilities.

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