arXiv subjects

B. Sharmila

Publications and source records attributed to B. Sharmila.

14 recordsLinked to original sources

arXiv2026

Estimating spacetime fluctuation strength in SU(1,1) and SU(2) interferometers

High-precision laser interferometers are commonly used to search for signatures of random spacetime fluctuations, in an attempt to understand the fundamental nature of gravity. Conventionally, $SU(2)$ interferometers have been used in such experimental investigations. Motivated by [K. Zheng \textit{et al.}, Photon. Res. \textbf{8}, 1653 (2020)], I assess if there is any advantage to be gained by using an $SU(1,1)$ interferometer instead of the $SU(2)$ interferometer. To this end, I compute the quantum Fisher information for estimating the strength and the correlation length of the spacetime fluctuations, and the corresponding classical Fisher information considering different experimentally relevant measurement schemes, in both types of interferometers. I compare the information metrics corresponding to different parameter regimes that are relevant to both current and possible future experimental setups. This helps me assess if and when the $SU(1,1)$ interferometer offers any advantage over the $SU(2)$ interferometer for estimating either the fluctuation strength or correlation length of the spacetime fluctuations.

quant-ph↗

arXiv2025

Signatures of Correlation of Spacetime Fluctuations in Laser Interferometers

Spacetime fluctuations (SFs), a common feature of different proposed gravity models, could be detected using laser interferometers. In the search for SFs, a correspondence between the expected output signals and different gravity models is needed, both for guiding the design of future interferometers, and for identifying the signal in experimental data. In this work, we provide such a correspondence for some classes of SFs and geometries of the interferometers. We consider three different classes of SFs, characterised by the decay behaviours and symmetries of their two-point correlation functions. Our approach applies to Michelson laser interferometers with Fabry-P\'erot arm cavities such as the km-long LIGO detectors and those without arm cavities such as the laboratory-scale setups QUEST and GQuEST. Analysing the expected interferometer output signals, we identify three characteristic signatures for each class of SF. The designed broadband sensitivity of the laboratory-scale instruments would allow all characteristic signatures of the different classes of SFs to be observed, and such observations could provide more information on the nature of the SFs than those from LIGO. On the other hand, we find that LIGO is better suited for detecting the bare presence or absence of SFs.

gr-qc↗

arXiv2025

Signatures of a gravitational quantum vacuum on dynamics of massive particles

We study the interaction of two massive particles with a quantised gravitational field in its vacuum state using two different position observables: (i) a frame-dependent coordinate separation and (ii) a frame-independent geodesic separation. For free particles, (i) leads to purely unitary dynamics but (ii) leads to dissipation. For two particles coupled through a linear spring, (i) and (ii) lead to different cut-off dependences in the frequency shift harmonic ladder of the differential motional mode. Our findings highlight the subtle consequences of different position observables at the interface of quantum mechanics and gravity.

quant-ph↗

arXiv2024

Detecting quantum vacuum fluctuations of the electromagnetic field

Quantum vacuum fluctuations of the electromagnetic field result in two signatures on a harmonically trapped charged particle: a shift from the natural trap frequency and generation of quantum coherences. We assess the role of the long-wavelength and rotating-wave approximations in estimating this frequency shift. We estimate the magnitude of the frequency shift using parameters from a single-electron cyclotron experiment and also demonstrate how the dependence of the frequency shift on the magnetic field of the cyclotron is tied to the rotating-wave approximation. We expect the frequency shift to be observable in future experiments. We also suggest a possible route to detecting vacuum-generated quantum coherences. These experiments should settle the debate on the choice of approximations and gauge in capturing the effect of the quantum vacuum fluctuations.

quant-ph↗

arXiv2024

A Cooper-pair beam splitter as a feasible source of entangled electrons

We investigate the generation of an entangled electron pair emerging from a system composed of two quantum dots attached to a superconductor Cooper pair beam splitter. We take into account three processes: Crossed Andreev Reflection, cotuneling, and Coulomb interaction. Together, these processes play crucial roles in the formation of entangled electronic states, with electrons being in spatially separated quantum dots. By using perturbation theory, we derive an analytical effective model that allows a simple picture of the intricate process behind the formation of the entangled state. Several entanglement quantifiers, including quantum mutual information, negativity, and concurrence, are employed to validate our findings. Finally, we define and calculate the covariance associated with the detection of two electrons, each originating from one of the quantum dots with a specific spin value. The time evolution of this observable follows the dynamics of all entanglement quantifiers, thus suggesting that it can be a useful tool for mapping the creation of entangled electrons in future applications within quantum information protocols.

quant-ph↗

arXiv2024

Extracting electromagnetic signatures of spacetime fluctuations

We present a formalism to discern the effects of fluctuations of the spacetime metric on electromagnetic radiation. The formalism works via the measurement of electromagnetic field correlations, while allowing a clear assessment of the assumptions involved. As an application of the formalism, we present a model of spacetime fluctuations that appear as random fluctuations of the refractive index of the vacuum in single, and two co-located Michelson interferometers. We compare an interferometric signal predicted using this model to experimental data from the Holometer and aLIGO. We show that if the signal manifests at a frequency at which the interferometers are sensitive, the strength and scale of possible spacetime fluctuations can be constrained. The bounds, thus obtained, on the strength and scale of the spacetime fluctuations, are also shown to be more stringent than the bounds obtained previously using astronomical observation at optical frequencies. The formalism enables us to evaluate proposed experiments such as QUEST for constraining quantum spacetime fluctuations and to design new ones.

gr-qc↗

arXiv2021

Tomographic entanglement indicators from NMR experiments

In recent years, the performance of different entanglement indicators obtained directly from tomograms has been assessed in continuous-variable and hybrid quantum systems. In this paper, we carry out this task in the case of spin systems. We compute the entanglement indicators from actual experimental data obtained from three liquid-state NMR experiments, and compare them with standard entanglement measures calculated from the corresponding density matrices, both experimentally reconstructed and numerically computed. The gross features of entanglement dynamics and spin squeezing properties are found to be reproduced by these entanglement indicators. However, the extent to which these indicators and spin squeezing track the entanglement during time evolution of the multipartite systems in the NMR experiments is very sensitive to the precise nature and strength of interactions as well as the manner in which the full system is partitioned into subsystems. We also use the IBM quantum computer to implement equivalent circuits that capture the dynamics of the multipartite system in one of the NMR experiments. We compute and compare the entanglement indicators obtained from the tomograms corresponding to the experimental execution and simulation of these equivalent circuits. This exercise shows that these indicators can estimate the degree of entanglement without necessitating detailed state reconstruction procedures, establishing the advantage of the tomographic approach.

quant-ph↗

arXiv2021

Exact eigenvalue order statistics for the reduced density matrix of a bipartite system

We consider the reduced density matrix $ρ_{A}^{(m)}$ of a bipartite system $AB$ of dimensionality $mn$ in a Gaussian ensemble of random, complex pure states of the composite system. For a given dimensionality $m$ of the subsystem $A$, the eigenvalues $λ_{1}^{(m)},\ldots, λ_{m}^{(m)}$ of $ρ_{A}^{(m)}$ are correlated random variables because their sum equals unity. The following quantities are known, among others: The joint probability density function (PDF) of the eigenvalues $λ_{1}^{(m)},\ldots, λ_{m}^{(m)}$ of $ρ_{A}^{(m)}$, the PDFs of the smallest eigenvalue $λ_{\rm min}^{(m)}$ and the largest eigenvalue $λ_{\rm max}^{(m)}$, and the family of average values $\langle \mathrm{Tr}\big(ρ_{A}^{(m)}\big)^{q}\rangle$ parametrised by $q$. Using values of $m$ running from $2$ to $6$ for definiteness, we show that these inputs suffice to identify and characterise the eigenvalue order statistics, i.e., to obtain explicit analytic expressions for the PDFs of each of the $m$ eigenvalues arranged in ascending order from the smallest to the largest one. When $m = n$ (respectively, $m < n$) these PDFs are polynomials of order $m^{2}-2$ (respectively, $mn - 2$) with support in specific sub-intervals of the unit interval, demarcated by appropriate unit step functions. Our exact results are fully corroborated by numerically generated histograms of the ordered set of eigenvalues corresponding to ensembles of over $10^{5}$ random complex pure states of the bipartite system. Finally, we present the general solution for arbitrary values of the subsystem dimensions $m$ and $n$, namely, formal exact expressions for the PDFs of every ordered eigenvalue.

quant-ph↗

arXiv2021

Tomographic entanglement indicators in frequency combs and Talbot carpets

Recent theoretical investigations on tomographic entanglement indicators have showcased the advantages of the tomographic approach in the context of continuous-variable (CV), spin and hybrid quantum systems. Direct estimation of entanglement using experimental data from the IBM quantum computing platform and NMR experiments have also been carried out in earlier work. A similar investigation in the context of CV systems is necessary to fully assess the utility of our tomographic approach. In this paper, we highlight the advantages of our approach in the context of experiments reported in the literature on two CV systems, namely, entangled Talbot carpets and biphoton frequency combs. We use the tomographic entanglement indicator to assess the extent of entanglement between a pair of Talbot carpets and demonstrate that this provides a simpler and more direct procedure compared to the one suggested in the experiment. We also establish that the tomograms corresponding to the two biphoton frequency combs carry clear entanglement signatures that distinguish between the two states.

quant-ph↗

arXiv2020

Signatures of nonclassical effects in tomograms

The thesis showcases the importance of tomograms in quantifying nonclassical effects such as wavepacket revivals, squeezing, and quantum entanglement in continuous-variable, hybrid quantum, and qubit systems. This approach avoids error-prone statistical methods used in state reconstruction procedures. The performance of many bipartite entanglement indicators obtained directly from tomograms is examined both in the case of known quantum states and experimental data reported in the literature.

quant-ph↗

arXiv2020

Signatures of avoided energy-level crossings in entanglement indicators obtained from quantum tomograms

Extensive theoretical and experimental investigations on multipartite systems close to an avoided energy-level crossing reveal interesting features such as the extremisation of entanglement. Conventionally, the estimation of entanglement directly from experimental observation involves either one of two approaches: Uncertainty-relation-based estimation that captures the linear correlation between relevant observables, or rigorous but error-prone quantum state reconstruction on tomograms obtained from homodyne measurements. We investigate the behaviour, close to avoided crossings, of entanglement indicators that can be calculated directly from a numerically-generated tomogram. The systems we study are two generic bipartite continuous-variable systems: a Bose-Einstein condensate trapped in a double-well potential, and a multi-level atom interacting with a radiation field. We also consider a multipartite hybrid quantum system of superconducting qubits interacting with microwave photons. We carry out a quantitative comparison of the indicators with a standard measure of entanglement, the subsystem von Neumann entropy (SVNE). It is shown that the indicators that capture the nonlinear correlation between relevant subsystem observables are in excellent agreement with the SVNE.

quant-ph↗

arXiv2019

Tomographic entanglement indicators in multipartite systems

We assess the performance of an entanglement indicator which can be obtained directly from tomograms, avoiding state reconstruction procedures. In earlier work, we have examined this tomographic entanglement indicator, and a variant obtained from it, in the context of continuous variable systems. It has been shown that, in multipartite systems of radiation fields, these indicators fare as well as standard measures of entanglement. In this paper, we assess these indicators in the case of two generic hybrid quantum systems, the double Jaynes-Cummings model and the double Tavis-Cummings model using, for purposes of comparison, the quantum mutual information as a standard reference for both quantum correlations and entanglement. The dynamics of entanglement is investigated in both models over a sufficiently long time interval. We establish that the tomographic indicator provides a good estimate of the extent of entanglement both in the atomic subsystems and in the field subsystems. An indicator obtained from the tomographic indicator as an approximation, however, does not capture the entanglement properties of atomic subsystems, although it is useful for field subsystems. Our results are inferred from numerical calculations based on the two models, simulations of relevant equivalent circuits in both cases, and experiments performed on the IBM computing platform.

quant-ph↗

arXiv2019

Estimation of entanglement in bipartite systems directly from tomograms

We investigate the advantages of extracting the degree of entanglement in bipartite systems directly from tomograms, as it is the latter that are readily obtained from experiments. This would provide a superior alternative to the standard procedure of assessing the extent of entanglement between subsystems after employing the machinery of state reconstruction from the tomogram. The latter is both cumbersome and involves statistical methods, while a direct inference about entanglement from the tomogram circumvents these limitations. In an earlier paper, we had identified a procedure to obtain a bipartite entanglement indicator directly from tomograms. To assess the efficacy of this indicator, we now carry out a detailed investigation using two nonlinear bipartite models by comparing this tomographic indicator with standard markers of entanglement such as the subsystem linear entropy and the subsystem von Neumann entropy and also with a commonly-used indicator obtained from inverse participation ratios. The two model systems selected for this purpose are a multilevel atom interacting with a radiation field, and a double-well Bose-Einstein condensate. The role played by the specific initial states of these two systems in the performance of the tomographic indicator is also examined. Further, the efficiency of the tomographic entanglement indicator during the dynamical evolution of the system is assessed from a time-series analysis of the difference between this indicator and the subsystem von Neumann entropy.

quant-ph↗

arXiv2016

Signatures of nonclassical effects in optical tomograms

Several nonclassical effects displayed by wave packets subject to generic nonlinear Hamiltonians can be identified and assessed directly from tomograms without attempting to reconstruct the Wigner function or the density matrix explicitly. We have demonstrated this for both single-mode and bipartite systems. We have shown that a wide spectrum of effects such as the revival phenomena, quadrature squeezing and Hong-Mandel and Hillery type higher-order squeezing in both the single-mode system and the double-well Bose-Einstein condensate can be obtained from appropriate tomograms in a straightforward manner. We have investigated entropic squeezing of the subsystem state of a bipartite system as it evolves in time, solely from tomograms. Further we have identified a quantifier of the extent of entanglement between subsystems which can be readily obtained from the tomogram and which mirrors the qualitative behavior of other measures of entanglement such as the subsystem von Neumann entropy and the subsystem linear entropy. The procedures that we have demonstrated can be readily adapted to multimode systems.

quant-ph↗