arXiv subjects

Aparajita Bhattacharyya

Publications and source records attributed to Aparajita Bhattacharyya.

At least 19 recordsLinked to original sources

arXiv2026

Quantum transistors for heat flux in and out of working substance parts: harmonic vs transmon and Kerr environs

Quantum thermal transistors have been widely studied in the context of three-qubit systems, where each qubit interacts separately with a Markovian harmonic bath. Markovianity is an assumption that is imposed on a system if the environment loses its memory within short while, while non-Markovianity is a general feature, inherently present in a large fraction of realistic scenarios. Instead of Markovian environments, here we propose a transistor in which the interaction between the working substance and an environment comprising of an infinite chain of qutrits is based on periodic collisions. We refer to the device as a working-substance thermal transistor, since the model focuses on heat currents flowing in and out of each individual qubit of the working substance to and from different parts of the system and environment. We find that the transistor effect prevails in this apparatus and we depict how the amplification of heat currents depends on the temperature of the modulating environment, the system-environment coupling strength and the interaction time. We further show that there exists a non-zero amplification even if one of the environments, that is not the modulating one, is detached from the system. Additionally, the environment, being comprised of three-level systems, allows us to consider the effects of frail perturbations in the energy-spacings of the qutrit, leading to a non-linearity in the environment. We consider non-linearities that are either of transmon- or of Kerr-type. We find parameter ranges where there is a significant amplification for both transmon- and Kerr-type non-linearities in the environment. Finally, we detect the non-Markovianity induced in the system from a non-monotonic behavior of the amplification observed with respect to time, and quantify it using the distinguishability-based measure of non-Markovianity.

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arXiv2026

Positive and non-positive measurements in energy distillation from quantum batteries

We investigate energy distillation from quantum batteries within the framework of generalized quantum measurements, including both positive operator-valued measurements (POVMs) and physically realizable non-positive operator-valued measurements (NPOVMs) performed on an auxiliary system coupled to the battery. Two classes of NPOVMs, namely type-1 and type-2, are analyzed in the presence of environmental noise acting on the auxiliary system. We derive general expressions for the distillable energy corresponding to positive and non-positive measurements and show that the distillable energy obtained via NPOVMs remains robust against environmental noise. For a specific model, we demonstrate that the energy extracted using type-1 NPOVMs exceeds or equals that obtained via POVMs under amplitude-damping, dephasing, and bit-flip noise, while type-2 NPOVMs outperform POVMs under amplitude-damping noise. These results establish a clear operational advantage of NPOVMs for energy extraction. We also analyze restricted measurement settings and compare the accessible distillable energy for constrained positive and type-1 non-positive measurements.

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arXiv2026

Quantum refrigerator embedded in spin-star environments: Scalings of temperature and refrigeration time

We examine a quantum absorption refrigerator that comprises three qubits, each of which is connected with a separate spin-star environment, with the three qubit-bath units coupled through an effective six-body interaction. The refrigerator exhibits the feature of transient cooling, i.e., lowering of the temperature of the first qubit in sufficiently small timescales, rather than steady-state refrigeration. A key advantage of our model is that the symmetries of the Hamiltonian enable a semi-analytic solution of the reduced density matrices of the refrigerator qubits, even in the presence of a large number of environmental spins. We derive the condition for autonomous refrigeration and analyze how the optimal cold-qubit temperature scales with the number of bath spins. We find a power-law scaling towards a constant asymptotic value. We also find the scaling of the minimum time required for optimal cooling as a function of the number of bath spins. Furthermore, we quantify the non-Markovianity of the cold-qubit dynamics using a restricted Breuer-Laine-Piilo information-backflow measure and observe that stronger backflow correlates with lower transient minimum temperatures across the sampled parameter regime. The transient-cooling performance is found to be robust under broad parameter variations. Compared to a conventional Markovian three-qubit refrigerator, the CSQAR achieves lower cold-qubit temperatures on shorter timescales. We further analyze the heat currents associated with the three qubits and their respective baths.

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arXiv2026

Remote predictability of quantum measurement outcomes

Predicting quantum measurement outcomes for a local observer makes sense before, but not after, the measurement. However, for a remote observer, predicting measurement outcomes even after measurement remains a valid question. We define remote predictability as the degree to which one observer can predict a measurement outcome of a spatially separated observer, given full knowledge of the shared quantum state and measurement setting. We show that the remote predictability before and after the measurement remains the same for product states, whereas it increases for all pure entangled states and for some classically correlated states. Perfect remote predictability for arbitrary projective measurements occurs only for maximally entangled states among all pure states, underscoring their special role. Comparing pure entangled states with their dephased versions, we find that dephasing on one subsystem can enhance remote predictability for a broad class of states and measurements - a counterintuitive, noise-induced advantage that vanishes for maximally entangled states under any projective measurement.

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arXiv2026

Distributed Quantum Dense Coding Enhanced With Non-classical Routing

We propose a distributed quantum dense coding protocol that uses a control system to superpose two dense coding processes, allowing us to simultaneously and coherently encode and non-classically route the sender's single-qubit system to two receiver labs. We find that dense coding with coherently controlled encoding and routing performs better than the standard dense coding protocol in both global and one-way local decoding strategies employed by receiver labs in weak entanglement regimes and with noisy mixed states. We extend the protocol to incorporate noisy channels and show that, there exists a certain noise strength below which our protocol outperforms the noiseless standard dense coding protocol in presence of quantum dephasing noise. We extend our protocol to an arbitrary number of receivers and analyse its performance under a global decoding strategy.

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arXiv2026

Role of energy-invariant assistants in energy extraction from quantum batteries

We investigate the role of energy-invariant assistants in energy extraction from quantum batteries. To this end, for energy extraction, we restrict ourselves to unitaries that jointly act on the battery and the assistant but preserve the energy of the assistant. We demonstrate that, in the presence of an energy-invariant assistant having the same dimension as the battery, all stored energy of the battery can always be extracted, transforming the battery into its ground state when an appropriate joint unitary and assistant state are employed. Additionally, we provide a necessary and sufficient condition for a battery to be unable to provide any energy, i.e., to be inactive, even when an energy-invariant assistant is present and prepared in an arbitrary but fixed state.

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arXiv2026

Enhanced quantum state discrimination under general measurements with entanglement and nonorthogonality restrictions

The minimum error probability for distinguishing between two quantum states is bounded by the Helstrom limit, derived under the assumption that measurement strategies are restricted to positive operator-valued measurements. We explore scenarios in which the error probability for discriminating two quantum states can be reduced below the Helstrom bound under some constrained access of resources, indicating the use of measurement operations that go beyond the standard positive operator-valued measurements framework. We refer to such measurements as non-positive operator-valued measurements. While existing literature often associates these measurements with initial entanglement between the system and an auxiliary, followed by joint projective measurement and discarding the auxiliary, we demonstrate that initial entanglement between system and auxiliary is not necessary for the emergence of such measurements in the context of state discrimination. Interestingly, even initial product states can give rise to effective non-positive measurements on the subsystem, and achieve sub-Helstrom discrimination error when discriminating quantum states of the subsystem.

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arXiv2025

Auxiliary-assisted energy distillation from quantum batteries

We discuss the idea of extracting energy from a quantum battery, applying a projective measurement on an auxiliary system. The battery is initially connected to the auxiliary system and allowed to interact with it. After some time, we execute a measurement on the auxiliary system which probabilistically projects the setup to a particular state, and the corresponding state of the battery is the final state. We consider the sum of the product of the energy difference between the initial and final states of the battery with the probability of getting that final state, where the sum is taken over all the preferable outcomes, that is, the outcomes which reduce the energy of the battery. We define the maximum value of this quantity as the distillable energy, where the maximization is taken over the time of interaction and auxiliary state and measurement basis parameters. Restricting ourselves to a particular uncountable set of states, we find that distillable energy is always higher than the ergotropy of the battery, irrespective of the presence or absence of entanglement between battery and auxiliary. We also compare the distillable energy with the energy extracted using the interaction between the battery and the auxiliary, without any measurements. In comparison with the measurement-free scenario, we show that while measurement-based protocols do not provide any enhancement in the amount of extractable energy, they do yield a distinct advantage in terms of power, most notably in the case of distillable power, surpassing the power obtained without measurements.

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arXiv2025

Nonlinearity-assisted advantage for open Dicke-quantum batteries

We analyze the performance of a quantum battery in terms of energy storage and energy extraction, assisted by nonlinearities in a Dicke quantum battery utilizing an open-system approach. In particular, we consider two types of nonlinearities in the system, viz. nonlinearity in the coupling between battery and the cavity, and the cavity itself comprising an anharmonic oscillator. In both these scenarios, the cavity is connected to an environment, and is driven by an external laser source. We derive the Markovian master equation for the dynamics of the combined cavity-battery system in presence of the environment. When the cavity is non-linearly coupled to a battery, we find an enhancement in the steady state ergotropy over the linearly-coupled case. We further see that the times at which steady state ergotopy is attained get decreased in the presence of nonlinear coupling. We additionally find a complementarity between the nonlinear interaction strength and the coherent-drive strength in the steady-state ergotropy values, i.e. the same ergotropy as that obtained using weak nonlinearity and strong coherent drive can be obtained utilizing an opposite order of their values. We also identify the nonlinear interaction where the cavity can be considered as a ``charger" and show an advantage of using the nonlinear coupling over linear ones. In case when the cavity is modeled as a multi-level transmon with an inherent anharmonicity, we find an advantage in maximum ergotropy values over the harmonic instance. Unlike the non-linearly coupled batteries, the ones mediated by an anharmonic cavity prove to be useful in the transient regime. We also determine values of the coherent drive for which the anharmonic battery is beneficial over the harmonic one at all timescales. Further, we observe a distinct region of the anharmonic strength and coherent drive where ergotropy values reach their maxima.

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arXiv2025

Quantum sensing of even- versus odd-body interactions

We analyze the scaling of quantum Fisher information with the number of system particles in the limit of large number of particles, as a function of the number of parties interacting with each other, for encoding Hamiltonians having arbitrary-body interactions. We find that estimation of coupling strength of such arbitrary-body encoding Hamiltonians provide a super-Heisenberg scaling that increases monotonically with an increase in the number of interacting particles, in the limit of large number of system particles. Moreover, we also find that the optimal probes corresponding to Hamiltonians that contain even-body interaction terms, may be entangled, but certainly not so in all bipartitions, and particularly, it is possible to attain optimal precision using asymmetric probes. Thereby we find a complementarity in the requirement of asymmetry and genuine entanglement in optimal probes for estimating strength of odd- and even-body interactions respectively. Additionally, we provide an upper bound on the number of parties up to which one can always obtain an asymmetric product state that gives the best metrological precision for even-body interactions. En route, we find the quantum Fisher information in closed form for two- and three-body interactions for arbitrary number of parties. We also provide an analysis of the case when the Hamiltonian contains local fields and up to k-body interaction terms, where the strength of interaction gradually decreases with an increase in the number of parties interacting with each other. Interestingly, we find a similar dichotomy in the nature of the optimal probe in this case as well. Further, we identify conditions on the local component of the Hamiltonian, for which this dichotomy is still shown to exist for two- and three-body encoding Hamiltonians with arbitrary local dimensions.

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arXiv2025

Precision in estimating independent local fields: attainable bound and indispensability of genuine multiparty entanglement

Estimation of local quantum fields is a crucial aspect of quantum metrology applications, and often also forms the test-bed to analyze the utility of quantum resources, like entanglement. However, so far, this has been analyzed using the same local field for all the probes, and so, although the encoding process utilizes a local Hamiltonian, there is an inherent "nonlocality" in the encoding process in the form of a common local field applied on all the probes. We show that estimation of even independent multiple field strengths of a local Hamiltonian, i.e., one formed by a sum of single-party terms, necessitates the utility of genuine multipartite entangled state as the input probe. The feature depends on the choice of the weight matrix considered, which is full-rank and contains non-vanishing off-diagonal terms. We obtain this result by providing a lower bound on the precision of multiparameter estimation, optimized over input probes, for an arbitrary positive semi-definite weight matrix. We show that there exists a weight matrix for which this bound is always attainable by the Greenberger-Horne-Zeilinger (GHZ) state, chosen in a certain basis. Furthermore, we find the parametric form of the most general optimal state for three parties. We also show that no pure product state can achieve the lower bound. Finally, for an arbitrary weight matrix and an arbitrary multiparty local encoding Hamiltonian, we prove that using a probe that is in any mixed state provides a precision lower than that obtainable using pure states. To emphasize the importance of the weight matrix considered, we also prove that the choice of identity operator as the same - thereby ignoring the "off-diagonal" covariances in the precision matrix - does not require the use of genuine multiparty entanglement in input probes for attaining the best precision, and the optimal probe can be a pure product.

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arXiv2025

Transient effects in quantum refrigerators with finite environments

We explore a small quantum refrigerator consisting of three qubits, each of which is kept in contact with an environment. We consider two settings: one is when there is necessarily transient cooling and the other is when both steady-state and transient coolings prevail. Our primary focus, however, is on the transient cooling phenomena. We show that in the transient regime, the temperature of the cold qubit can decrease further compared to the case where all qubits are connected to Markovian environments, by replacing the environment attached to the cold qubit with a finite-size spin environment, modeled by a few quantum spins interacting with the cold qubit. We also consider refrigeration with more than one finite-size spin environments of the three-qubit refrigerating device. As expected, a steady temperature is reached only if there are at least two Markovian baths, regardless of whether the cold qubit is attached to an environment. We investigate the effects of the finite-size environments on the cooling of the cold qubit when one, two, or all Markovian baths are replaced by finite-size environments. Additionally, we examine this effect in two- and single-qubit self-sustained devices connected to one or more finite-size environments. In deriving the dynamical equations for the qubits connected to finite-size environments, we made no assumptions about the Markovian nature of the environment. As a result, these finite-size environments inherently exhibit information backflow from the environment to the system, a hallmark of non-Markovianity. Hence, we propose a witness to detect non-Markovianity in such systems. Finally, the cooling processes are studied in presence of Markovian noise, and we analyse the response on the refrigeration of the noise strength. In particular, we find the noise strength until which refrigeration remains possible.

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arXiv2024

Modified Landauer's principle: How much can the Maxwell's demon gain by using general system-environment quantum state?

The Landauer principle states that decrease in entropy of a system, inevitably leads to a dissipation of heat to the environment. This statement is usually established by considering the system to be in contact with an environment that is initially in a thermal state, with the system-environment initial state being in a product state. Here we show that a modified Landauer principle, with correction terms, still holds even if the system and environment are initially correlated and the environment is in an athermal state. This is the most general quantum mechanically allowed operation in the Maxwell demon's arsenal, and, in particular, includes non-completely positive but physically realizable maps on the system. The correction terms provide an advantage: they reduce the work required by the Maxwell's demon to erase its memory. The modified principle also incorporates the possibility of arbitrary charge flows, including the usual heat flow, between system and environment. Furthermore, we consider a case where the system is in contact with a large initially-decoupled athermal environment, and we derive the finite-time modified Landauer's bound for the same.

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arXiv2024

Approximate second laws and energy extraction from quantum batteries

Conservation of energy under thermal operations, \textbf{TO}, is ensured by commutation of the unitary generating such operations with the total Hamiltonian. However in realistic scenarios, perturbations or disturbances in the system are unavoidable, which in turn may alter the commutation relation and hence in succession may affect the physical processes governed by \textbf{TO}. We call the altered set of operations as approximate thermal operations, \textbf{TO}$_ε$, where $ε$ denotes a degree of disturbance. We provide state transformation conditions under such operations, providing what can be referred to as approximate second laws. We show that in presence of feeble perturbations in the system's Hamiltonian, the states transform in such a way that diagonal elements of the system states start talking not only with each other but also with the off-diagonal elements. In parallel, the off-diagonal elements transform in a way such that they start connecting with diagonal elements and other off-diagonal elements. Such cross-talk is disallowed in the unperturbed second laws. As an application, we show that approximate thermal operations may lead to finite ergotropy extraction from quantum batteries, something that the exact ones are unable to.

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arXiv2024

Enhancing precision of atomic clocks by tuning disorder in accessories

We find that a quantum device having an accessory involving precision measurement can have an enhancement of its metrological precision in estimating an unknown parameter of the quantum system by insertion of glassy disorder, accidental or engineered. We clearly mention how an unbiased estimator can also be identified in a disordered situation, and how the precision thereof can be bounded by the quantum Cr{á}mer-Rao inequality. We compare the Fisher information-based lower bound of the minimum standard deviation of an unbiased estimator, in presence of glassy disorder in the system, with the same of an ideal, viz. disorder-free, situation. The phenomenon can boost the efficiency of certain measuring devices, such as atomic clocks. The precision of these clocks, when measuring time, hinges on the precise determination of the frequency of a two-level atom. In cases where impurities are present in the atom, and can be modeled as a disorder parameter, it is possible for the measurement of frequency to be more accurate than in an ideal, disorder-free scenario. Moreover, disorder insertion can reduce the requirement of entanglement content of the initial probes, which are copies of two-qubit states, along with providing a disorder-induced enhancement.

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arXiv2024

Noncompletely Positive Quantum Maps Enable Efficient Local Energy Extraction in Batteries

Energy extraction from quantum batteries by means of completely positive trace-preserving (CPTP) maps leads to the concept of CPTP-local passive states, which identify bipartite states from which no energy can be squeezed out by applying any CPTP map to a particular subsystem. We prove, for arbitrary dimension, that if a state is CPTP-local passive with respect to a Hamiltonian, then an arbitrary number of copies of the same state - including an asymptotically large one - is also CPTP-local passive. We show further that energy can be extracted efficiently from CPTP-local passive states employing NCPTP but still physically realizable maps on the same part of the shared battery on which operation of CPTP maps were useless. Moreover, we provide the maximum extractable energy using local-CPTP operations, and then, we present an explicit class of states and corresponding Hamiltonians, for which the maximum can be outperformed using physical local NCPTP maps. We provide a necessary and sufficient condition and a separate necessary condition for an arbitrary bipartite state to be unable to supply any energy using non-completely positive trace-preserving (NCPTP) operations on one party with respect to an arbitrary but fixed Hamiltonian. We build an analogy between the relative status of CPTP and NCPTP operations for energy extraction in quantum batteries, and the association of distillable entanglement with entanglement cost for asymptotic local manipulations of entanglement. The surpassing of the maximum energy extractable by NCPTP maps for CPTP-passive as well as for CPTP non-passive battery states can act as detectors of non-CPTPness of quantum maps.

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arXiv2024

Restoring metrological quantum advantage of measurement precision in noisy scenario

We show that in presence of a local and uncorrelated dephasing noise, quantum advantage can be obtained in the Fisher information-based lower bound of the minimum uncertainty in estimating parameters of the system Hamiltonian. The quantum advantage refers here to the benefit of initiating with a maximally entangled state instead of a product one. This quantum advantage was known to vanish in the same noisy scenario for a frequency estimation protocol. Restoration of the better precision in frequency estimation with maximally entangled probes can be obtained by incorporating an interaction between the system particles. The interaction examined here is Ising in nature, and is considered with or without a transverse magnetic field. There are instances, e.g. where frequency estimation in presence of a transverse field is considered and quantum advantage is not restored. A quantum advantage can also be obtained while estimating the strength of the introduced magnetic field along the transverse direction, whereas for the instances considered, using uncorrelated probes is better in measuring the coupling parameter of the Ising interaction. We also investigate the dependence of measurement precision on the entanglement content, which is not necessarily maximal, of the initial state. The precision in estimation of coupling constant decreases monotonically with the increase of entanglement content of the initial state, while the same for frequency estimation is independent of the entanglement content of the inputs.

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arXiv2023

Kerr-type nonlinear baths enhance cooling in quantum refrigerators

We study the self-contained three-qubit quantum refrigerator, with a three-body interaction enabling cooling of the target qubit, in presence of baths composed of anharmonic quantum oscillators with Kerr-type nonlinearity. We show that such baths, locally connected to the three qubits, opens up the opportunity to implement superior steady-state cooling compared to using harmonic oscillator baths, aiding in access to the free energy required for empowering the refrigerator function autonomously. We find that in spite of providing significant primacy in steady-state cooling, such anharmonic baths do not impart much edge over using harmonic oscillator baths if one targets transient cooling. However, we gain access to steady-state cooling in the parameter region where only transient cooling could be achieved by using harmonic baths. Subsequently, we also study the scaling of steady-state cooling advantage and the minimum attainable temperature for varying levels of anharmonicity present in the bath oscillators. Finally, we analyse heat currents and coefficients of performance of quantum refrigerators using bath modes involving Kerr-type nonlinearity, and present a comparison with the case of using bosonic baths made of simple harmonic oscillators. On the way, we derive the decay rates in the Gorini-Kossakowski-Sudarshan-Lindblad quantum master equation for Kerr-type anharmonic oscillator baths.

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