arXiv subjects

Srijon Ghosh

Publications and source records attributed to Srijon Ghosh.

At least 19 recordsLinked to original sources

arXiv2025

Efficient Estimation of Multiple Temperatures via a Collisional Model

We present a quantum thermometric protocol for the estimation of multiple temperatures within the collisional model framework. Employing the formalism of multiparameter quantum metrology, we develop a systematic strategy to estimate the temperatures of several thermal reservoirs with minimal estimation error. We prove a necessary and sufficient condition for the singularity of the Fisher information matrix for a bi-parametrized qubit state. By using controlled rotations of ancillary systems between successive interaction stages, we eliminate parameter interdependencies, thereby rendering the quantum Fisher information matrix non-singular. Remarkably, we demonstrate that precision enhancement in the joint estimation of multiple temperatures can be achieved even in the absence of correlations among the ancillas, surpassing the corresponding thermal Fisher information limits. Exploiting correlations within the ancillary system yields additional enhancement of Fisher information. Finally, we identify the dimensionality of the ancillary systems as a key factor governing the efficiency of multiparameter temperature estimation.

quant-ph↗

arXiv2025

Optimal phase estimation in the presence of correlated dephasing

We investigate optimal metrological protocols for phase estimation in the presence of correlated dephasing noise, including spin-squeezed states sensing strategies as well as parallel and adaptive protocols optimized using tensor-network based numerical methods. The results are benchmarked against fundamental bounds obtained either via a latest quantum comb extension method or an optimized classical simulation method. We find that the spin-squeezed offer practically optimal performance in the regime where phase fluctuations are positively correlated, but can be outperformed by tensor-network optimized strategies for negatively correlated fluctuations.

quant-ph↗

arXiv2024

Operational Ergotropy: suboptimality of the geodesic drive

We put forth a notion of optimality for extracting ergotropic work, derived from an energy constraint governing the necessary dynamics for work extraction in a quantum system. Within the traditional ergotropy framework, which predicts an infinite set of equivalent pacifying unitaries, we demonstrate that the optimal choice lies in driving along the geodesic connecting a given state to its corresponding passive state. Moreover, in a practical scenario where unitaries are inevitably affected by environmental factors, we refine the existing definition of ergotropy and introduce the notion of operational ergotropy. It enables the characterization of work extraction in noisy scenarios. We find that for certain typical noise models, the optimal choice which governs the Schrodinger part of the dynamics, aligns with the optimal drive in the unperturbed scenario. However, we demonstrate that such optimality is not universal by presenting an explicit counterexample. Additionally, within this generalized framework, we discuss the potential for faster work extraction from quantum systems in the presence of noise.

quant-ph↗

arXiv2024

Dimensional gain in sensing through higher-dimensional quantum spin chain

Recent breakthroughs in quantum technology pave the way for extensive utilization of higher-dimensional quantum systems, which outperform their qubit counterparts in terms of capabilities and versatility. We present a framework for accurately predicting weak external magnetic fields using a higher-dimensional many-body quantum probe. We demonstrate that dimension serves as a valuable resource for quantum sensing when a transverse spin-s Ising chain interacts locally with a magnetic field whose strength has to be determined. We observe the distinct performance of sensors for spin chains with half-integer and integer spins. Furthermore, we highlight that the time duration appropriate for quantum-enhanced sensing increases with the increase of dimension. Additionally, we observe that, in addition to nearest-neighbor interactions, incorporating interactions between the next nearest-neighbor sites increases sensing precision, particularly for spin chains with integer spins. We also prove the dimensional-dependence of the bound on quantum Fisher information which provides the limit on the precision in estimating parameters.

quant-ph↗

arXiv2023

Quantum-enhanced sensing with variable-range interactions

The typical bound on parameter estimation, known as the standard quantum limit (SQL), can be surpassed by exploiting quantum resources such as entanglement. To estimate the magnetic probe field, we propose a quantum sensor based on a variable-range many-body quantum spin chain with a moderate transverse magnetic field. We report the threefold benefits of employing a long-range system as a quantum sensor. First, sensors with quasi-long-range interactions can always beat the SQL for all values of the coordination number, while a sensor with long-range interactions does not have this ubiquitous quantum advantage. Second, a long-range Hamiltonian outperforms a nearest-neighbor (NN) Hamiltonian in terms of both estimating precision and system-size scaling. Finally, we observe that the system with long-range interactions can go below the SQL in the presence of a high temperature of the initial state, while sensors having NN interactions cannot. Furthermore, a sensor based on the long-range Ising Hamiltonian proves to be robust against impurities in the magnetic field and when the time-inhomogeneous dephasing noise acts during interaction of the probe with the system.

quant-ph↗

arXiv2023

Framework of dynamical transitions from long-range to short-range quantum systems

A quantum many-body system undergoes phase transitions of distinct species with variations of local and global parameters. We propose a framework in which a dynamical quantity can change its behavior for quenches across global (coarse-grained criterion) or local system parameters (fine-grained criterion), revealing the global transition points. We illustrate our technique by employing the long-range extended Ising model in the presence of a transverse magnetic field. We report that by distinguishing between algebraic and exponential scaling of the total correlation in the steady state, one can identify the first transition point that conventional indicators such as the rate function fail to detect. To determine the second one, we exploit the traditional local quenches. During quenches with and without crossing the critical points along the local parameter, total correlation follows either the same or different scaling laws depending on its global phase.

quant-ph↗

arXiv2022

Superiority in dense coding through non-Markovian stochasticity

We investigate the distributed dense coding (DC) protocol, involving multiple senders and a single or two receivers under the influence of non-Markovian noise, acting on the encoded qubits transmitted from senders to the receiver(s). We compare the effects of non-Markovianity on DC both for the dephasing and depolarising channels. In the case of dephasing channels, we illustrate that for some classes of states, high non-Markovian strength can eradicate the negative influence of noisy channels which is not observed for depolarizing noise. Furthermore, we incorporate randomness into the noise models by replacing the Pauli matrices with random unitaries and demonstrate the constructive impact of stochastic noise models on the quenched averaged dense coding capacity. Interestingly, we report that the detrimental effect of non-Markovian depolarising channels in the DC protocol can be eliminated when randomness is added to the channel.

quant-ph↗

arXiv2022

Multimode advantage in continuous variable quantum battery

We provide an architecture for a multimode quantum battery (QB) based on the framework of continuous variable (CV) systems. We examine the performance of the battery by using a generic class of multimode initial states whose parameters can be tuned to produce separable as well as entangled states and that can be charged locally as well as globally by Gaussian unitary operations. Analytical calculations show that a separable state is equally advantageous to an entangled one for two- and three-mode batteries when taking the figures of merit as the second moments of the change in energy. In order to produce a stable quantum battery consisting of an arbitrary number of modes, we derive compact analytical forms of the energy fluctuations and prove that for a multimode separable Gaussian initial state, fluctuations decrease as the number of modes increases, thereby obtaining a scaling analysis. Moreover, we demonstrate that local displacement as a charger is better for minimizing the fluctuations in energy than that involving the squeezing unitary operation.

quant-ph↗

arXiv2022

Mimicking quantum correlation of a long-range Hamiltonian by finite-range interactions

The quantum long-range extended Ising model possesses several striking features that cannot be observed in the corresponding short-range model. We report that the pattern obtained from the entanglement between any two arbitrary sites of the long-range model can be mimicked by the model having a finite range of interactions provided the interaction strength is moderate. On the other hand, we illustrate that when the interactions are strong, the entanglement distribution in the long-range model does not match the class of a model with a few interactions. We also show that the monogamy score of entanglement is in good agreement with the behavior of pairwise entanglement. Specifically, it saturates when the entanglement in the finite-range Hamiltonian behaves similarly to the long-range model, while it decays algebraically otherwise.

quant-ph↗

arXiv2022

Refrigeration via purification through repeated measurements

We design a measurement-based quantum refrigerator with an arbitrary number of qubits situated in a one-dimensional array that interact through variable-range XY interactions. The method proposed is based on repeated evolution followed by a measurement on the single accessible qubit, which has the potential to reduce the temperature in the rest of the subsystems, thereby demonstrating cooling in the device. The performance of the refrigerator is quantified by the fidelity of each local subsystem with the ground state of the local Hamiltonian and the corresponding probability of success. We identify system parameters, which include the interaction strength, range of interactions, initial temperature of each qubit, and the position of the measured qubit, so that the fidelities of all the unmeasured qubits approach unity with a nonvanishing probability. We observe that although strong interactions during evolution are required to achieve cooling, the long-range interactions typically deteriorate the performance of the refrigerator, which indicates that interactions are not ubiquitous. We report the scalability and the saturation property of the success probability with respect to the system size, which turns out to be independent of the involved system parameters and the number of repeated measurements. Furthermore, we show that the number of subsystems which can be cooled changes depending on the odd or even number of sites in the refrigerator. We argue that the distribution of entanglement between unmeasured qubits can give a possible explanation of the dependence of cooling process on the measured and unmeasured sites.

quant-ph↗

arXiv2021

Designing refrigerators in higher dimensions using quantum spin models

We design quantum refrigerators based on spin-j quantum XYZ and bilinear-biquadratic models with individual spins attached to bosonic thermal baths. By considering both local and global master equations, we illustrate an enhancement in the performance of the refrigerators with an increase in the spin dimension irrespective of the choice of the spin models. To assess the performance of the refrigerators, we introduce a distance-based measure to quantify the local temperature of a particle with arbitrary spin quantum number j. Interestingly, we find that the local temperature quantifier, defined via minimizing the distance between a spin-j thermal state and the evolved state of the spin-j particle in the steady state, coincides with the population-based definition of local temperature known in the literature for spin-1/2 particles. Moreover, we demonstrate that the qualitative behavior of the distance-based local temperature is independent of the choice of the distance measure by comparing the trace distance, Uhlmann's fidelity and relative entropy distance. We further observe by computing local master equation that the quantum refrigerator consisting of a spin-1/2 and a spin-j particle can lead to a lower local temperature compared to a refrigerator with two identical spin-j particles following the XYZ interactions.

quant-ph↗

arXiv2021

Circulating Genuine Multiparty Entanglement in Quantum Network

We propose a deterministic scheme of generating genuine multiparty entangled states in quantum networks of arbitrary size having various geometric structures -- we refer to it as entanglement circulation. The procedure involves optimization over a set of two-qubit arbitrary unitary operators and the entanglement of the initial resource state. We report that the set of unitary operators that maximize the genuine multipartite entanglement quantified via generalized geometric measure (GGM) is not unique. We prove that the GGM of the resulting state of arbitrary qubits coincides with the minimum GGM of the initial resource states. By fixing the output state as the six-qubit one, we find the optimal way to create such states according to the available resource. Moreover, we show that the method proposed here can be implemented by using logic gates, or by using the time dynamics of realizable spin Hamiltonians. In case of an ordered system, GGM varies periodically with time while the evolution via disordered models lead to a low but constant multipartite entanglement in outputs at a critical time, which decreases exponentially with the increase of the strength of the disorder.

quant-ph↗

arXiv2021

Quantum Battery with Ultracold Atoms: Bosons vs. Fermions

We design a quantum battery made up of bosons or fermions in an ultracold-atom setup, described by Fermi-Hubbard and Bose-Hubbard models, respectively. We compare the performance of bosons and fermions to determine which can function as a quantum battery more effectively given a particular on-site interaction and initial state temperature. The performance of a quantum battery is quantified by the maximum energy stored per unit time over the evolution under an on-site charging Hamiltonian. We report that when the initial battery state is in the ground state, fermions outperform bosons in a certain configuration over a large range of on-site interactions which are shown analytically for a smaller number of lattice sites and numerically for a considerable number of sites. Bosons take the lead when the temperature is comparatively high in the initial state for a longer range of on-site interaction. We study a number of up and down fermions as well as the number of bosons per site to find the optimal filling factor for maximizing the average power of the battery. We also introduce disorder in both on-site and hopping parameters and demonstrate that the maximum average power is robust against impurities. Moreover, we identify a range of tuning parameters in the fermionic and bosonic systems where the disorder-enhanced power is observed.

cond-mat.quant-gas↗

arXiv2021

Designing Robust Quantum Refrigerators in Disordered Spin Models

We explore a small quantum refrigerator in which the working substance is made of paradigmatic nearest-neighbor quantum spin models, the XYZ and the XY model with Dzyaloshinskii-Moriya interactions, consisting of two and three spins, each of which is in contact with a bosonic bath. We identify a specific range of interaction strengths which can be tuned appropriately to ensure a cooling of the selected spin in terms of its local temperature in the weak-coupling limit. Moreover, we report that in this domain, when one of the interaction strengths is disordered, the performance of the thermal machine operating as a refrigerator remains almost unchanged instead of degradation, thereby establishing the flexibility of this device. However, to obtain a significant amount of cooling via ordered as well as disordered spin models, we observe that one has to go beyond the weak-coupling limit and compute the figures of merits by using global master equations.

quant-ph↗

arXiv2021

Dimensional enhancements in a quantum battery with imperfections

Power storage devices are shown to increase their efficiency if they are designed by using quantum systems. We show that the average power output of a quantum battery based on a quantum interacting spin model, charged via a local magnetic field, can be enhanced with the increase of spin quantum number. In particular, we demonstrate such increment in the power output when the initial state of the battery is prepared as the ground or canonical equilibrium state of the spin-j XY model and the bilinear-biquadratic spin-j Heisenberg chain (BBH) in presence of the transverse magnetic field. Interestingly, we observe that in the case of the XY model, a trade-off relation exists between the range of interactions in which the power increases and the dimension while for the BBH model, the improvements depend on the phase in which the initial state is prepared. Moreover, we exhibit that such dimensional advantages persist even when the battery-Hamiltonian has some defects or when the initial battery-state is prepared at finite temperature.

quant-ph↗

arXiv2020

Decoherence-free mechanism to protect long-range entanglement against decoherence

Quantum spin models with variable-range interactions can exhibit certain quantum characteristics that a short-ranged model cannot possess. By considering the quantum XYZ model whose interaction strength between different sites varies either exponentially or polynomially, we report the creation of long-range entanglement in dynamics both in the absence and presence of system-bath interactions. Specifically, during closed dynamics, we determine a parameter regime from which the system should start its evolution so that the resulting state after quench can produce a high time-averaged entanglement having low fluctuations. Both in the exponential and power-law decays, it occurs when the magnetic field is weak and the interactions in the z-direction are nonvanishing. When part of the system interacts with the bath repeatedly or is attached to a collection of harmonic oscillators along with dephasing noise in the z-direction, we observe that long-range entanglement of the subparts which are not attached with the environment remains constant with time in the beginning of the evolution, known as freezing of entanglement, thereby demonstrating a method to protect long-range entanglement. We find that the frozen entanglement content in any length and the time up to which freezing occurs called the freezing terminal to follow a complementary relation for all ranges of interactions. However, we find that for a fixed range of entanglement, there exists a critical value of interaction length which leads to the maximum freezing terminal.

quant-ph↗

arXiv2020

Distribution of entanglement with variable range interactions

Distribution of quantum entanglement is investigated for an anisotropic quantum XY model with variable range interactions and in the presence of a uniform transverse magnetic field. We report the possibility of \emph{qualitative} growth in the entanglement between distant sites with an increase in the range of interactions that vary either exponentially or polynomially as the distance between the sites increases. Interestingly, we find that such entanglement enhancement is not ubiquitous and is dependent on the factorization points, a specific set of system parameters where the zero-temperature state of the system is fully separable. In particular, we observe that at zero-temperature, when the system parameters are chosen beyond the pair of factorization points, the increments in entanglement length due to variable range interactions are more pronounced compared to the situation when the parameters lie in between the factorization points. By employing the sum of all the bipartite entanglements with respect to a single site, we also show that the shareability of the bipartite entanglements are constrained, thereby establishing their monogamous nature. Furthermore, we note that the factorization points get reallocated depending on the laws of interaction fall-offs and provide an ansatz for the same. We reveal that the temperature at which the canonical equilibrium state becomes entangled from an unentangled one increases with the increase in the range of interactions, thereby demonstrating enhanced robustness in entanglement against temperature in the presence of long-range interactions and only when the system parameters are chosen between the pair of factorization points. We apply an energy-based entanglement witness to provide a justification to the observed robustness with temperature.

quant-ph↗

arXiv2020

Fast charging of quantum battery assisted by noise

We investigate the performance of a quantum battery exposed to local Markovian and non-Markovian dephasing noises. The battery is initially prepared as the ground state of a one-dimensional transverse $XY$ model with open boundary condition and is charged (discharged) via interactions with local bosonic reservoirs. We show that in the transient regime, quantum battery (QB) can store energy faster and has a higher maximum extractable work, quantified via ergotropy, when it is affected by local phase-flip or bit-flip Markovian noise compared to the case when there is no noise in the system. In both the charging and discharging processes, we report the enhancement in work-output as well as in ergotropy when all the spins are affected by non-Markovian Ohmic bath both in the transient and the steady-state regimes, thereby showing a counter-intuitive advantage of decoherence in QB. Both in Markovian and non-Markovian cases, we identify the system parameters and the corresponding noise models which lead to maximum enhancement of work-output and ergotropy. Moreover, we show that the benefit due to noise persists even with the initial state being prepared at a moderate temperature.

quant-ph↗