arXiv subjects

Anumita Mukhopadhyay

Publications and source records attributed to Anumita Mukhopadhyay.

8 recordsLinked to original sources

arXiv2026

Spectrally Selective Charging of an Interacting Quantum Battery via an Anharmonic Mediator

We investigate a finite-time quantum-battery charging architecture in which a driven two-level working system transfers nonequilibrium resources to an interacting two-qubit battery through a weakly anharmonic three-level mediator. Finite-time driving generates coherence and ergotropy, while excitation-number conservation organizes the transfer into distinct dynamical sectors. The mediator then provides spectrally selective charging channels: its lower transition enables nearly complete single-excitation transfer with negligible residual mediator energy, whereas initial mediator loading activates a cross-resonant two-excitation cascade toward the doubly excited battery state. The latter exhibits a modest reduction in transfer efficiency due to the unequal collective matrix elements of the effective three-state chain. Detuning scans quantify the spectral tolerance, and the full mixed-state dynamics reveals distinct energy- and ergotropy-transfer profiles. Finite-time compression, thermal reset, and switching work close the cycle-level energy balance to numerical precision. The resulting scheme combines nonequilibrium resource generation, spectral selectivity, coherent multilevel charging, and thermodynamic cycle closure within a single architecture.

quant-ph↗

arXiv2026

Efficient algorithm for fidelity estimation of two quantum states

The fidelity estimation between two quantum states is crucial for quantum computation and information science. However, an efficacious method for this, especially for mixed states and higher-dimensional density matrices, remains elusive. While there are many existing algorithms on computing the fidelity between two pure states, there is not much work on how to obtain the fidelity between two mixed states. Here, an efficient quantum algorithm for the fidelity estimation is proposed, based primarily on the density matrix exponentiation and interferometeric scheme for mixed states, with a time complexity of $O(κ^2N^2/ε^7)$, where $N$ is the system size, $κ$ is the larger of the condition number of the density matrices and $ε$ is a precision error. This algorithm may serve as a resource-efficient technique to deduce fidelity of any two (pure or mixed) unknown or known quantum states, when the density matrices of the quantum states commute with each other.

quant-ph↗

arXiv2026

Cross-Spectral Reservoir Correlations as a Resource for Finite-Time Quantum Otto Engines

We investigate the thermodynamic consequences of longitudinal-transverse cross-spectral reservoir correlations in a finite-time quantum Otto engine with a two-level working medium. Each reservoir couples through excitation-relaxation and dephasing channels whose fluctuations are characterized by a Hermitian positive-semidefinite spectral-density matrix, with the off-diagonal elements encoding their cross correlations. The finite-time isochoric dynamics is derived within the second-order time-convolutionless framework, without imposing the Markov limit at the outset, so that finite reservoir-memory effects can enter through time-dependent dissipative and reservoir-induced coherent contributions. The resulting dynamics is then recast in Bloch-vector form to construct the stroke-resolved cycle dynamics. At fixed auto-spectral densities, cross-spectral correlations modify the populations and coherences of the working medium and thereby its thermodynamic performance. Increasing the correlation strength can enhance the output power, with the enhancement controlled by the cross-spectral phase and characteristic frequency scale. The correlations also reshape the transient cycle-to-cycle evolution and the approach to periodic operation, while the limit-cycle efficiency remains fixed at the Otto value for the population-preserving unitary strokes considered here. These results establish off-diagonal reservoir spectra as an additional resource for controlling finite-time quantum thermal machines.

quant-ph↗

arXiv2026

Estimation of trace distance between two arbitrary quantum states

When it comes to discriminating between two quantum states, trace distance is one of the well-known metrics used in quantum computation and quantum information theory. While there are several quantum algorithms for calculating the trace distance between two quantum states, computing it for any two general density matrices remains computationally demanding. In this paper, we propose a quantum algorithm based on the exponentiation of the density matrix and the improved quantum phase estimation (IQPE) to determine the trace distance for both pure and mixed states, with a time complexity of $O(N^2/\varepsilon^6)$ where $N$ is the number of qubits of the given states and $\varepsilon$ is the simulation or estimation precision error. We demonstrate its ability to predict the quantity with proof-of-principle simulations and also quantum hardware computations on the IBM quantum computers, confirming its promise for near-term quantum devices.

quant-ph↗

arXiv2026

P-divisibility conditions on subsystems in quantum dynamics

We investigate the constraints imposed by global unitary dynamics on the P-divisibility of local subsystems in a bipartite system-environment setting. Using the trace distance as a measure of state distinguishability and exploiting its conservation under unitary evolution, we develop a fully symmetric framework that simultaneously tracks information flow in both the open system $S$ and its environment $E$. We first show that for initially uncorrelated states the maximum net non-Markovian gain in one subsystem is rigidly bounded by the initial distinguishability of the other (Theorem~1). We then establish a two-sided Correlation Window (Theorem~2) that confines the sum of net changes in local distinguishability: the upper edge is set by the total initial marginal distinguishability, while the lower edge is governed exactly by the dynamically generated bipartite correlation norm, proving that any mutual loss of local distinguishability is necessarily encoded into correlations. Finally, we generalize both results to initially correlated states (Theorem~3), showing that initial bipartite correlations augment the capacity for non-Markovian gain and that simultaneous P-indivisibility of both subsystems is funded by the degradation of the initial correlation reservoir. These results are validated on four-qubit GHZ and W circuit examples, demonstrating subsystem locking, asymmetric single-subsystem backflow, and correlation-funded simultaneous back-flow.

quant-ph↗

arXiv2026

Complete Positivity of Subsystems in Quantum Dynamics

Although many quantum channels satisfy Completely Positive Trace Preserving (CPTP) condition, there are valid quantum channels that can be non-completely positive (NCP). As memory effects can provide advantages in the dynamics of noisy quantum systems, we investigate the relative CP condition and the CP-divisibility condition of the system and environment subsystems of a joint system-environment quantum state evolving noiselessly. We show that the system and environment channels must be both CP (NCP) or CP-divisible (CP-indivisible) for the evolution in the joint system-environment space to be unitary. We illustrate our results with examples of Bell state created from $|00\rangle$, GHZ state created from $|000\rangle$, W state created from $|100\rangle$, and the partial transpose (PT) operation acting on the Bell state

quant-ph↗

arXiv2025

Unitality Conditions on Subsystems in Quantum Dynamics

It is known that non-unital noise such as the amplitude damping can sometimes increase quantum correlations, while unital noise such as the dephasing usually decreases quantum correlations. It is, therefore, important to delineate the conditions, when noise can enhance the quantumness of the system. Here, we show that if the noise acting on the system is unital (non-unital), then the noise acting on the environment must also be unital (non-unital), for the evolution to be unitary in the joint system-environment space. For example, if the first two qubits are treated as system and the third qubit is treated as environment, then both the system and the environment evolve unitally in case of a three-qubit GHZ state, and both of them evolve non-unitally in case of a three-qubit W state. Our result may be of interest in quantum information, and we anticipate it to be useful in various contexts, such as to better tackle noise in quantum computing and quantum information processing.

quant-ph↗

arXiv2025

Efficient learning of arbitrary single-copy quantum states

Quantum state tomography is the problem of estimating a given quantum state. Usually, it is required to run the quantum experiment - state preparation, state evolution, measurement - several times to be able to estimate the output quantum state of the experiment, because an exponentially high number of copies of the state is required. In this work, we present an efficient algorithm to estimate with a small but non-zero probability of error the output state of the experiment using a single copy of the state, without knowing the evolution dynamics of the state. It also does not destroy the original state, which can be recovered easily for any further quantum processing. As an example, it is usually required to repeat a quantum image processing experiment many times, since many copies of the state of the output image are needed to extract the information from all its pixels. The information from $\mathcal{N}$ pixels of the image can be inferred from a single run of the image processing experiment in our algorithm, to efficiently estimate the density matrix of the image state.

physics.gen-ph↗