arXiv subjects

Anindita Sarkar

Publications and source records attributed to Anindita Sarkar.

4 recordsLinked to original sources

arXiv2026

Surpassing thermal-state limit in thermometry via non-completely positive quantum encoding

Conventional quantum thermometry assumes completely positive (CP) encoding maps, where the probe is initially uncorrelated with the environment. We consider realistic scenarios with initial probe-environment correlations leading to physically realizable non-completely positive (NCP) encoding, and show how such encodings can significantly impact temperature estimation of the environment. We first consider pure entangled probe-environment initial states (Type-I NCP encoding) and analytically show that for probes and environments of equal but arbitrary dimension, the maximum achievable precision matches the thermal-state bound, as in the CP case. However, upon relaxing the constraint of pure probe-environment states and considering general correlated initial states (Type-II NCP encoding), we demonstrate that the estimation precision can surpass the thermal-state limit. This establishes a clear advantage of NCP encoding in enhancing thermometric performance. We illustrate the results using qubit probes interacting with qubit environments via XY interactions.

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arXiv2025

Comparing physical quantities with finite-precision: beyond standard metrology and an illustration for cooling in quantum processes

We propose a general framework to compare the values of a physical quantity pertaining to two - or more - physical setups, in the finite-precision scenario. Such a situation requires us to compare between two "patches" on the real line instead of two numbers. Identification of extent of the patches is typically done via standard deviation, as obtained within usual quantum metrological considerations, but can not be always applied, especially for asymmetric error distributions. The extent can however be universally determined by utilizing the concept of percentiles of the probability distribution of the corresponding estimator. As an application, we introduce the concept of finite-precision cooling in a generic quantum system. We use this approach in the working of a three-qubit quantum refrigerator governed by Markovian dynamics, and demonstrate the occurrence of cooling within finite precision for both transient and steady-state regimes, across strong- and weak-coupling limits of the inter-qubit interaction.

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arXiv2025

Fluctuation in energy extraction from quantum batteries: How open should the system be to control it?

We ask whether there exists a relation between controllability of the fluctuations in extractable energy of a quantum battery and (a) how open an arbitrary but fixed battery system is and (b) how large the battery is. We examine three classes of quantum processes for the energy extraction: unitary operations, completely positive trace-preserving (CPTP) maps, and arbitrary quantum maps, including physically realizable non-CPTP maps. We show that all three process classes yield the same average extractable energy from a fixed quantum battery. Moreover, open systems are better at controlling fluctuations in fixed quantum batteries: while random unitary operations result in nonzero fluctuation in the extractable energy, the remaining two classes lead to vanishing fluctuations in extractable energy. Furthermore, when the auxiliary system used to implement the non-unitary physically realizable maps is restricted up to a dimension $n$, fluctuation in extractable energy scales as $1/n$ for CPTP maps, outperforming the $\ln{n}/n$ scaling observed for general quantum maps. Even within open dynamics, therefore, energy extraction via random CPTP maps exhibits greater resilience to fluctuation compared to processes based on arbitrary quantum maps. We subsequently obtain that fluctuations in extractable energy scale as the inverse of the battery's dimension for all three process classes. Unitary maps, therefore, perform - in the sense of as low fluctuation as possible - equally well as more resource-intensive open maps, provided we have access to large quantum batteries. The results underscore a fundamental trade-off between performance of a battery and the resource cost of implementing the extraction processes.

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arXiv2025

Non-Markovianity vs athermality: perturbation-enhanced information backflow

Non-Markovianity and athermality are useful resources in quantum technologies, and it is therefore important to understand the relations between the two, for general quantum dynamics. We propose three measures of non-Markovianity, first within the ambit of thermal operations, and then beyond it, that result from unavoidable perturbations in system's Hamiltonian and that leads to violations of conservation of total energy characterizing any thermal operation. The proposed measures are based respectively on system-environment entanglement, total correlation in the system-environment partition, and on a concept of distance defined on the sets of usual and approximate thermal operations. We investigate the response of non-Markovianity to the athermality-inducing perturbations, using all the three measures. For the entanglement and distance-based measures, we derive upper bounds on the response by a quantity that depends on the perturbative Hamiltonian. For the total correlation-based measure, we are able to compute the exact response. We present examples of qubit-qubit and qubit-qutrit systems for which perturbation leads to enhancement of non-Markovianity, as quantified by the entanglement and total correlation-based measures.

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