arXiv subjects

Jithin G. Krishnan

Publications and source records attributed to Jithin G. Krishnan.

4 recordsLinked to original sources

arXiv2026

Logarithmic growth of peripheral entanglement concentrated via noisy measurements in a star network of spins

In a star-network of qubits interacting via Heisenberg interaction of XYZ-type, we demonstrate a logarithmic growth of the localizable bipartite peripheral entanglement with increasing periphery-size and vanishing xy-anisotropy. This feature disappears when xy-anisotropy becomes non-zero, exhibiting an anisotropy effect, which can be negated by taking the system out of equilibrium by a qubit-local magnetic field. In the large-center and the competing-center limits of the model, the behaviour of LBPE is qualitatively different from that of the large-periphery limit. Also, the bipartite peripheral entanglement computed via a partial trace-based approach behaves qualitatively similarly to the LBPE in the large periphery limit, while in the other two limits, it behaves differently. We further consider the generalized description of localizable entanglement using unsharp measurements, and demonstrate that the logarithmic growth of LBPE is present for all noise strengths in the large-periphery limit, while in the competing-center limit, it does not.

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arXiv2026

Controlling gain with loss: Bounds on localizable entanglement in multi-qubit systems

We investigate the relation between the amount of entanglement localized on a chosen subsystem of a multi-qubit system via local measurements on the rest of the system, and the bipartite entanglement that is lost during this measurement process. We study a number of paradigmatic pure states, including the generalized GHZ, the generalized W, Dicke, and the generalized Dicke states. For the generalized GHZ and W states, we analytically derive bounds on localizable entanglement in terms of the entanglement present in the system prior to the measurement. Also, for the Dicke and the generalized Dicke states, we demonstrate that with increasing system size, localizable entanglement tends to be equal to the bipartite entanglement present in the system over a specific partition before measurement. We extend the investigation numerically in the case of arbitrary multi-qubit pure states. We also analytically determine the modification of these results, including the proposed bounds, in situations where these pure states are subjected to single-qubit phase-flip noise on all qubits. Additionally, we study one-dimensional paradigmatic quantum spin models, namely the transverse-field XY model and the XXZ model in an external field, and numerically demonstrate a cubic dependence of the localized entanglement on the lost entanglement. We show that this relation is robust even in the presence of disorder in the strength of the external field.

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arXiv2025

Block entanglement bounds distribution of regionally localized entanglement

In quantum networks, eliminating connections between nodes is crucial to mitigate the effects of decoherence, often achieved by performing measurements on nodes that are idle, or vulnerable to noise. To characterize the entanglement content of the resulting smaller network, we introduce the notion of ``regionally localized entanglement", defined as the average entanglement concentrated over a two-qubit region in a multi-qubit system. Hence, the total regionally localized entanglement can be obtained by considering all two-qubit regions sharing a common qubit, referred to as the ``hub". We prove that the total regionally localized entanglement corresponding to a specific hub is bounded above and below via the localizable block entanglement shared between the hub and the rest of the multi-qubit system for a number of paradigmatic pure quantum states, including permutation-symmetric states and arbitrary superposition of states from a specific magnetization sector. Numerical simulations confirm that the bounds for permutation-symmetric pure states remain valid even for Haar-uniformly generated pure states, and when each of the qubits is sent through local phase-flip channels of Markovian and non-Markovian types, except when the system-size is small. On the other hand, arbitrary states from a particular magnetization sector yield bounds on total regionally localized entanglement that are distinct from the permutation-symmetric states, highlighting the structurally unique entanglement properties of the former.

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arXiv2024

Simultaneous cooling of qubits via a quantum absorption refrigerator and beyond

We design a quantum thermal device that can simultaneously and dynamically cool multiple target qubits. Using a setup with three bosonic heat baths, we propose an engineering of interaction Hamiltonian using operators on different subspaces of the full Hilbert space of the system labelled by different magnetizations. We demonstrate, using the local as well as global quantum master equations, that a set of target qubits can be cooled simultaneously using these interaction Hamiltonians, while equal cooling of all target qubits is possible only when the local quantum master equation is used. However, the amount of cooling obtained from different magnetization subspaces, as quantified by a distance-based measure of qubit-local steady-state temperatures, may vary. We also investigate cooling of a set of target qubits when the interaction Hamiltonian has different magnetization components, and when the design of the quantum thermal device involves two heat baths instead of three. Further, we demonstrate, using local quantum master equation, that during providing cooling to the target qubits, the designed device operates only as a quantum absorption refrigerator. In contrast, use of the global quantum master equation indicates cooling of the target qubits even when the device works outside the operation regime of a quantum absorption refrigerator. We also extend the design to a star network of qubits interacting via Heisenberg interaction among each other, kept in contact with either three, or two heat baths, and discuss cooling of a set of target qubits using this device.

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