arXiv subjects

Ewelina Lange

Publications and source records attributed to Ewelina Lange.

4 recordsLinked to original sources

arXiv2026

Improving quantum-battery charging via unidirectional quantum jumps to metastable state

In the process of charging a quantum battery, quantum jumps play a detrimental role because they induce transitions from higher to lower energy levels, leading to energy dissipation. This be- havior directly opposes the fundamental objective of the charging protocol, which is to increase the population of higher-energy states and store usable energy in the system. Furthermore, decoherence introduced by randomness of quantum jumps leads to a mixed state, thereby reducing the amount of energy that can be extracted from the charged quantum battery. Here, we propose a quantum battery comprising an ensemble of three-level atoms in the Λ configuration to store energy in the metastable state over a long time. We demonstrate that, when spontaneous atomic transitions are restricted to occur solely from the excited state to a metastable state, with decay to the ground state completely suppressed, quantum jumps acquire a constructive character. Under these con- ditions, they drive the quantum battery to a fully charged state, render the entire stored energy extractable, and shorten the duration of the second stage of the charging protocol.

quant-ph↗

arXiv2023

The effect of thermal photons on exceptional points in coupled resonators

We analyse two quantum systems with hidden parity-time (PT) symmetry: one is an optical device, whereas another is a superconducting microwave-frequency device. To investigate their symmetry, we introduce an equilibrium frame, in which loss and gain terms for a given Hamiltonian are balanced. We show that the non-Hermitian Hamiltonians of both systems can be tuned to reach an exceptional point (EP), i.e., the point in parameter space at which a transition from broken to unbroken hidden PT symmetry takes place. We calculate a degeneracy of a Liouvillian superoperator, which is called the Liouvillian exceptional point (LEP), and show that, in the optical domain, LEP is equivalent to EP obtained from the non-Hermitian Hamiltonian (HEP). We also report breaking the equivalence between LEP and HEP by a non-zero number of thermal photons for the microwave-frequency system.

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arXiv2021

Equilibrium frame reveals hidden PT symmetry of passive systems

We discuss how introducing an equilibrium frame, in which a given Hamiltonian has balanced loss and gain terms, can reveal PT symmetry hidden in non-Hermitian Hamiltonians of dissipative systems. Passive PT-symmetric Hamiltonians, in which only loss is present and gain is absent, can also display exceptional points, just like PT-symmetric systems, and therefore are extensively investigated. We demonstrate that non-Hermitian Hamiltonians, which can be divided into a PT-symmetric term and a term commuting with the Hamiltonian, possess hidden PT symmetries. These symmetries become apparent in the equilibrium frame. We also show that the number of eigenstates having the same value in an exceptional point is usually smaller in the initial frame than in the equilibrium frame. This property is associated with the second part of the Hamiltonian.

quant-ph↗

arXiv2020

Rotation-time symmetry in bosonic systems and the existence of exceptional points in the absence of $\mathcal{PT}$ symmetry

We study symmetries of open bosonic systems in the presence of laser pumping. Non-Hermitian Hamiltonians describing these systems can be parity-time (${\cal{PT}}$) symmetric in special cases only. Systems exhibiting this symmetry are characterised by real-valued energy spectra and can display exceptional points, where a symmetry-breaking transition occurs. We demonstrate that there is a more general type of symmetry, i.e., rotation-time (${\cal{RT}}$) symmetry. We observe that ${\cal{RT}}$-symmetric non-Hermitian Hamiltonians exhibit real-valued energy spectra which can be made singular by symmetry breaking. To calculate the spectra of the studied bosonic non-diagonalisable Hamiltonians we apply diagonalisation methods based on bosonic algebra. Finally, we list a versatile set rules allowing to immediately identifying or constructing ${\cal{RT}}$-symmetric Hamiltonians. We believe that our results on the ${\cal{RT}}$-symmetric class of bosonic systems and their spectral singularities can lead to new applications inspired by those of the ${\cal{PT}}$-symmetric systems.

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