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arXiv:1508.04349

Thermalization and long-lived quantum memory in the multi-atomic ensembles

S. A. Moiseev, Vladimir A. Skrebnev

Published Aug 18, 2015• 0 citations
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Abstract

We have studied the role of thermalization in lifetime reduction of quantum memory based on multi-atomic ensembles. Herein, it is shown to be impossible to remove the thermaization-caused decoherence in such systems using the methods of dynamical decoupling. We have analyzed the existing models of the thermalization and have proposed a new understanding of the thermalization as a result of the internal processes which are not described in the unitary quantum dynamics formalism. The possible ways for reducing the negative influence of thermalization on the quantum memory lifetime are also discussed.

DOI
10.48550/arxiv.1508.04349

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We have studied the role of thermalization in lifetime reduction of quantum memory based on multi-atomic ensembles. Herein, it is shown to be impossible to remove the thermaization-caused decoherence in such systems using the methods of dynamical decoupling. We have analyzed the existing models of the thermalization and have proposed a new understanding of the thermalization as a result of the internal processes which are not described in the unitary quantum dynamics formalism. The possible ways for reducing the negative influence of thermalization on the quantum memory lifetime are also discussed.
The influence of decoherence on quantum memories for photons based on atomic ensembles is discussed. It is shown that despite the large entanglement of the collective storage states, corresponding to single photons or nonclassical states of light, the sensitivity to decoherence does not scale with the number of atoms. This is due to the existence of equivalence classes of storage states, which have the same projection onto the relevant quasiparticle mode (dark-state polariton). Several decoherence processes resulting from uncorrelated individual reservoir couplings are analyzed in detail: single-atom spin flips and dephasing, atom loss, and motion of atoms. Furthermore, it is shown that the sensitivity to collective decoherence processes that affect all polariton modes with comparable strength does also not increase with the number of atoms.
Thermalization in quantum many-body systems is a central problem of quantum mechanics. Despite the unitary nature of quantum evolution, isolated many-body systems often appear to relax toward states that are locally described by statistical ensembles. However, phenomena such as integrability and quantum many-body scars demonstrate that thermalization is not universal and can fail in unexpected ways, leading to nonergodic dynamics and long-lived nonequilibrium behavior. In this thesis, the problem is addressed from multiple complementary perspectives, including thermalization, integrability breaking, constrained dynamics, and quantum many-body scars, together with the development of new tensor-network methods for nonequilibrium quantum systems and quantum metrological applications. First, we study energy-filtering protocols for the preparation of thermal states, analyzing the possibility of simulating them classically through the behavior of entanglement and correlations. The effects of weak integrability breaking are then investigated in models relevant to Rydberg-atom experiments, leading to the identification of novel scattering mechanisms and the introduction of subsystem variance as a sensitive probe of weak integrability breaking. A part of the thesis is devoted to quantum many-body scars and their consequences for relaxation and transport phenomena. Their dynamical signatures are studied in both closed and open quantum systems, revealing new mechanisms that sustain long-lived nonequilibrium behavior. The thermalization processes arising during the quasi-adiabatic preparation of the ground state in the trapped Bose-Hubbard model are also examined, showing how transport constraints can stabilize low-entropy states under experimentally realistic conditions. Finally, a novel tensor-network approach based on Tree Tensor Operators is introduced for the efficient computation of the Quantum Fisher Information of generic mixed states. Applicable to both closed and open quantum systems, the method provides a promising numerical tool for quantum sensing and metrological applications.
We discuss the influence of atomic thermal motion on the efficiency of multimode quantum memory in two configurations: over the free expand of atoms cooled beforehand in a magneto-optical trap, and over complete mixing of atoms in a closed cell at room temperature. We consider the high-speed quantum memory, and assume that writing and retrieval are short enough, and the displacements of atoms during these stages are negligibly small. At the same time we take in account thermal motion during the storage time, which, as well known, must be much longer than durations of all the other memory processes for successful application of memory cell in communication and computation. We will analyze this influence in terms of eigenmodes of the full memory cycle and show that distortion of the eigenmodes, caused by thermal motion, leads to the efficiency reduction. We will demonstrate, that in the multimode memory this interconnection has complicated character.
Quantum systems are affected by interactions with their environments, causing decoherence through two processes: pure dephasing and energy relaxation. For quantum information processing it is important to increase the coherence time of Josephson qubits and other artificial two-level atoms. We show theoretically that if the coupling between these qubits and a cavity field is longitudinal and in the ultrastrong-coupling regime, the system is strongly protected against relaxation. Vice versa, if the coupling is transverse and in the ultrastrong-coupling regime, the system is protected against pure dephasing. Taking advantage of the relaxation suppression, we show that it is possible to enhance their coherence time and use these qubits as quantum memories. Indeed, to preserve the coherence from pure dephasing, we prove that it is possible to apply dynamical decoupling. We also use an auxiliary atomic level to store and retrieve quantum information.