arXiv subjects

T. T. Sergeev

Publications and source records attributed to T. T. Sergeev.

9 recordsLinked to original sources

arXiv2025

Non-Markovian protection of states from decay in quasi-PT-symmetric systems

We consider a quasi-PT-symmetric system of two resonators, one of which interacts with a finite-size environment. The interaction with the environment leads to energy losses in the resonators, and the finite size of the environment leads to a non-Markovian dynamics of the relaxation process. We demonstrate that non-Markovian processes in the quasi-PT-symmetric system can make the states of the system infinitely living, loss-protected states, even in the absence of gain. There is a critical value of the interaction between the resonator and the environment below which any state of the system is loss-protected. When the interaction magnitude is greater than the critical value, depending on the coupling strength between the resonators, either one or both states are unprotected. We show that the boundaries of regions with different numbers of protected states are determined by the relaxation rates in the quasi-PT-symmetric system, calculated in the Markovian approximation. By changing the coupling strength between the resonators and the interaction magnitude between the resonator and the environment, the system switches between modes with two, one, or no loss-protected states. This makes it possible to realize stable PT-symmetric devices based on purely dissipative systems. The obtained results are applicable to quantum systems with single excitations, allowing the concept of PT symmetry to be extended to such systems.

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arXiv2025

Spontaneous symmetry emergence in a Hermitian system of coupled oscillators without symmetry

Spontaneous symmetry breaking in systems with symmetry is a cornerstone phenomenon accompanying second-order phase transitions. Here, we predict the opposite phenomenon, namely, spontaneous symmetry emergence in a system that lacks symmetry. In the example of two coupled oscillators interacting non-symmetrically with a set of oscillators whose frequencies uniformly fill a finite frequency range, we demonstrate that the system state can acquire symmetry that is not inherent in the system Hamiltonian. The emergence of symmetry is manifested as a change in the system dynamics, which can be interpreted as a phase transition in a Hermitian system that lacks symmetry.

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arXiv2025

Anomalous dependence of sensitivity on observation time caused by memory retention in the time crystal

In this work, we consider a composite atom-cavity system interacting with a ring resonator. In such a structure, time crystal regime can be observed. We show that a quadratic observation time dependence of the system's sensitivity to perturbations takes place in the time crystal regime and also in the transition area to the normal state. This dependence is due to ability of the system to retain the memory of the atom's initial state. Outside these areas, the system is not able to retain the memory of the atom's initial state and the sensitivity scales linearly on the observation time. Our results open up a new way for implementation of discrete time crystals in sensing and metrology.

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arXiv2025

Transition from non-ergodic to ergodic dynamics in an autonomous discrete time crystal

We consider an autonomous system of two coupled single-mode cavities, one of which interacts with a multimode resonator. We demonstrate that for small coupling strengths between single-mode cavities, the Loschmidt echo oscillates periodically in time and spontaneous breaking of time translation symmetry takes place. The Loschmidt echo behavior is an indication of the non-ergodic nature of the system when its evolution is time-reversible and the system retains a memory of the initial state under the action of small perturbations. This behavior reveals the presence of a time crystalline order in the autonomous system. In this regime, the system is a new class of time crystals - autonomous discrete time crystals. An increase in the coupling strength leads to a transition from periodic oscillations to an exponential decay in time of the Loschmidt echo. This corresponds to the transition from non-ergodic behavior to ergodic one in the system, and is accompanied by the disappearance of time crystalline order. We demonstrate that at the transition point the time-averaged variance of the number of photons reaches a maximum, which serves as a signature of the transition. We show that such a transition can also be observed when changing the number of degrees of freedom in the resonator, which is achieved by changing its length.

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arXiv2024

Spontaneous breaking of time translation symmetry in a system without periodic external driving

It is known that the spontaneous time translation symmetry breaking can occur in systems periodically driven at a certain period. We predict a spontaneous breaking of time translation symmetry in an atom-cavity system without external driving, in which a time scale is determined by the time of light bypass of the resonator. We demonstrate that there is a parameter range, in which a system state returns to its initial state only after two bypasses of the resonator. We believe that the predicted phenomenon opens a way to a new direction in the time crystal field.

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arXiv2023

Signature of exceptional point phase transition in Hermitian systems

Exceptional point (EP) is a spectral singularity in non-Hermitian systems. The passing over the EP leads to a phase transition, which endows the system with unconventional features that find a wide range of applications. However, the need of using the dissipation and amplification limits the possible applications of systems with the EP. In this work, we demonstrate an existence of signature of exceptional point phase transition in Hermitian systems that are free from dissipation and amplification. We consider a composite Hermitian system including both two coupled oscillators and their environment consisting only of several tens of degrees of freedom. We show that the dynamics of such a Hermitian system demonstrate a transition, which occurs at the coupling strength between oscillators corresponding to the EP in the non-Hermitian system. This transition manifests itself even in the non-Markovian regime of the system dynamics in which collapses and revivals of the energy occur. Thus, we demonstrate that the phase transition occurring at the passing over the EP in the non-Hermitian system manifests itself in the Hermitian system at all time. We discuss the experimental scheme to observe the signature of EP phase transition in the non-Markovian regime.

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arXiv2023

Self-consistent description of relaxation processes in systems with ultra- and deep-strong coupling

An ultra-strong coupling regime takes place in a compound system when a coupling strength between the subsystems exceeds one tenth of the system eigenfrequency. It transforms into a deep-strong coupling regime when the coupling strength exceeds the system eigenfrequency. In these regimes, there are difficulties with description of relaxation processes without explicit considering of environment degrees of freedom. To correctly evaluate the relaxation rates, it is necessary to consider the interaction of the system with its environment taking into account the counter-rotating wave and diamagnetic terms. We develop a self-consistent theory for calculation of the relaxation rates in the systems, in which the coupling strength is of the order of the system eigenfrequency. We demonstrate that the increase in the coupling strength can lead to a significant decrease in the relaxation rates. In particular, we show that for frequency-independent density of states of the environment, the relaxation rates decrease exponentially with the increase in the coupling strength. This fact can be used to suppress losses by tuning the strength coupling and the environment states.

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arXiv2022

Environment-assisted strong coupling regime

Strong coupling regime takes place in open hybrid systems consisting of two or more physical subsystems when the coupling strength between subsystems exceeds the relaxation rate. The relaxation arises due to the interaction of the system with environment. For this reason, it is usually believed that the enhancement of the interaction with environment inevitably leads to a transition of the system from the strong to weak coupling regime. In this paper, we refute this common opinion. We demonstrate the interaction of the coupled system with environment induces an additional coupling between the subsystems that contribute to retention the system in the strong coupling regime. We show that the environmental-induced coupling strength is proportional to the product of the Rabi coupling strength by the gradient of the density of states of the reservoir. There is a critical Rabi coupling strength above which the environmental-induced coupling ensures that the system remains in the strong coupling regime at any relaxation rate. In this case, the strong coupling regime takes place even when the relaxation rate is significantly above the Rabi coupling strength between the subsystems. The critical coupling depends on the gradient of the reservoir density of states. We demonstrate that managing this gradient can serve as an additional tool to control the properties of the coupled systems.

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arXiv2021

A new type of non-Hermitian phase transition in open systems far from thermal equilibrium

We demonstrate a new type of non-Hermitian phase transition in open systems far from thermal equilibrium, which takes place in coupled systems interacting with reservoirs at different temperatures. The frequency of the maximum in the spectrum of energy flow through the system plays the role of the order parameter, and is determined by an analog of the -potential. The phase transition is exhibited in the frequency splitting of the spectrum at a critical point, the value of which is determined by the relaxation rates and the coupling strengths. Near the critical point, fluctuations of the order parameter diverge according to a power law. We show that the critical exponent depends only on the ratio of reservoir temperatures. This dependence indicates the non-equilibrium nature of the phase transition at the critical point. This new non-Hermitian phase transition can take place in systems without exceptional points.

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