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Spontaneous symmetry breaking is a phenomenon of an alteration of a state symmetry without a change in the system symmetry. A transition from a state with unbroken symmetry to a state with broken symmetry leads to a qualitative change in system properties, and respective states are called as symmetric and non-symmetric phases of the system. Usually, the change in the system parameters is necessary for the transition between symmetric and non-symmetric phases. In this letter, we consider the behavior of an optomechanical system with an exceptional point under the influence of noise. We demonstrate that below the exceptional point, PT-symmetric and non-PT-symmetric phases can coexist. In this regime, a noise can lead to random transitions between the symmetric and non-symmetric phases. These transitions are accompanied by the abrupt changes in the intensities of photon and phonon modes. The predicted phenomenon opens up the possibility of studying the kinetics of phase transitions caused by spontaneous breaking of PT symmetry.

A phonon laser is an important device that can generate coherent sound waves at gigahertz frequency. In this paper, we demonstrate that in a phonon laser, the radiation line width can depend on the pumping non-monotonically. This is because there are two different types of solutions whose spectra depend differently on the noise amplitude. The first solution is a zero solution, which is stable before the generation threshold and is nondegenerate. The second solution is a nonzero solution that is stable above the generation threshold and is degenerate with respect to the phase. The line width of peaks in the spectrum of the zero solution does not depend on the noise amplitude. While, the line width of the peak in the spectrum of the nonzero solution increases with increasing noise amplitude. The noise amplitude increases with temperature, and, as a result, there is a temperature above which the transition from the zero solution to the nonzero solution leads to an increase in the radiation line width. In this case, the radiation line width non-monotonically depends on the pumping.

We demonstrate theoretically and experimentally that linear coupling due to the backscattering between two counterpropagating modes in a Kerr microresonator leads to extreme sensitivity of the intensity asymmetry of nonlinear light states to the relative phase between the bidirectional pumps of equal power. In the absence of linear coupling, the relative pump phase does not affect counterpropagating intraresonator intensities, and two asymmetric states arise due to spontaneous symmetry breaking. Contrariwise, in the presence of weak linear intermode coupling, the asymmetry of the states is deterministically controlled via changes in phase (for all phases except 0 and $\pi$). Spontaneous symmetry breaking at zero phase is suppressed when the linear intermode coupling is increased, while for the $\pi$ phase it can be enhanced, so that the overall threshold for the spontaneous symmetry breaking can be significantly lower for nonzero linear coupling. These results are important for fundamental understanding of the processes in Kerr resonators and other systems with Kerr-like nonlinearities and linear intermode coupling and have high prospects for the development of photonic devices such as ultrasensitive sensors., Comment: Main text was shortened (details moved to Supplementary materials), new ref. [25] was added

In this work, we develop an optomechanical formalism for macroscopic quantum states in exciton-polariton systems with strong exciton-phonon interactions. We show that polariton optomechanical interactions induce dynamical backaction, resulting in dispersive and dissipative shifts in the complex vibrational response functions. Unlike conventional optomechanical systems, polariton optomechanics features high-dimensionality and phase-space confinement due to the dispersion relations of exciton-polaritons. Consequently, vibrational modes exhibit effective positive or negative mass depending on the detuning parameter, and are capable for the nonequilibrium vibrational Bose-Einstein condensation under the resonant conditions [arXiv:2309.08498]. We demonstrate the potential for vibrational control of polariton condensates at room temperature.

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.

The dynamics of open quantum systems connected with several reservoirs attract great attention due to its importance in quantum optics, biology, quantum thermodynamics, transport phenomena, etc. In many problems, the Born approximation is applicable which implies that the influence of the open quantum system on the reservoirs can be neglected. However, in the case of a long-time dynamics or mesoscopic reservoir, the reverse influence can be crucial. In this paper, we investigate the transient dynamics of several bosonic reservoirs connected through an open quantum system. We use an adiabatic approach to study the temporal dynamics of temperatures of the reservoirs during relaxation to thermodynamic equilibrium. We show that there are various types of temperature dynamics that strongly depend on the values of dissipative rates and initial temperatures. We demonstrate that temperatures of the reservoirs can exhibit non-monotonic behavior. Moreover, there are moments of time during which the reservoir with initially intermediate temperature becomes the hottest or coldest reservoir. The obtained results pave the way for managing energy flows in mesoscale and nanoscale systems.

The emission linewidth in active medium emerges due to homogeneous and inhomogeneous broadening. We demonstrate that in lasers with inhomogeneous broadening there is a critical pump rate, above which the special mode forms. This mode consists of locked-in oscillations of cavity mode and of the active particles with different transition frequencies. Below the critical value of the pump rate, the radiation spectrum of the laser has a Gaussian profile, provided that inhomogeneous broadening is dominant. Above the critical value of pump rate, the special mode mostly determines the laser radiation spectrum. As the result, the spectrum attains Lorentz shape characteristic for homogeneous broadening. We demonstrate that the formation of the special mode precedes lasing and that the critical pump rate plays the role of lasing prethreshold. We obtain expressions for the threshold and generation frequency of single-mode laser where both homogeneous and inhomogeneous broadening are present.

Polariton thermalization is a key process in achieving light-matter Bose--Einstein condensation, spanning from solid-state semiconductor microcavities at cryogenic temperatures to surface plasmon nanocavities with molecules at room temperature. Originated from the matter component of polariton states, the microscopic mechanisms of thermalization are closely tied to specific material properties. In this work, we investigate polariton thermalization in strongly-coupled molecular systems. We develop a microscopic theory addressing polariton thermalization through electron-phonon interactions (known as exciton-vibration coupling) with low-energy molecular vibrations. This theory presents a simple analytical method to calculate the temperature-dependent polariton thermalization rate, utilizing experimentally accessible spectral properties of bare molecules, such as the Stokes shift and temperature-dependent linewidth of photoluminescence, in conjunction with well-known parameters of optical cavities. Our findings demonstrate qualitative agreement with recent experimental reports of nonequilibrium polariton condensation in both ground and excited states, and explain the thermalization bottleneck effect observed at low temperatures. This study showcases the significance of vibrational degrees of freedom in polariton condensation and offers practical guidance for future experiments, including the selection of suitable material systems and cavity designs.

System with strong photon-phonon interaction and optomechanical instability are perspective for generation of coherent phonons and photons. Typically, above the threshold of optomechanical instability, the photon intensity increases linearly with pumping. We demonstrate that in such systems, it is possible to achieve hard mode of excitation when jump increase in the photon intensity takes place. We derive the analytical expression determining conditions for such a jump increase. We demonstrate that the hard excitation mode in system with optomechanical instability arises due to an additional phase condition for the existence of a nonzero solution. The discovered hard excitation mode paves the way for creation highly sensitive sensors and optical transistors.

The quantum regression theorem is a powerful tool for calculating the muli-time correlators of operators of open quantum systems which dynamics can be described in Markovian approximation. It enables to obtain the closed system of equation for the multi-time correlators. However, the scope of the quantum regression theorem is limited by a particular time order of the operators in multi-time correlators and does not include out-of-time-ordered correlators. In this work, we obtain an adjoint master equation for multi-time correlators that is applicable to out-of-time-ordered correlators. We show that this equation can be derived for various approaches to description of the dynamics of open quantum systems, such as the global or local approach. We show that the adjoint master equation for multi-time correlators is self-consistent. Namely, the final equation does not depend on how the operators are grouped inside the correlator, and it coincides with the quantum regression theorem for the particular time ordering of the operators., Comment: 11 pages

Exceptional points show great prospects for applications such as imaging, sensing and designing lasers. Recently, systems with amplifying active medium exhibiting pump-induced exceptional points have attracted much attention due to possibility of controlling strong coupling between light and matter with the aid of pumping. In such structures, the interaction of active molecules with external degrees of freedom, such as phonons or impurities, leads to the destruction of the correlation between polarizations of different molecules. We study the effect of the correlations decay on a system behavior near pump-induced exceptional point. We show that strong coupling persists and eigenvectors together with eigenfrequencies coalesce at a negative value of population inversion, regardless of correlation decay magnitude. Thus, we show that exceptional points are robust to correlation decay, which is crucial for designing systems with exceptional points.

We demonstrate a macro-coherent regime in exciton-polariton systems, where nonequilibrium polariton Bose--Einstein condensation coexists with macroscopically occupied vibrational states. Strong exciton-vibration coupling induces an effective optomechanical interaction between cavity polaritons and vibrational degrees of freedom of molecules, leading to vibrational amplification in a resonant blue-detuned configuration. This interaction provide a sympathetic mechanism to achieve vibrational condensation with potential applications in cavity-controlled chemistry, nonlinear and quantum optics.

We perform canonical analysis of an action in which 2+1-dimensional gravity with negative cosmological constant is coupled to cylindrically symmetric dust shell. The resulting phase space is finite dimensional having geometry of SO(2; 2) group manifold. Representing the Poisson brackets by commutators results in the algebra of observables which is a quantum double D(SL(2)q). Deformation parameter q is real when the total energy of the system is below the threshold of a black hole formation, and a root of unity when it is above. Inside the black hole the spectra of the shell radius and time operator are discrete and take on a finite set of values. The Hilbert space of the black hole is thus finite-dimensional., Comment: 8 pages

Strong and ultra-strong coupling in "cavity - single atom" system are of great interest for both fundamental and applied physics. It is considered that the increase in the coupling strength between a cavity mode and an atom leads, first, to transition from weak to strong coupling and, second, to ultra-strong coupling regime. In this letter, we refute this common opinion and demonstrate that the transitions between the coupling regimes occur in different sequence for the correlations' functions of different orders. We show that for n-th order correlations' functions, the transition to the strong coupling regime requires the coupling strength approximately by $n^{2/3}$ times greater than the one for first order correlations' functions. In contrast, the transition to the ultra-strong coupling regime manifests in the dynamics of n-th order correlations' functions at the less coupling strength than in the dynamics of first order correlations' functions. As a result, there is the order of correlations' functions, above which the increase in the coupling strength leads, first, to the transition from the weak coupling first to the ultra-strong coupling regime, and second to the strong coupling regime. We argue that the measurement of high orders correlations' functions makes it possible to observe the ultra-strong coupling in "cavity mode - single atom" when the coupling strength is much less than one tenth of the oscillation frequency.

We discuss the connection between the vacuum energy problem and the number of dark matter sectors and its relation to the cancellation of quadratic divergences both in the cosmological constant and simultaneously in the polarization tensor renormalizing the Higgs meson mass. In the absence of global supersymmetry, the cancellations of quartic and quadratic divergences call for a dark Higgs scalar sector. We estimate the number of hidden(dark) gauged Higgs field sectors to be equal to five. The masses of dark bosons may be of the order of the Standard Model boson masses. Then the following picture of the Standard Model emerges: 6 leptons and 6 quarks are accompanied by 6 boson sectors, only one of the latter is realized in the conventional Standard Model., Comment: 8 pages, 1 figure

Polarization-preserving fibers maintain the two polarization states of an orthogonal basis. Quantum communication, however, requires sending at least two nonorthogonal states and these cannot both be preserved. We present a new scheme that allows for using polarization encoding in a fiber not only in the discrete, but also in the continuous-variable regime. For the example of a helically twisted photonic-crystal fibre, we experimentally demonstrate that using appropriate nonorthogonal modes, the polarization-preserving fiber does not fully scramble these modes over the full Poincar\'e sphere, but that the output polarization will stay on a great circle; that is, within a one-dimensional protected subspace, which can be parametrized by a single variable. This will allow for more efficient measurements of quantum excitations in nonorthogonal modes., Comment: 7 pages, 4 figures, accepted in Phys. Rev. Applied

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.

The work describes a system for converting VLBI observation data using the algorithms of coherent dedispersion and compensation of two-bit signal sampling. Coherent dedispersion is important for processing pulsar observations to obtain the best temporal resolution, while correction for signal sampling makes it possible to get rid of a number of parasitic effects that interfere with the analysis of the diffraction pattern of pulsars. A pipeline has been established that uses the developed converter and the ASC Software Correlator, which will allow reprocessing all archived data of Radioastron pulsar observations and to conduct a search for giant pulses, which requires the best temporal resolution., Comment: 16 pages, 4 figures

Scintillation of pulsar radio signals caused by the interstellar medium can in principle be used for interstellar interferometry. Changes of the dynamic spectra as a function of pulsar longitude were in the past interpreted as having spatially resolved the pulsar magnetosphere. Guided by this prospect we used VLBI observations of PSR B1237+25 with the Arecibo and Green Bank radio telescopes at 324 MHz and analyzed such scintillation at separate longitudes of the pulse profile. We found that the fringe phase characteristics of the visibility function changed quasi-sinusoidally as a function of longitude. Also, the dynamic spectra from each of the telescopes shifted in frequency as a function of longitude. Similar effects were found for PSR B1133+16. However, we show that these effects are not signatures of having resolved the pulsar magnetosphere. Instead the changes can be related to the effect of low-level digitizing of the pulsar signal. After correcting for these effects the frequency shifts largely disappeared. Residual effects may be partly due to feed polarization impurities. Upper limits for the pulse emission altitudes of PSR B1237+25 would likely be well below the pulsar light cylinder radius. In view of our analysis we think that observations with the intent of spatially resolving the pulsar magnetosphere need to be critically evaluated in terms of these constraints on interstellar interferometry., Comment: 19 pages, 11 figures, published in The Astrophysical Journal

Bright squeezed light can be generated in optical fibers utilizing the Kerr effect for ultrashort laser pulses. However, pulse propagation in a fiber is subject to nonconservative effects that deteriorate the squeezing. Here, we analyze two-mode polarization squeezing, which is SU(2)-invariant, robust against technical perturbations, and can be generated in a polarization-maintaining fiber. We perform a rigorous numerical optimization of the process and the pulse parameters using our advanced model of quantum pulse evolution in the fiber that includes various nonconservative effects and real fiber data. Numerical results are consistent with experimental results., Comment: 9 pages, 3 figures

Recently, systems with strong coupling between electronic and vibrational degrees of freedom attract a great attention. In this work, we consider the transient dynamics of the system consisting of strongly coupled vibron and exciton driven by external monochromatic field. We show that under coherent pumping, polarization of exciton exhibits complex quantum dynamics which can be divided into three stages. At the first stage, exciton oscillations at its eigenfrequency relax due to the transition to set of shifted Fock states of vibrons. We demonstrate that these shifted Fock states play the role of an effective reservoir for the excited exciton state. The time of relaxation to this reservoir depends on exciton-vibron coupling. At the second stage, excitation, transferred to the reservoir of the vibronic shifted states at the first stage, returns into electronic degrees of freedom and revival of oscillations at exciton eigenfrequency appears. Thus, the dynamics of molecular polarization exhibit collapses and revivals. At the final stage, these collapses and revivals dissipate and polarization exhibits Rayleigh response at the frequency of the external field. Discovered collapses and revivals manifest in radiation spectrum as multiple splitting of the spectral line near the exciton transition frequency.

Dephasing processes significantly impact the performance of deterministic single-photon sources. Dephasing broadens the spectral line and suppresses the indistinguishability of the emitted photons, which is undesirable for many applications, primarily for quantum computing. We consider a light emitted by a two-level system with a pulsed incoherent pump in the presence of the spectral filter. The spectral filter allows control of the second-order autocorrelation function, indistinguishability, and quantum yield. We show that narrow spectral filters can increase the indistinguishability of the emitted light while undermining the quantum yield. The influence of the spectral filter on the second-order correlation function depends on the duration of the pump. When the pumping pulse is long compared to the lifetime of the two-level system, the narrow spectral filters lead to a rapid increase in the second-order autocorrelation function. In this limit, the statistics of the light from the two-level system inherit the statistics of the incoherent pump. In the case of the short duration of the pump pulse, it is possible to preserve single-photon properties to some degree for the sub-lifetime width of the spectral filter. Moreover, when the light emitted by the single-photon source is used to control a quantum system, e.g., cavity, the single-photon properties of the light manifest themselves differently, depending on the response time of the quantum system. In particular, in the case of long response time, the spectral filter with sub-lifetime width can provide the near-zero second-order autocorrelation function.