arXiv subjects

A. A. Pukhov

Publications and source records attributed to A. A. Pukhov.

At least 19 recordsLinked to original sources

arXiv2026

Thermodynamic coprocessor for linear operations with input-size-independent calculation time based on open quantum system

Linear operations, e.g., vector-matrix and vector-vector multiplications, are core operations of modern neural networks. To diminish computational time, these operations are implemented by parallel computations using different coprocessors. In this work we show that an open quantum system consisting of bosonic modes and interacting with bosonic reservoirs can be used as an analog thermodynamic coprocessor implementing multiple vector-matrix multiplications with stochastic matrices in parallel. Input vectors are encoded in occupancies of reservoirs, and the output result is presented by stationary energy flows. The operation takes time needed for the system's transition to a non-equilibrium stationary state independently on the number of the reservoirs, i.e., on the input vector dimension. With technological limitations being considered, a device of $5\times5$ cm$^2$ area covered with the coprocessors can conduct of the order of $10^{11}$ operations per second per a mode of the OQS. The computations are accompanied by an entropy growth. We construct a direct mapping between open quantum systems and electrical crossbar structures frequently used in analog vector-matrix multiplication, showing that dissipation rates multiplied by open quantum system's modes frequencies can be seen as conductivities, reservoirs' occupancies can be seen as potentials, and stationary energy flows can be seen as electric currents.

quant-ph↗

arXiv2026

Measurement-based acceleration of optical computations

Analog coprocessors are intensively developing nowadays with the aim to optimize energy computations of neural networks. In this work we focus on the possibility of using detection of collective oscillations in optical systems for computational purposes. We show that in a system of coupled resonators, collective oscillations can be used to implement matrix-vector multiplication. The matrix is formed by the coupling constants between the resonators, and the input vector is formed by the initial occupancies of the involved modes. The frequency of the collective oscillations is growing with the number of the involved modes, similarly to Rabi oscillations. The time needed for their detection, i.e., averaging, decreases with an increase in the input vector dimension. We discuss the limitations imposed on parallel computation in the system by restriction of the allowed optical frequency band.

physics.optics↗

arXiv2025

Optical coprocessor based on spontaneous Brillouin scattering

Analog coprocessors for neural networks are an intensively developing field. They provide approximate results of computations for relatively low energy cost and at high speed. We show that a set of ring resonators with Brillouin interaction between photons and phonons, being coupled to a waveguide, can be used to implement matrix-vector multiplication. The input vector is formed by occupancies of the anti-Stokes optical modes pumped via spontaneous Brillouin scattering, i.e, scattering on thermal phonons. Brillouin scattering rates and coupling constants between ring resonators and the waveguide form the matrix. The system allows for parallel computations in frequency band.

physics.optics↗

arXiv2024

Second-law-allowed temporal cooling of the coldest reservoir without external refrigeration

Non-equilibrium quantum thermodynamics is an intensively developing field with many existing applications. We study the dynamics of temperatures and chemical potentials of fermionic reservoirs coupled to an open quantum system. We show that heat transfer from the coldest reservoir to the hottest one is allowed by the Clausius inequality and results in transient cooling of the coldest reservoir without additional external refrigeration. We show that during the establishment of thermal and chemical equilibrium, non-monotone evolution of reservoirs' temperatures and chemical potentials is possible, including changes in reservoirs' temperatures and chemical potentials orderliness. Achieved results can be used in the design of quantum thermal machines and nanoelectronic devices.

quant-ph↗

arXiv2023

Transient Temperature Dynamics of Reservoirs Connected Through an Open Quantum System

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.

quant-ph↗

arXiv2022

Energy Transport Induced by Transition from Weak to Strong Coupling Regime Between Non-Hermitian systems

Recently, strong coupling between non-Hermitian physical systems of different nature is widely investigated due to it endows them with new properties. In this work, we investigate the energy transport between strongly coupled systems. We use a partial-secular approach for the description of an open quantum system to investigate the system dynamics during the transition from a weak to a strong coupling regime with an increase of coupling between subsystems. On the example of strongly coupled two-level atoms, we show that near the transition point enhancement of energy transport between the system and reservoirs takes place. This manifests in the fact that energy flow normalized to the coupling constant reaches the maximum both in the cases of zero and non-zero frequency detuning. We show that maximization of normalized energy flow can be used for the determination of the transition to the strong coupling regime in the case of non-zero detuning when there is no clear transition point from the weak to strong coupling regime. The suppression of the energy flow at high relaxation is demonstrated.

quant-ph↗

arXiv2022

Operational regimes of lasers based on gain media with a large Raman scattering cross-section

We report on unusual regimes of operation of a laser with a gain medium with a large Raman scattering cross-section, which is often inherent in new types of gain media such as colloidal and epitaxial quantum dots and perovskite materials. These media are characterized by a strong electron-phonon coupling. Using the Fröhlich Hamiltonian to describe the electron-phonon coupling in such media, we analyze the operation of the system above the lasing threshold. We show that below a critical value of the Fröhlich constant, the laser can only operate in the conventional regime: namely, there are coherent cavity photons but no coherent phonons. Above the critical value, a new pump rate threshold appears. Above this threshold, either joint self-oscillations of coherent phonons in the gain medium and photons in a cavity or a chaotic regime are established. We also find a range of the values of the Fröhlich constant, the pump rate, and the resonator eigenfrequency, in which more than one dynamical regime of the system is stable. In this case the laser dynamics is determined by the initial values of the resonator field, the active medium polarization, the population inversion, and phonon amplitude.

physics.optics↗

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.

quant-ph↗

arXiv2020

Quantum theory of Rayleigh scattering

We develop a quantum theory of atomic Rayleigh scattering. Scattering is considered as a relaxation of incident photons from a selected mode of free space to the reservoir of the other free space modes. Additional excitations of the reservoir states which appear are treated as scattered light. We show that an entangled state of the excited atom and the incident photon is formed during the scattering. Due to entanglement, a photon is never completely absorbed by the atom. We show that even if the selected mode frequency is incommensurable with any atomic transition frequency, the scattered light spectrum has a maximum at the frequency of the selected mode. The linewidth of scattered light is much smaller than that of the spontaneous emission of a single atom, therefore, the process can be considered as elastic. The developed theory does not use the phenomenological concept of virtual level.

quant-ph↗

arXiv2020

Cascade Brillouin scattering as a mechanism for photoluminescence from rough surfaces of noble metals

In surface-enhanced Raman scattering experiments that use plasmonic nanostructures as substrates, the scattering spectrum contains a broad background usually associated with photoluminescence. This background exists above and below the frequency of the incident wave. The low-frequency part of this background is similar to the scattering spectrum of a plasmon nanoparticle, while the high-frequency part follows the Gibbs distribution. We develop a theory that explains experimentally observed features in both the high- and low-frequency parts of the photoluminescence spectrum from a unified point of view. We show that photoluminescence is attributed to the cascade Brillouin scattering of the incident wave by metal phonons under the plasmon resonance conditions. The theory is in good agreement with our measurements over the entire frequency range of the background.

cond-mat.mes-hall↗

arXiv2020

Exceptional points as lasing pre-thresholds in open-cavity lasers

The genesis of lasing, as an evolution of the laser hybrid eigenstates comprised of electromagnetic modes and atomic polarization, is considered. It is shown that the start of coherent generation at the laser threshold is preceded by the formation of a special hybrid state at the lasing pre-threshold. This special state is characterized by an enhanced coupling among excited atoms and electromagnetic modes. This leads to an increase in the rate of stimulated emission in the special state and, ultimately, to lasing. At the lasing pre-threshold, the transformation of hybrid eigenstates has the features of an exceptional point (EP) observed in non-Hermitian systems. The special state is formed when eigenfrequencies of two hybrid states coalesce or come close to each other. Below the pre-threshold, lifetimes of all hybrid states grow with increasing pump rate. When the pump rate crosses the pre-threshold, resonance trapping occurs with the lifetime of the special state continuing to increase while the lifetimes of all other eigenstates begin to decrease. Consequently, the latter eigenstates do not participate in the lasing. Thus, above the pre-threshold, a laser transitions into the single-mode regime.

physics.optics↗

arXiv2019

Self-formation of coherent emission in a cavity-free system

It is commonly accepted that a collection of pumped atoms without a resonator, which provides feedback, cannot lase. We show that intermodal coupling via active atoms pulls the frequencies of the free-space modes towards the transition frequency of the atoms. Although at a low pump rate mode phases randomly fluctuate, phase realizations at which interference of pulled modes is constructive emerge. This results in an increase of stimulated emission into such realizations and makes their lifetime longer. Thus, mode pulling provides positive feedback. When the pump rate exceeds a certain threshold, the lifetime of one of the realizations diverges, and radiation becomes coherent.

physics.optics↗

arXiv2019

Enhancement of the Raman Effect by Infrared Pumping

We propose a method for increasing the Raman scattering from an ensemble of molecules by up to four orders of magnitude. Our method requires an additional coherent source of IR radiation with the half-frequency of the Stokes shift. This radiation excites the molecule electronic subsystem that in turn, via Fröhlich coupling, parametrically excites nuclear oscillations at a resonant frequency. This motion is coherent and leads to a boost of the Raman signal in comparison to the spontaneous signal because its intensity is proportional to the squared number of molecules in the illuminated volume.

quant-ph↗

arXiv2018

Photon statistics of amplified spontaneous emission

Developments in quantum technologies lead to new applications that require radiation sources with specific photon statistics. A widely used Poissonian statistics are easily produced by lasers; however, some applications require super- or sub-Poissonian statistics. Statistical properties of a light source are characterized by the second-order coherence function g^(2)(0). This function distinguishes stimulated radiation of lasers with g^(2)(0)=1 from light of other sources. For example, g^(2)(0)=2 for black-body radiation, and g^(2)(0)=0 for single-photon emission. One of the applications requiring super-Poissonian statistics (g^(2)(0)>1) is ghost imaging with thermal light. Ghost imaging also requires light with a narrow linewidth and high intensity. Currently, rather expensive and inefficient light sources are used for this purpose. In the last year, a superluminescent diode based on amplified spontaneous emission (ASE) has been considered as a new light source for ghost imaging. Even though ASE has been widely studied, its photon statistics has not been settled - there are neither reliable theoretical estimates of the second-order coherence function nor unambiguous experimental data. Our computer simulation clearly establishes that coherence properties of light produced by ASE are similar to that of a thermal source with g^(2)(0)=2 independent of pump power. This result manifests the fundamental difference between ASE and laser radiation.

physics.optics↗

arXiv2018

Exciting surface plasmon polaritons in the Kretschmann configuration by light beam

We consider exciting surface plasmon polaritons in the Kretschmann configuration. Contrary to common belief, we show that a plane wave incident at an angle greater than the angle of total internal reflection does not excite surface plasmon polaritons. These excitations do arise, however, if the incident light forms a narrow beam composed of an infinite number of plane waves. The surface plasmon polariton is formed at the geometrical edge of the beam as a result of interference of reflected plane waves.

cond-mat.other↗

arXiv2018

Zeroth law of thermodynamics for thermalized open quantum systems having integrals of motion

We study the evolution of an open quantum system described by a dynamical semigroup having the Lindblad superoperator as a generator. This generator may have an eigenfunction with a unity eigenvalue, referred to as a constant of motion (COM). An open quantum system has a unique stationary state if and only if it has no COMs. A system with multiple stationary states has a basis of COMs; any COM of the system is a linear combination of the basis COMs. The basis divides the space of system states into subspaces. Each subspace has its own stationary state, and any stationary state of the system is a linear combination of these states. Usually, neither the basis of COMs nor even the number of COMs is known. We demonstrate that finding the stationary state of the system does not require looking for the COMs. Instead, one can construct a set of invariant subspaces. If the system evolution begins from one of these subspaces, the system will remain in it, arriving at a stationary state independent of evolution in other subspaces. We suggest a direct way of finding the invariant subspaces by studying the evolution of the system. We show that the sets of invariant subspaces and subspaces generated by the basis of COMs are equivalent. A stationary state of the system is a weighted sum of stationary states in each invariant subspace; the weighted factors are determined by the initial state of the system.

quant-ph↗

arXiv2017

Mode cooperation in two-dimensional plasmonic distributed-feedback laser

Plasmonic distributed-feedback lasers based on a two-dimensional periodic array of metallic nanostructures are the main candidate for nanoscale sources of coherent electromagnetic field. Strong localization of the electromagnetic field and the large radiation surface are good opportunities for achieving an ultrashort response time to the external actions and creating beam directionality. At the same time, the investigation of such a system is a challenging problem. In this paper, we present an exhaustive study of the operation of a two-dimensional plasmonic distributed-feedback laser. We show that the complex structure of the modes of a periodic plasmonic array and the nonlinear interaction between the modes through the active medium lead to a new effect, namely, mode cooperation. Mode cooperation is manifested as the generation of the modes in an allowed band with a high threshold instead of modes localized near the band gap with a low threshold. Suppression of lasing of the modes at the edge of the band gap results in widening of the radiation pattern above the generation threshold. This paves the way for effective control and manipulation of the radiation pattern of nanoscale systems, which is of great importance for applications in spectroscopy and optoelectronics.

physics.optics↗

arXiv2017

Inversion of a two-level atom by quantum superoscillations

We show that a two-level atom with a high transition frequency ω_SO can be inverted via non-radiative interaction with a cluster of excited low-frequency two-level atoms or quantum oscillators whose transition frequencies are smaller than ω_SO. This phenomenon occurs due to the Förster resonant energy transfer arising during a train of quantum superoscillation of low-frequency two-level atoms. The suggested model could explain the mechanism of biophoton emission.

physics.optics↗