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We present an expansion of a many-body correlation function in a sum of pseudomodes - exponents with complex frequencies that encompass both decay and oscillations. We demonstrate that, typically, it is enough to take a few first terms of this infinite sum to obtain an excellent approximation to the correlation function at any time, with the large time behavior being determined solely by the first pseudomode. The pseudomode expansion emerges in the framework of the Heisenberg version of the recursion method. This method essentially solves Heisenberg equations in a Lanczos tridiagonal basis constructed in the Krylov space of a given observable. To obtain pseudomodes, we first add artificial dissipation satisfying the dissipative generalization of the universal operator growth hypothesis, and then take the limit of the vanishing dissipation strength. Fast convergence of the pseudomode expansion is facilitated by the localization in the Krylov space, which is generic in the presence of dissipation and can survive the limit of the vanishing dissipation strength. As an illustration, we apply the pseudomode expansion to calculate infinite-temperature autocorrelation functions in the quantum Ising and $XX$ spin-$1/2$ models on the square lattice.

We present an updated catalog of sources detected by the Mikhail Pavlinsky ART-XC telescope aboard the Spektrum-Roentgen-Gamma (SRG) observatory during its all-sky survey. It is based on the data of the first four and the partially completed fifth scans of the sky (ARTSS1-5). The catalog comprises 1545 sources detected in the 4-12 keV energy band. The achieved sensitivity ranges between $\sim 4\times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$ near the ecliptic plane and $\sim 7\times 10^{-13}$ erg s$^{-1}$ cm$^{-2}$ near the ecliptic poles, which is a $\sim$30-50% improvement over the previous version of the catalog based on the first two all-sky scans (ARTSS12). There are $\sim 130$ objects, excluding the expected contribution of spurious detections, that were not known as X-ray sources before the SRG/ART-XC all-sky survey. We provide information, partly based on our ongoing follow-up optical spectroscopy program, on the identification and classification of the majority of the ARTSS1-5 sources (1463), of which 173 are tentative at the moment. The majority of the classified objects (964) are extragalactic, a small fraction (30) are located in the Local Group of galaxies, and 469 are Galactic. The dominant classes of objects in the catalog are active galactic nuclei (911) and cataclysmic variables (192)., Comment: Accepted for publication in A&A, 67 pages, 11 figures, the catalog of sources is included

This article presents a study of the passage of dimuonium through the foil of ordinary matter. First, we provide an overview of how dimuonium is planned to be produced for such a type of experiment and how it is expected to interact with the ordinary atoms -- predominantly electromagnetically via the screened coulomb potential of the atomic nuclei. Then, we describe the transport equations that represent the evolution of dimuonium states during the passage and their solution methods. Finally, for three different foils (Beryllium, Aluminium and Lead), we present the results of this study. To estimate impact of uncertainties in the potential of a target atom, we study 15 different approximations of the atomic potential and show that the corresponding atomic-potential-model-dependent error in the yields of the low lying states of dimuonium is quite small within the framework of the applied Born approximation. The convergence of the results after truncation of the infinite system of transport equations to the finite number of quantum states of dimuonium is also studied, and good convergence for the yields of low-lying states is demonstrated., Comment: 22 pages, 7 figures, 4 tables, published version

We present the results of the optical identification and spectroscopic redshift measurements of 216 galaxy clusters detected in the SRG/eROSITA all-sky X-ray survey. The spectroscopic observations were performed in 2020-2023 with the 6-m BTA telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences, the 2.5-m telescope at the Caucasus Mountain Observatory of the Sternberg Astronomical Institute of the Moscow State University, the 1.6-m AZT-33IK telescope at the Sayan Solar Observatory of the Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences, and the 1.5-m Russian-Turkish telescope (RTT-150) at the T\"{U}B\.{I}TAK Observatory. For all of the galaxy clusters presented here the spectroscopic redshift measurements have been obtained for the first time. Of these, 139 galaxy clusters have been detected for the first time in the SRG/eROSITA survey and 22 galaxy clusters are at redshifts $z_{spec} \gtrsim 0.7$, including three at $z_{spec} \gtrsim 1$. Deep direct images with the rizJK filters have also been obtained for four distant galaxy clusters at $z_{spec} > 0.7$. For these observations the most massive clusters are selected. Therefore, most of the galaxy clusters presented here most likely will be included in the cosmological samples of galaxy clusters from the SRG/eROSITA survey.

In the fall of 2019, during the in-flight calibration phase of the SRG observatory, the onboard eROSITA and Mikhail Pavlinsky ART-XC telescopes carried out a series of observations of PG 1634+706 - one of the most luminous (an X-ray luminosity $\sim 10^{46}$ erg/s) quasars in the Universe at $z<2$. Approximately at the same dates this quasar was also observed by the XMM-Newton observatory. Although the object had already been repeatedly studied in X-rays previously, its new observations allowed its energy spectrum to be measured more accurately in the wide range $1-30$ keV (in the quasar rest frame). Its spectrum can be described by a two-component model that consists of a power-law continuum with a slope $\Gamma\approx 1.9$ and a broadened iron emission line at an energy of about 6.4 keV. The X-ray variability of the quasar was also investigated. On time scales of the order of several hours (here and below, in the source rest frame) the X-ray luminosity does not exhibit a statistically significant variability. However, it changed noticeably from observation to observation in the fall of 2019, having increased approximately by a factor of 1.5 in 25 days. A comparison of the new SRG and XMM-Newton measurements with the previous measurements of other X-ray observatories has shown that in the entire 17-year history of observations of the quasar PG 1634+706 its X-ray luminosity has varied by no more than a factor of 2.5, while the variations on time scales of several weeks and several years are comparable in amplitude., Comment: 16 pages, 9 figures

We report an implementation of the recursion method that addresses quantum many-body dynamics in the nonperturbative regime. The method essentially amounts to constructing a Lanczos basis in the space of operators and solving coupled Heisenberg equations in this basis. The reported implementation has two key ingredients: a computer-algebraic routine for symbolic calculation of nested commutators and a procedure to extrapolate the sequence of Lanczos coefficients according to the universal operator growth hypothesis. We apply the method to calculate infinite-temperature correlation functions for spin-$1/2$ systems on one- and two-dimensional lattices. In two dimensions the accessible timescale is large enough to essentially embrace the relaxation to equilibrium. The method allows one to accurately calculate transport coefficients. As an illustration, we compute the diffusion constant for the transverse-field Ising model on a square lattice.

We find that the spontaneous and collective emissions have a strong influence on the excitation of two-level absorbers (atoms, molecules) interacting in resonance with the plasmonic mode near the metal nanoparticle. The spontaneous and collective emissions limit the absorption enhancement by the plasmonic mode and make the enhancement possible only with a fast, picosecond population relaxation of the upper absorbing states. Conditions for the maximum of plasmon-enhanced absorption in the presence of spontaneous and collective emissions are found. The nonlinearity in the nanoparticle-absorber interaction and in collective emission causes the bistability in the plasmon-enhanced absorption at high external field intensities and the plasmonic mode excitation., Comment: 11 pages, 7 figures

We found that fluctuations in the number of emitters lead to a super-thermal photon statistics of small LEDs in a linear regime, with a strong emitter-field coupling and a bad cavity favorable for collective effects. A simple analytical expression for the second-order correlation function g_2 is found. g_2 increase up to g_2=6 in the two-level LED model is predicted. The super-thermal photon statistics is related to the population fluctuation increase of the spontaneous emission to the cavity mode., Comment: 22 pages, 5 figures

In this paper we are focusing on entanglement control problem in a three-qubit system. We demonstrate that vector representation of entanglement, associated with SO(6) representation of SU(4) two-qubit group, can be used to solve various control problems analytically including (i) the transformation between a W-type states and GHZ state, and (ii) manipulating bipartite concurrences and three-tangle under a restricted access to only two qubits, and (iii) designing USp(4)-type quaternionic operations and quantum states.

We consider in sufficient detail how the Berry phase arises in a rotating electric field in a model system with spin one. The goal is to help the student who first encountered this interesting problem, which is fraught with some subtleties that require attention in order not to go astray., Comment: 11 pages, no figures, published version

We present the results of our identification of 14 X-ray sources detected in the eastern Galactic sky ($010^{22}$ cm$^{-2}$ at a 90% confidence level, with one of them being probably heavily obscured ($N_{\rm H}>5\times 10^{22}$ cm$^{-2}$ with 90% confidence). This paper continues our series of publications on the identification of hard X-ray sources detected during the all-sky survey with the SRG orbital X-ray observatory., Comment: Published in Astronomy Letters, 2023, Vol. 49, No. 2. arXiv admin note: text overlap with arXiv:2208.10963

We investigate the bistability in a small Fabry-Perot interferometer (FPI) with the optical wavelength size cavity, the nonlinear Kerr medium and only a few photons, on average, excited by the external quantum field. Analytical expressions for the stationary mean photon number, the bistability domain, the field and the photon number fluctuation spectra are obtained. Multiple stationary states of the FPI cavity field with different spectra are possible at realistic conditions, for example, in the FPI with the photonic crystal cavity and the semiconductor-doped glass nonlinear medium., Comment: 20 pages, 6 figures

We consider the small, of the size of the order of the wavelength, interferometer with the main mode excited by a quantum field from a nano-LED or a laser. The input field is detuned from the interferometer mode with, on average, a few photons. We find the field and the photon number fluctuation spectra inside and outside the interferometer and identify the contributions of quantum and classical noise in the spectra. Structures of spectra are different for the field, the photon number fluctuations inside the interferometer; for the transmitted, and the reflected fields. We note asymmetries in spectra. Differences in the spectra are related to the colored (white) quantum noise inside (outside) the interferometer. We calculate the second-order time correlation functions; they oscillate and be negative under certain conditions. Results help the study, design, manufacture, and use small elements of quantum optical integrated circuits, such as delay lines and optical transistors., Comment: 16 pages, 9 figures

We present the results of our identification of 17 X-ray sources detected in the 4-12 keV energy range by the Mikhail Pavlinsky ART-XC telescope during the first year of the SRG all-sky survey. Three of them have been discovered by the ART-XC telescopes, while the remaining ones have already been known previously as X-ray sources, but their nature has remained unknown. We took optical spectra for nine sources located in the northern sky $\delta > -20$ deg with the 1.6-m AZT-33IK telescope at the Sayan Observatory (the Institute of Solar-Terrestrial Physics, the Siberian Branch of the Russian Academy of Sciences) and the 1.5-m Russian-Turkish telescope at the TUBITAK National Observatory. For the remaining objects we have analyzed the archival optical spectra taken during the 6dF survey. All of the investigated objects have turned out to be Seyfert galaxies (eight of type 1, seven of type 2, and two of intermediate type 1.8) at redshifts up to $z\approx 0.15$. Based on data from the eROSITA and ART-XC telescopes onboard the SRG observatory, we have obtained X-ray spectra in the energy range 0.2-20 keV for eight sources. A significant intrinsic absorption ($N_H > 10^{22}$ cm$^{-2}$) has been detected in three of them, with two of them being probably strongly absorbed ($N_H \sim 10^{23}$ cm$^{-2}$). This paper is a continuation of the series of publications on the optical identification of active galactic nuclei detected by the ART-XC telescope., Comment: Published in Astronomy Letters, 2022, Vol. 48, No. 2

A laser model is formulated in terms of quantum harmonic oscillators. Emitters in the low lasing states are usual harmonic oscillators, and emitters in the upper states are inverted harmonic oscillators. Diffusion coefficients, consistent with the model and necessary for solving quantum nonlinear laser equations analytically, are found. Photon number fluctuations of the lasing mode and fluctuations of the population of the lasing states are calculated. Collective Rabi splitting peaks are predicted in the intensity fluctuation spectra of the superradiant lasers. Population fluctuation mechanisms in superradiant lasers and lasers without superradiance are discussed and compared with each other., Comment: Preprint: 18 pages, 6 figures, 1 table

The hot electron generation in plasmonic nanoparticles is the key to efficient plasmonic photocatalysis. In the paper, we study theoretically for the first time the effect of Tamm states (TSs) at the interface metal-semiconductor on hot electron generation and Landau damping (LD) in metal nanoparticles. TSs can lead to resonant hot electron generation and to the LD rate enhanced by several times. The resonant hot electron generation is reinforced by the transition absorption due to the jump of the permittivity at the metal-semiconductor interface., Comment: 8 pages, 5 figures

In this article, we present MuMuPy, a computational library and cloud-based tool for calculating cross sections for the interaction of dimuonium (true muonium) with matter. MuMuPy calculates corresponding form factors and allows one to find the probabilities of dimuonium transitions in the electric field of the nucleus. MuMuPy was developed in the context of the $\mu\mu$-tron facility, the project of a low-energy electron-positron collider for production and experimental study of dimuonium, proposed in our home institute, Budker Institute of Nuclear Physics. The reliability of MuMuPy was verified by three independent methods, one of which was developed by the authors earlier., Comment: Accepted in Computer Physics Communications Added one new reference in this version

Nonlinear Heisenberg-Langevin equations are solved analytically by operator Fourier-expansion for the laser in the LED regime. Fluctuations of populations of lasing levels are taken into account as perturbations. Spectra of operator products are calculated as convolutions, preserving Bose commutations for the lasing field operators. It is found that fluctuations of population significantly affect spontaneous and stimulated emissions into the lasing mode, increase the radiation rate, the number of lasing photons and broad the spectrum of a bad cavity thresholdless and the superradiant lasers. The method can be applied to various resonant systems in quantum optics., Comment: 13 pages and 5 figures

Landau Damping (LD) mechanism of the Localized Surface Plasmon (LSP) decay is studied for the hybrid nanoplasmonic (metal core/dielectric shell) structures. It is shown that LD in hybrid structures is strongly affected by permittivity and electron effective mass in the dielectric shell in accordance with previous observations by Kreibig, and the strength of LD can be enhanced by an order of magnitude for some combinations of permittivity and effective mass. The physical reason for this effect is identified as electron spillover into the dielectric where electric field is higher than in the metal and the presence of quasi-discrete energy levels in the dielectric. The theory indicates that the transition absorption at the interface metal-dielectric is a dominant contribution to LD in such hybrid structures. Thus, by judicious selection of dielectric material and its thickness one can engineer decay rates and hot carrier production for important applications, such as photodetection and photochemistry.

We present the first sample of tidal disruption events (TDEs) discovered during the SRG all-sky survey. These 13 events were selected among X-ray transients detected in the 0

We report the discovery of three previously unknown cataclysmic variables in the data of the first year of the all-sky X-ray survey by the SRG orbital observatory. The sources were selected due to their brightness in the 4--12 keV band in the data of the Mikhail Pavlinsky ART-XC telescope. They are also detected by the eROSITA telescope, which provided accurate localizations and spectral data for broad-band spectral analysis. All three objects had been previously known as X-ray sources from the ROSAT all-sky survey and XMM-Newton slew survey, but their nature remained unknown. The X-ray spectra obtained by eROSITA and ART-XC are consistent with optically thin thermal emission with a temperature kT>~15 keV for SRGAJ194638.9+704552 and SRGAJ225412.8+690658 and kT>~5 keV for SRGAJ204547.8+672642. This, together with the inferred high X-ray luminosities ($2\times 10^{32}$-$3\times 10^{33}$ erg s$^{-1}$), strongly suggests that all three sources are CVs. We have obtained optical photometry and spectroscopy for these objects using the AZT-33IK 1.6-m telescope of the Sayan Observatory. The optical properties confirm the CV nature of the objects. We conclude that SRGAJ194638.9+704552 is an intermediate polar, SRGAJ204547.8+672642 is most likely a polar or an intermediate polar, and SRGAJ225412.8+690658 can be either a magnetic or a non-magnetic CV. We also measured an orbital period of 2.98~hours for SRGAJ204547.8+672642, based on TESS data. Three out of the planned eight SRG all-sky surveys have now been completed. We expect to find plenty of new CVs during the survey and to continue our optical follow-up program., Comment: Accepted for publication in A&A Special Issue: The Early Data Release of eROSITA and Mikhail Pavlinsky ART-XC on the SRG Mission. 9 pages, 5 figures

We present the results of our identification of eight objects from a preliminary catalogue of X-ray sources detected in the 4-12 keV energy band by the Mikhail Pavlinsky ART-XC telescope aboard the SRG observatory during its first all-sky survey. Three of them (SRGAJ005751.0+210846,SRGAJ014157.0-032915, SRGAJ232446.8+440756) have been discovered by ART-XC, while five were already known previously as X-ray sources, but their nature remained unknown. The last five sources have also been detected in soft X-rays by the eROSITA telescope of the SRG observatory. Our optical observations were carried out at the 1.6-m AZT-33IK telescope of the Sayan Observatory and the 1.5-m Russian-Turkish telescope (RTT-150). All of the investigated objects have turned out to be active galactic nuclei (AGNs) at redshifts from 0.019 to 0.283. Six of them are Seyfert 2 galaxies(including one Seyfert 1.9 galaxy), one (SRGAJ005751.0+210846) is a "hidden" AGN (in an edge-on galaxy), and one (SRGAJ224125.9+760343) is a narrow-line Seyfert 1 galaxy. The latter object is characterized by a high X-ray luminosity (~(2-13)*10^44 erg/s in the 4-12 keV band) and,according to our black hole mass estimate (~2*10^7 M_sun), an accretion rate close to the Eddington limit. All three AGNs discovered by the ART-XC telescope (which are not detected by the eROSITA telescope) are characterized by a high absorption column density (N_H > 3*10^23 cm^-2). The results obtained confirm the expectations that the ART-XC telescope is an efficient instrument for searches of heavily obscured and other interesting AGNs in the nearby (z<0.3) Universe. The SRG all-sky survey will last for more than 3 years more, which will allow many such objects to be discovered.

A new approach for analytically solving quantum nonlinear Langevin equations is proposed and applied to calculations of spectra of superradiant lasers where collective effects play an important role. We calculate lasing spectra for arbitrary pump rates and recover well-known results such as the pump dependence of the laser linewidth across the threshold region. We predict new sideband peaks in the spectrum of superradiant lasers with large relaxation oscillations as well as new nonlinear structures in the lasing spectra for weak pump rates. Our approach sheds new light on the importance of population fluctuations in the narrowing of the laser linewidth, in the structure of the lasing spectrum, and in the transition to coherent operation., Comment: 19 pages, 10 figures

Quantum efficiencies of surface (SPE) and volume (VPE) photo-emissions from metal nanoparticles are calculated by quantum mechanical perturbation theory and compared with each other. Along with discontinuities in the potential barrier and dielectric function, the discontinuity in electron effective mass on the metal-environment interface is taken into account. General formulas for quantum efficiencies of SPE and VPE are derived. An example of spherical gold particles with rectangular potential barrier on the interface is considered, analytical formulas for quantum efficiencies of SPE and VPE on the red border of photoemission are derived. It is found that the efficiency of SPE is less decreased with the reduction of the electron effective mass than the efficiency of VPE, so SPE is more efficient that VPE for small particles and large discontinuity in effective mass. Nanoparticle size, when SPE is more efficient than VPE, is found to be tens of nm or less., Comment: preprint

We investigate the influence of a static electric field on the incoherent nonlinear response of an unloaded electrically-contacted nanoscale optical gap antenna. Upon excitation by a tightly focused near-infrared femtosecond laser beam, a transient elevated temperature of the electronic distribution results in a broadband emission of nonlinear photoluminescence (N-PL). We demonstrate a modulation of the yield at which driving photons are frequency up-converted by means of an external control of the electronic surface charge density. We show that the electron temperature and consequently the N-PL intensity can be enhanced or reduced depending on the command polarity and the strength of the control static field. A modulation depth larger than 100\% is observed for activation voltages of a few volts., Comment: 12 pages, 10 figures

We suggest a dynamical vector model of entanglement in a three qubit system based on isomorphism between $su(4)$ and $so(6)$ Lie algebras. Generalizing Pl\"ucker-type description of three-qubit local invariants we introduce three pairs of real-valued $3D$ vector (denoted here as $A_{R,I}$ , $B_{R,I}$ and $C_{R,I}$). Magnitudes of these vectors determine two- and three-qubit entanglement parameters of the system. We show that evolution of vectors $A$, $B$ , $C$ under local $SU(2)$ operations is identical to $SO(3)$ evolution of single-qubit Bloch vectors of qubits $a$, $b$ and $c$ correspondingly. At the same time, general two-qubit $su(4)$ Hamiltonians incorporating $a-b$, $a-c$ and $b-c$ two-qubit coupling terms generate $SO(6)$ coupling between vectors $A$ and $B$, $A$ and $C$, and $B$ and $C$, correspondingly. It turns out that dynamics of entanglement induced by different two-qubit coupling terms is entirely determined by mutual orientation of vectors $A$, $B$, $C$ which can be controlled by single-qubit transformations. We illustrate the power of this vector description of entanglement by solving quantum control problems involving transformations between $W$, Greenberg-Horne-Zeilinger ($GHZ$ ) and biseparable states., Comment: 26 pages, no figures; Accepted Phys. Rev. A 29Jul2020