We study the tunneling of slow quantum packets through a high Coulomb barrier. We show that the transmission coefficient can be quite different from the standard expression obtained in the plane wave ...(WKB) approximation (and larger by many orders of magnitude), even if the momentum dispersion is much smaller than the mean value of the momentum.
•Tunneling of quantum packets through high Coulomb barrier.•Transmission probability is much higher than for plain waves.•Transmission probability strongly depends on the packet shape.•Transmission probability can depend on the momentum variance only.•The concept of effective Planck constant does not work.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
A two-parameter family of quantum states preserving the mean value of the magnetic moment (proportional to the kinetic angular momentum) is found for a charged particle in a constant homogeneous ...magnetic field in the presence of an isotropic two-dimensional parabolic potential, which can be either attractive or repulsive (the case of the Penning trap). The evolution of such states in a specific time-dependent magnetic field of the Epstein–Eckart form is studied, with an emphasis on the limit cases of the sudden jump and adiabatic approximations. The behavior in the case of magnetic field inversion is shown to be qualitatively different from the case when the field does not change its sign. The case of time-dependent vector potentials with a constant magnetic field (arising due to deformations of the shape of a solenoid) is considered, as well.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
We give a short review of known exact inequalities that can be interpreted as 'energy-time' and 'frequency-time' uncertainty relations. In particular we discuss a precise form of signals minimizing ...the physical frequency-time uncertainty product. Also, we calculate the 'stationarity time' for mixed Gaussian states of a quantum harmonic oscillator, showing explicitly that pure quantum states are 'more fragile' than mixed ones with the same value of the energy dispersion. The problems of quantum evolution speed limits, time operators and measurements of energy and time are briefly discussed, too.
We study numerically the evolution of the cavity electromagnetic field mode which is in resonance with an oscillating boundary (dynamical Casimir effect), taking into account the interaction between ...the field and a two-level atom, that may or not be continuously monitored by a coupled atomic excitation detector. We analyze the behavior of the field statistics and the quadrature squeezing properties in different regimes, demonstrating that at the expense of decreasing the number of produced photons and the degree of squeezing, one can create qualitatively new types of cavity field states.
► We study the statistics of photons created in a cavity via dynamical Casimir effect. ► We take into account the interaction with a two-level atom placed inside the cavity. ► The field–atom dynamics is calculated numerically for the Rabi coupling. ► The interaction with a detector can totally change the statistics of created photons. ► The statistics can vary from weakly super-Poissonian to strong “hyper-Poissonian”.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
ABSTRACT
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° < l < 180° ...hemisphere by eROSITA during its second sky survey (2020 June 10 to December 14) and confirmed by optical follow-up observations. The most distant event occurred at z = 0.581. One TDE continued to brighten at least 6 months. The X-ray spectra are consistent with nearly critical accretion on to black holes of a few ×103 to $10^8\, \mathrm{ M}_\odot$, although supercritical accretion is possibly taking place. In two TDEs, a spectral hardening is observed 6 months after the discovery. Four TDEs showed an optical brightening apart from the X-ray outburst. The other nine TDEs demonstrate no optical activity. All 13 TDEs are optically faint, with Lg/LX < 0.3 (Lg and LX being the g band and 0.2–6 keV luminosity, respectively). We have constructed a TDE X-ray luminosity function, which can be fit by a power law with a slope of −0.6 ± 0.2, similar to the trend observed for optically selected TDEs. The total rate is estimated at (1.1 ± 0.5) × 10−5 TDEs per galaxy per year, an order of magnitude lower than inferred from optical studies. This suggests that X-ray bright events constitute a minority of TDEs, consistent with models predicting that X-rays can only be observed from directions close to the axis of a thick accretion disc formed from the stellar debris. Our TDE detection threshold can be lowered by a factor of ∼2, which should allow a detection of ∼700 TDEs by the end of the SRG survey.
•It is studied a circuit QED system with two modes and a qubit with time-modulated frequency.•Qubit and two cavity modes can be simultaneously excited from vacuum for fast harmonic perturbation.•The ...scheme relies on three-photon resonances and precise tuning of the ratio of modes' frequencies.•Only a few photons are generated in each mode, and strong tripartite correlations are formed.
We revisit the dynamical Casimir effect in high-quality multimode cavities, namely, generation of photons from vacuum due to fast external variation of system parameters. In one-dimensional case the spectrum of modes is equidistant, so for harmonic modulation an infinite number of modes become excited. But only one mode is usually excited in cavities with nonequidistant spectrum, apart from some exceptional configurations for which two or three modes are populated. Here is unveiled another scenario in which a small number of photons is generated from vacuum in two specific modes of a cavity with special spectrum profile. The scheme relies on strongly coupling the field to an initially unexcited qubit, whose frequency undergoes a weak harmonic perturbation. When the ratio between two modes' frequencies is accurately tuned in the vicinity of 3, these modes and the qubit can be driven to tripartite entangled states due to narrow three-photon resonances.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Dynamical Casimir effect (DCE) denotes a plethora of phenomena characterized by generation of quanta (usually photons) from vacuum due to modulation of parameters of some neutral system. Its ...counterpart, or Anti-DCE, can be defined as a process in which photons, as well as overall system excitations, are annihilated from the thermal state of the field due to variation of system parameters. In this paper is proposed a new fast scheme for Anti-DCE based on coupling a single-mode cavity to a qubit with time-dependent coupling strength g=εsinηt, where the modulation frequency η is a linear or quadratic function of time and ε lies in the ultrastrong coupling regime. Numeric solutions of time-dependent Rabi Hamiltonian illustrate some convenient forms of η that reduce the average number of system excitations for initial thermal and Poissonian mixed states, and modification of the photon statistics is discussed.
•Novel proposal to reduce the average numbers of photons and overall system excitations.•Quantum Rabi model with time-dependent atom-field coupling in the ultrastrong coupling regime.•Modulation frequency is a linear or quadratic function of time.•Examples illustrate modification of photon statistics for thermal and Poissonian mixed states.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We study the influence of the non-equidistancy of the frequency spectrum on the Dynamical Casimir effect in a rectangular cavity with a harmonically oscillating ideal wall. The transition from the ...linear growth of the mean photon number of photons created from vacuum in the cavity with equidistant spectrum to the exponential growth in a weakly non-equidistant case is shown explicitly.
► We consider the dynamical Casimir effect in different rectangular cavities. ► One-dimensional models predict the linear growth of the number of photons. ► Three-dimensional models predict the exponential growth. ► We show how one model is transformed continuously into the other. ► The key parameter is the degree of non-equidistancy of the frequency spectra.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We have studied the nonstationary circuit QED system in which N qubits or a (N + 1)-level ladder atom interact with a single mode of the cavity field in the presence of external one- and two-photon ...drivings. It is assumed that all the system parameters can be simultaneously time-modulated by arbitrary external modulations. We have derived new analytical expressions for the system dynamics in the dressed-states basis and have shown that one can implement different regimes of light-matter interaction by adjusting the modulation form.
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