Radiophysics and Quantum Electronics最新文献

We give a brief review of the current status of world research in the field of creating quantum sensors of DC and RF electric fields based on highly excited Rydberg atoms with principal quantum number n ≫ 1. Such atoms have large dipole moments, which increase as n² with increasing n. Due to this fact, electric polarizabilities of Rydberg levels increase as n⁷ and exceed the polarizabilities of low-excited atoms by many orders of magnitude. This forms the basis for creating highly sensitive quantum sensors. For their implementation, alkali-metal atoms in compact optical vapor cells are excited to Rydberg states. When microwave radiation with a frequency above 1 GHz should be detected, splitting of a single resonance of electromagnetically induced transparency (EIT), which appears under two-photon laser excitation of Rydberg states, into two resonances due to the Autler-Townes (AT) effect in a microwave radiation field is measured. With Rydberg sensors, it is possible to construct both metrological standards for measuring electric field strength and highly sensitive detectors of RF fields for various applications. Our first experimental results on the observation of EIT resonances under two-photon laser excitation 5S_1/2 → 5P_3/2 → nS_1/2 of ⁸⁵Rb Rydberg states in an optical cell and on the observation of AT splitting in the field of microwave radiation with a frequency of 58.17 GHz, which was in resonance with 41S_1/2 → 41P_3/2 transition between the neighboring Rydberg states, are also presented.

We demonstrate the storage and readout of the polarization state of light with a memory based on the atomic frequency comb protocol in a ¹⁵³Eu:Y₂SiO₅ crystal, both in the bright-pulse regime and at the single photon level. We reconstruct the χ-matrix of the process corresponding to our quantum memory and demonstrate the recall fidelity for the quantum state of about 80% at the maximum efficiency of about 26%. It is also shown that the quantum memory stores the phase difference between the stored pulses with high fidelity, which is essential for working with time-bin qubits.

We review attacks on the technical implementation of continuous-variable quantum key distribution systems, considering such attack types as the Trojan-horse attack, the induced-photorefraction attack, and the laser-damage attack. The attacks on the specific features of continuous-variable quantum key distribution systems are also considered.

We propose a new tomography protocol for reconstruction of the polarization qutrit state that does not require the conversion to a ququart. We show that a set of unitary two-mode polarization transformations is sufficient to perform a full set of measurements. In the experimental realization of the protocol, this transformation is implemented through a quarter-wave plate oriented at different angles. It is shown that this protocol allows one to reconstruct both pure and mixed states.

We describe the results of studying the small-scale processes in the boundary layer of the atmosphere, including their contribution to the momentum exchange under the storm conditions. The main research method is laboratory modeling performed in the High-Speed Wind-Wave Channel of the Large Thermostratified Tank of the Institute of Applied Physics of the Russian Academy of Sciences (Nizhny Novgorod, Russia). A detailed description of this unique facility and the measurement methods is presented. The dependence of the aerodynamic drag coefficient on the parameters of waves and the foam coverage is studied. A theoretical model is proposed for the description of this dependence. The results of generation of the splashes, which are ripped off from breaking waves by a strong wind, are also presented. Direct-shadow visualization methods in combination with high-speed filming made it possible to classify the fragmentation mechanisms, identify the dominant one, and construct the spray generation function. The issues related to the transfer of the obtained results to the natural conditions on the basis of whitecap method are discussed in the conclusion section.

This paper gives a comparative analysis of various techniques of array signal processing in cases where the spatial coherence of the useful signal at the input of the receiving array is greatly weakened and the coherence length is small compared to the array size. The main focus is on realistic estimates of the practical possibilities of achieving high values of the array gain for suboptimal processing, which are close to the maximum possible values for optimal processing in such conditions. For this, a generalized parametric model of the signal coherence function is used, which permits one to analyze various reception scenarios for coherence-degraded signals and obtain key dependences characterizing output performance in terms of the array gain in a wide range of specified parameters. It is shown that the choice of the suboptimal method crucially depends on two physical parameters, namely, the relative signal coherence length (compared to the array size) and the relative level of the coherent component in the signal field (compared to its total intensity). The results are considered to be useful for various applications in long-range sonar and radar, where the coherence degradation of the signal wave fronts becomes a typical environmental effect in large array operation.

The instability mechanism of plastic deformations in crystalline alloys is studied on the basis of the autowave approach. A mathematical model is proposed for the serrated flow behavior and localization of the plastic flow in the form of deformation bands—the Portevin–Le Chatelier effect, which manifests itself in a wide range of positive Celsius temperatures. A stationary solution of the initial system of equations at a constant load is found within the framework of the proposed model; the solution is the wave front of the plastic strain rate and is interpreted as a Lüders band. The numerical analysis of the initial model shows that the irregular variations of deforming stress and the spatial wave solutions take place in the instability region determined by the N-shaped dependence of the dislocation deceleration force on the dislocation velocity. This nonlinearity of the deceleration force is due to the peculiarities of the interaction of the dislocations with impurity atoms. The critical dimensionless parameters responsible for the variety of wave solutions of the equation system are determined. For the specific values of these parameters, the irregular oscillation waveform of deforming stress is found, as well as the shape and periodicity of the bursts in plastic strain rate with the oscillation waveform and bursts being strictly correlated and forming the Portevin–Le Chatelier bands, which can be relatively uniform or randomly distributed along the length of the crystal.

We analyze electron–nucleus collisions in a strongly magnetized ideal plasma with a low temperature at which the Coulomb interaction energy of particles at a distance of the order of the electron Larmor radius exceeds their thermal energy. In this limit, the motion of the electron in close collisions becomes quasibound. Analytical expressions for the spectral power of bremsstrahlung in the continuum at frequencies where quasibound motion makes a decisive contribution to the emission have been obtained. The proposed quantum approach is valid both in the limit of such a low energy of an electron that the latter can occupy only a fundamental Landau level before and after the collision and in the limit of classical particle motion. In the classical regime, the radiation losses approximately preserve the trajectory of the particle: the emission of a photon just changes the phase incursion of the electron wave function by the same amount within the cross section of a beam of rays approximating the state of the particle. A condition is formulated under which spontaneous emission puts the particle into a superposition of states essentially differing from the initial state by the probability for the electron to occupy different Landau levels after the collision. This emission regime takes place in the low-temperature plasma of the photosphere of a white dwarf in the infrared continuum.

We study the effectiveness of spatial signal processing at the apertures of the vertical and horizontal antenna arrays for acoustic waveguides with irregular free boundaries. The main attention is paid to a joint use of statistical models of signal and noise, which is generated near the waveguide boundary by dipole sources, and optimal algorithms for processing of partially coherent signals. The proposed signal and noise models allow for the effects of multiple scattering of acoustic modes. On the basis of the obtained theoretical results, we perform computations of the gain of the antenna arrays of various orientations for an oceanic waveguide with a wavy surface and regular parameters, which are typical of shallow sea. The dependences of the antenna-array gain on the distance and wind velocity are analyzed for various spatial-processing algorithms. The influence of the intermode signal correlations and the statistical model of noise on these results is discussed.

Superconducting circuits are among the most promising platforms for quantum computing. The milestone experiments demonstrating quantum advantage and suppression of quantum errors have already been performed on a simple and reliable transmon qubit. However, a transmon has a number of structural and technological features which limit the fidelity of basic operations required for a high-performance quantum computing device. Therefore, alternative superconducting qubits with a better protection from external noise are of growing interest. A fluxonium, which is characterized by significant anharmonicity and a large coherence time, is one of the most promising qubits. In this work, we describe the first experiments with an elementary unit cell of a quantum processor with planar fluxonium qubits. Methods of individual initialization and dispersive readout of qubits are demonstrated; single-qubit gates with a fidelity exceeding 99.96% and a two-qubit CZ gate with a fidelity of 99.22% are realized.