Radiophysics and Quantum Electronics最新文献

We consider the one-dimensional problem of propagation of a narrow-band signal in a homogeneous dispersive medium. Within the framework of the method of moments, simple relations are obtained which permit one to use integration of smooth (non-oscillatory) functions to find the midpoint and r.m. s. duration of an arbitrary signal at an arbitrary point on the path without any additional approximations. It is shown that if the absorption dispersion is negligible, the square of the r.m. s. duration of the signal depends on the path length according to the parabolic law, i.e., it has a single minimum (“focus” of the signal). The possibility of reducing the duration (and the corresponding power increase) of linear frequency-modulated (LFM) signals during propagation in a dispersive medium is considered. For a space-limited dispersive medium, using the example of a weakly colliding plasma, estimates are obtained for the maximum possible (for a given path length) reduction of the signal duration and increase in its power, as well as for the initial signal parameters at which these capabilities are realized.

Within the formalism of dynamic effects of directional coupling, or dynamic causal effects (DCEs), the relations between modern quantifiers of coupling between oscillatory systems (transfer entropy, Granger–Geweke causality spectrum, spectral DCEs, etc.) and the classical notions of the oscillation theory—namely, the concepts of “coupling” and “coupledness” by Mandelstam—are studied for an exemplary system of two linear dissipative stochastic oscillators. It is shown that the “coupledness” has two different meanings, which are expressed via different DCEs. A whole set of DCEs appears to be a detailed expression of the coupledness concept and provides a more accurate description of the different dynamic aspects of coupling.

We consider a mathematical description of signals and interference in broadband radio systems with digital linear and planar antenna arrays. The use of the models of spatiotemporal signals is discussed. The similarity of mathematical models of signals and external active noise interference, which have been filtered in the input amplifiers of receiving devices, is studied. Multidimensional Fourier transforms leading to multidimensional spatiotemporal spectra are considered. Their localization at certain frequencies, depending on the angular direction to the radiation source is shown. With allowance for the above said, to suppress active interference, it is proposed to use multidimensional spatiotemporal matched filtering. An example of the matched filtering of an impulse signal with linear frequency modulation at a plane antenna array is proposed.

Based on the approximation of the a posteriori probability density of a multidimensional Markov process by a system of a posteriori moments, an algorithm for nonlinear stochastic filtering of the antenna state vector of a radio-engineering system has been developed. The proposed suboptimal filter makes it possible to estimate the angles of head, roll, and pitch according to the indications of a global satellite navigation system using three spaced antennas. The advantage of using the proposed method compared with the widely used Kalman filter is shown in the case of non-Gaussian presentation of the state vector.

We present an analysis of a dipole antenna fed by a rectangular waveguide. The antenna is based on a recently proposed center-end-fed dipole radiator protected by the corresponding patent of the Russian Federation. The radiation resistance and the complex input impedance of such an antenna are determined by using the mirror image theory and the induced electromotive force method. The principles of choosing the optimal antenna-wire radii and the size of the waveguide wide wall are discussed. The experimental results confirm that the proposed approach is correct.

We demonstrate an implementation of the quantum memory based on the atomic frequency comb protocol in a YLiF₄ crystal doped with Nd ions. Two methods are used for creating the comb: one employs long pulses at different frequencies and the other involves a train of short monochromatic pulses. These methods are compared for two different samples and different numbers of radiation passes through the samples.

We propose a quantum key distribution protocol using axially symmetric polarization beams invariant to rotation of the radial coordinate in a plane that is normal to the beam propagation axis. A description and a method for generating vector beams with an axially symmetric polarization structure, which are formed by a vector superposition of the Hermite–Gaussian modes with indices 10 and 01 are given. It is shown that axial polarization symmetry renders such beams insensitive to rotations relative to the optical axis, which makes it possible to use them for transmitting information in cryptographic protocols in space communication systems. The mechanism of mode formation using radial polarizers and cube corner reflectors is shown in detail. Although a classical communication channel can be organized using radial polarizers, the inclusion of non-unitary elements in a quantum communication channel destroys its cryptographic strength. Therefore, we consider the procedure for detecting phase shifts in the configuration of the Mach–Zehnder interferometer whose elements are cube corner reflectors.

We propose a multiresonator quantum memory consisting of four mini-resonators coupled with a common resonator, which is connected with an external waveguide via a switch. Using the optimization methods, it is possible to effectively transmit a signal from the waveguide to the memory and its long-term multi-cycle storage when the switch is turned off. The advantages of the proposed memory and its potential and implementation range are discussed.

We demonstrate the implementation of a diode laser with high frequency stability at averaging times of 0.1 s to 100 s and low high-frequency phase noise (up to 5 MHz) to control the quantum state of optical qubits based on ¹⁷¹Yb⁺ ions. A high-finesse ULE glass optical cavity is used as an external reference to stabilize the laser frequency and as a spectral filter for high-frequency noise. The calculated suppression of laser phase noise by the cavity at a frequency of 1.2 MHz (where the laser noise density before the filtering reaches its maximum) is 49 dB. Injection locking makes it possible to amplify spectrally pure laser light and use it to control the quantum state of qubits. The spectral purity of the slave laser diode emission is studied by measuring the beat signal of the amplified emission relative to the light transmitted through the cavity. It is shown that the phase noise introduced by injection locking does not exceed -70 dBc in a bandwidth of up to 5 MHz, while an output power of up to 30 mW was achieved.

We optimize the geometric parameters of the single-photon source operated in the red spectral range, which is based on a photonic nanoantenna with InAs/AlGaAs quantum dots. This made it possible to determine the range of the transverse dimensions of the cylindrical nanoantenna, within which the maximum efficiency of radiation extraction into the numerical aperture NA = 0.42 is achieved. An increase in the extraction efficiency as compared to a planar structure was demonstrated for the optimized nanostructure. The statistics of single photon correlations under optical pumping with participation of acoustic phonons is studied. The value of the zero-delay second-order correlation function g⁽²⁾(0) was 0.065.