Quantum Electronics

A brief general review is presented of the theory of information transmission capacities of quantum communication channels, which is a development of the classical Shannon theory. Unlike a classical communication channel, a quantum channel is characterised by a whole set of different capacities, which depend on the type of transmitted information (classical or quantum) and on additional resources used during transmission. The main characteristics of a quantum channel are considered: classical capacity, capacity assisted by entanglement between the channel input and output, quantum capacity and secret classical capacity. The unique role of the quantum entanglement property, which manifests itself, in particular, in a nonclassical phenomenon of capacity superadditivity, is emphasised.

We consider main methods for detecting single photons used in quantum communications, including the quantum key distribution (QKD) technology. Two most promising single photon detectors (SPDs) based on superconducting nanowires and on a single-photon avalanche diode are described. The most effective SPD designs are presented and their advantages and disadvantages are analysed from the point of view of the possibility of their use in QKD devices. The results of the work of various scientific groups conducting research on QKD are discussed, which makes it possible to trace the trends in the global technological development of this industry over the past five years.

We report the results of experiments on implementing individually addressable one-qubit quantum gates on a microwave transition with two ⁸⁷Rb atoms in two optical dipole traps. Addressing is carried out using additional focused laser light, which results in a differential light shift of the microwave transition frequency. In the absence of addressing in each of the atoms, Rabi oscillations are obtained on the microwave clock transition 5S_1/2 (F = 2, m_F = 0) → 5S_1/2(F = 1, m_F = 0) between two working levels of qubits with a frequency of up to 5.1 kHz, a contrast up to 98 %, and a coherence time up to 4 ms. When addressing is turned on, the probability of a microwave transition in the addressed atom is suppressed to an average value of less than 5 %. The Rabi oscillations remaining in the other atom have the same contrast and correspond to the implementation of individually addressable basic one-qubit quantum operations (Hadamard gate and NOT gate) from different initial states of a qubit with an average fidelity of 92% ± 3 %. After renormalising this fidelity to the error in the preparation and measurement of quantum states of qubits, an estimate of 97% ± 3% is obtained for the fidelity of individual qubit rotations.

The paper describes the design of an optical coherence tomograph adapted for studying the state of the tympanic cavity of the human middle ear. We present the images obtained by the method of optical coherence tomography, which are characteristic of the norm and of the case of pathology – otitis media with effusion.

Phase locking of laser arrays is a promising approach for obtaining high-brightness light. A variety of experimental methods have been employed to ensure phase locking. Concurrently, complex theoretical models were developed and nontrivial physical effects were found. Here we review experimental data on passive phase locking and discuss current views on the potentialities of this method.

In this paper, we review photonic methods and technologies that are promising for monitoring the ocean and atmosphere and have been implemented mainly in recent years at the Institute of Automation and Control Processes, Far Eastern Branch, Russian Academy of Sciences. We present results of lidar studies that have made it possible to understand key features of ocean – atmosphere interaction processes under continent – ocean transition conditions, which determine specific features of the atmospheric aerosol distribution, small gaseous components of the atmosphere and its optical characteristics. We consider methods and tools for combined optical and laser fluorescence monitoring of the ocean surface. Particular attention is paid to results of research on remote methods and tools for real-time laser-induced and laser fluorescence environmental monitoring of the ocean, including specialised fibre-optic probes and mobile underwater robotic systems. We present results of the development and investigation of highly sensitive, noise-proof fibre-optic hydro- and seismic/acoustic sensors for remote monitoring of the ocean and robotic systems for underwater laser protection of marine vessels, hydraulic structures and oceanographic instruments against the negative impact of biofouling.

We have experimentally shown that nanosecond near-IR pulsed laser ablation of a thin amorphous carbon film produces carbon quantum dots with a graphite structure and nanodiamonds with a characteristic size of 20 – 500 nm on the substrate surface. The formation of these nanostructures is confirmed by electron microscopic images, luminescence spectra and Raman spectra. The mechanisms explaining the observed effects are proposed.

A theory of spontaneous four-wave mixing in a ring microcavity is developed. The rate of emission of biphotons for pulsed and monochromatic pumping with allowance for the dispersion of group velocities is analytically calculated. In the first case, pulses in the form of an increasing exponential are considered, which are optimal for excitation of an individual resonator mode. The behaviour of the group velocity dispersion as a function of the width and height of the waveguide is studied for a specific case of a ring microcavity made of silicon nitride. The results of the numerical calculation are in good agreement with the experimental data. The ring microcavity is made of two types of waveguides: completely etched and half etched. It is found that the latter allow for better control over the parameters in the manufacturing process, making them more predictable.

The work is dedicated to the further development of a compact quantum frequency standard based on a rubidium gas cell with a mixture of buffer gases. The results of frequency measurements and analysis of short-term frequency instability obtained on a laboratory prototype of a microwave rubidium atomic frequency standard (RAFS) with pulsed optical pumping (POP) are presented. The main in magnitude contributions to the overall frequency instability of the RAFS with POP are estimated. Short-term frequency instability expressed in terms of the Allan deviation and measured at averaging times τ up to several tens of seconds, σ_y(τ) = 2.5×10⁻¹³τ^−1/2, coincides satisfactorily with the calculated value of σ_y(τ) = 2.1×10⁻¹³τ^−1/2.

A frequency chain for converting the frequency of an optical clock based on ultracold ⁸⁷Sr atoms is updated for its comparison with the frequency of microwave standards from the State Primary Standard of time and frequency units and the national time scale, GET 1-2018. The results of the corresponding experiments are reported and analysed. An instrumental complex for reproducing and keeping the time and frequency units and the national time scale of the primary standard is described; this complex includes an optical clock based on strontium atoms and microwave standards of new generation. The order of the atomic time scale generation with application of optical clocks is also determined.

We examine the possibilities of refining an asymptotic description and quantitative calculations of the effects induced by thermal blackbody radiation (BBR) of the environment on the Rydberg states of atoms. Numerical values are calculated and asymptotic expressions are proposed for simplified estimates of natural lifetimes and threshold photoionisation cross sections for Rydberg states of rubidium and caesium atoms with large values of the principal quantum number, n ⩾ 20, and small orbital momenta, l = 0, 1, 2, 3. Based on analytical expressions, we present numerical estimates for the contributions of photoionisation probabilities to the BBR-induced broadening of the Rydberg energy level, as well as the contributions of continuum integrals to thermally induced shifts in the Rydberg-state energy levels.

We present a brief review of experimental work on the investigation of stimulated low-frequency Raman scattering of light in systems of submicron and nanosized particles of various physical nature.

We investigate displacement measurements of up to 17 μm on a heterodyne laser interferometer laboratory model. The measurement error for small (up to 200 nm) linear displacements is found to be 270 pm at a 10-s averaging time. The results obtained can be used for developing a space laser interferometric system for the global Earth's gravity field mapping.

We consider main methods for detecting single photons used in quantum communications, including the quantum key distribution (QKD) technology. Two most promising single photon detectors (SPDs) based on superconducting nanowires and on a single-photon avalanche diode are described. The most effective SPD designs are presented and their advantages and disadvantages are analysed from the point of view of the possibility of their use in QKD devices. The results of the work of various scientific groups conducting research on QKD are discussed, which makes it possible to trace the trends in the global technological development of this industry over the past five years.

This paper considers abnormal blocking of a guided mode propagating in a silicon optical waveguide with periodic tunnelling inserts. Using an independent two-dimensional analysis by the method of lines (MoL) and direct simulation by the finite-difference time-domain (FDTD) method, we have identified additional signal blocking bands, unrelated to Bragg conversion to backward guided modes of the parent silicon waveguide. These bands are due to the conversion of the incident wave energy to a leaky quasi-mode of the periodically segmented structure, which subsequently transfers the energy to the ambient medium in the form of radiation modes. A distinctive feature of this phenomenon is resonant coupling of the guided mode of the strip waveguide with its radiation modes, which is due to the weak tunnel coupling with the periodically segmented structure. This structure does not support independent guided propagation, so the energy stored in it is re-emitted to space. The abnormal blocking effect may find application in optical telecommunications elements and in the fabrication of optical sensors.

We report a numerical and experimental study of the laser beam propagation through a suspension of polystyrene microspheres in distilled water, showing the presence of higher-order centrally symmetric aberrations for the scatterer concentrations in the range from 1.3 × 10⁵ to 10⁶ mm⁻³ and analysing the dependence of the scattered light wavefront distortion on the concentration of particles in a turbid medium. The study has also shown the effectiveness of the compensation of the wavefront aberrations of a scattered laser beam using a bimorph adaptive mirror.

We consider image recording and reproduction using a reversed stimulated echo hologram, with a recording medium exposed to pulses of nonresonant electromagnetic standing waves. It is shown that the spatial intensity distribution in a stimulated echo hologram response depends on the electric field strength of nonresonant standing waves, which makes it possible to control reconstructed images.

Using a free electron laser developed in Novosibirsk, we have studied the reflection of monochromatic (λ = 130 μm) surface plasmon-polaritons (SPPs) from a plane mirror attached to a waveguiding surface. It is found that 100 % SPP reflection occurs not only in the perpendicular position of the mirror relative to the surface, but also when the mirror is deflected from the normal by the angle α being smaller than the limiting angle α* proportional to the SPP wave vector. When the mirror is deflected by the angle greater than α*, SPPs on a perfectly smooth surface must transform into a bulk wave, while, in the experiment, the SPP reflection coefficient decreases gradually to zero with increasing α, which is a manifestation of dispersion of the wave vector of monochromatic SPPs, caused by their scattering on the inhomogeneities of a real surface.