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 present the results of the aberration correction of laser radiation wavefront using a dynamic method for determining the reference wavefront. The method, which is based on the processing of synchronously obtained data on the near- and far-field zones, significantly improves the focusing quality with active wavefront correction, especially under conditions of dynamic aberrations. An increase in the Strehl number S from 0.7 to 0.86 is demonstrated when a beam 18 cm in diameter is focused by an F/2.5 parabolic mirror.

Lasing in a solid-state inhomogeneously diode end-pumped Nd : KGW/Cr4+ : YAG laser is obtained at wavelengths of 1067 and 1181 nm (Stokes shift 901 cm−1) under conditions of frequency degeneracy of cavity modes. In this case, the radiation has a characteristic ring spatial structure, which testifies to transverse mode locking of the fundamental and Stokes components. The possibility of controlling the Raman conversion efficiency by changing the position of an undoped KGW crystal additionally installed in the cavity is demonstrated. It is shown that the transverse mode locking is accompanied by the total longitudinal mode locking at λ = 1067 and 1181 nm.

We study the possibility of suppressing light shifts in Ramsey spectroscopy of coherent population trapping (CPT) using generalised autobalanced Ramsey spectroscopy (GABRS) and combined error signal in Ramsey spectroscopy (CESRS). We consider CPT resonances excited by a coherent bichromatic field in an open Λ-system with a ‘trap’ state. Using a rigorous mathematical proof and numerical calculations, these methods are shown to lead to complete suppression of the light shift and its fluctuations. Implementation of GABRS and CESRS in CPT clocks can markedly improve accuracy and long-term stability of these devices. These methods can also be applied in atomic CPT magnetometers and interferometers.

Spherical ultrasonic antennas are used in raster-scan optoacoustic (OA) angiography to record broadband signals generated by haemoglobin molecules in blood when they absorb pulsed optical radiation. Depending on the size of haemoglobin-containing structures, the characteristic frequencies of OA signals can vary quite significantly, ranging from hundreds of kilohertz to hundreds of megahertz. Meanwhile, the bandwidth of the receiving frequency band of standard piezoelectric sensors, as a rule, does not exceed the centre frequency value. It is possible to expand the receiving band of ultrasonic detectors to the required 0.1 kHz – 100 MHz values by using nonresonant piezomaterials based on polyvinidylene fluoride (PVDF). Two ultra-wideband detectors based on PVDF piezofilms of different thicknesses (9 μm and 25 μm) with different amplitude-frequency characteristics are experimentally compared. Comparative OA imaging of a tissue-like phantom demonstrates that the low-frequency sensor (film thickness l = 25 μm) has a greater depth of field, while the high-frequency sensor (l = 9 μm) has a better sensitivity in the range of 40 – 100 MHz. Using OA imaging of an experimental tumour in vivo, it is shown that a sensor with l = 25 μm is better suited for examining normal tissue containing relatively large blood vessels, while a sensor with l = 9 μm is better suited for studying tumour tissue containing a large number of multidirectional blood vessels of minimal size comparable to the maximum spatial resolution of the OA system.

Two-dimensional optical image edge enhancement is investigated using a spatial acousto-optical (AO) filter operating at two wavelengths of optical radiation, which significantly increases the reliability of measurements and also reduces the effect of noise. The filter operates basing on diffraction of light in two diffraction orders. It is shown that, in the general case, it is impossible to obtain a two-dimensional image contour simultaneously at two wavelengths at the same energy and frequency-angular parameters of the AO filter. A regime has been found that makes it possible to switch from one wavelength to another while maintaining the operation of edge enhancement by changing the acoustic power. This regime has been experimentally confirmed by the example of the formation of a two-dimensional image contour using two-colour radiation of an Ar laser emitting at wavelengths of 0.488 × 10−4 and 0.514 × 10−4 cm, and an AO cell made of TeO2, operating at a sound frequency of 57 MHz.

We report a study of the effect of postgrowth treatment of ZnGeP2 single crystals (low-temperature annealing, irradiation with fast electrons, polishing of working surfaces) and the conditions of exposure to repetitively pulsed laser radiation [wavelength (2091 or 1064 nm), pulse repetition rate, beam diameter, exposure time, sample temperature] on the laser-induced damage threshold (LIDT) of the surfaces of these crystals. It is found that thermal annealing of ZnGeP2 single crystals and their irradiation with a flux of fast electrons, which increase the LIDT at a wavelength of λ = 1064 nm, do not lead to a change in this threshold at λ = 2091 nm. It is shown that ZnGeP2 elements with lower optical losses in the spectral range 0.7 – 2.5 μm have a higher LIDT at λ = 2091 nm both immediately after fabrication and after postgrowth processing. An increase in the threshold energy density of laser radiation by a factor of 1.5 – 3 at λ = 2091 nm is revealed with a decrease in the crystal temperature from zero to –60 °C. The fact of reversible photodarkening of the propagation channel of laser radiation in ZnGeP2 in the predamage region of parameters is established by the method of digital holography.

We have developed and experimentally implemented an adaptive laser vector-phase hydroacoustic measuring system, which allows one to determine the total acoustic intensity vector. A receiving element of the measuring system comprises six spatially separated fibre-optic coil-type sensors. Signals from the sensors are phase demodulated by using a six-channel adaptive holographic interferometer based on dynamic holograms multiplexed in a photorefractive CdTe crystal. Performance of the developed measuring system has been experimentally tested by determining the bearing and localisation of a source of a weak hydroacoustic field.

We consider interference fluxes of energy, momentum and angular momentum, describing the interaction of electric charges with constant and variable electromagnetic fields. Absorption and stimulated emission of atoms in the field of free and evanescent waves are described. It is shown that in the case of interaction with reactive fields, the momentum of an atom during absorption and stimulated emission does not change. A wave derivation of Planck’s formula based on the interference of the fields of classical thermal radiation and quantum zero-point oscillations is presented.