For one-mode and multimode light, the photon-number tomograms of Gaussian quantum states are explicitly calculated in terms of multivariable Hermite polynomials. Positivity of the tomograms is shown to be a necessary condition for positivity of the density matrix.

We map the density matrix of the qubit (spin-1/2) state associated with the Bloch sphere and given in the tomographic probability representation onto vertices of a triangle determining Triada of Malevich's squares. The three triangle vertices are located on three sides of another equilateral triangle with the sides equal to root 2. We demonstrate that the triangle vertices are in one-to-one correspondence with the points inside the Bloch sphere and show that the uncertainty relation for the three probabilities of spin projections +1/2 onto three orthogonal directions has the bound determined by the triangle area introduced. This bound is related to the sum of three Malevich's square areas where the squares have sides coinciding with the sides of the triangle. We express any evolution of the qubit state as the motion of the three vertices of the triangle introduced and interpret the gates of qubit states as the semigroup symmetry of the Triada of Malevich's squares. In view of the dynamical semigroup of the qubit-state evolution, we constructed nonlinear representation of the group U(2).

We express the matrix elements of the density matrix of the qutrit state in terms of probabilities associated with artificial qubit states. We show that the quantum statistics of qubit states and observables is formally equivalent to the statistics of classical systems with three random vector variables and three classical probability distributions obeying special constrains found in this study. The Bloch spheres geometry of qubit states is mapped onto triangle geometry of qubits. We investigate the triada of Malevich’s squares describing the qubit states in quantum suprematism picture and the inequalities for the areas of the squares for qutrit (spin-1 system). We expressed quantum channels for qutrit states in terms of a linear transform of the probabilities determining the qutrit-state density matrix.

We introduce the probability distributions describing quantum observables in conventional quantum mechanics and clarify their relations to the tomographic probability distributions describing quantum states. We derive the evolution equation for quantum observables (Heisenberg equation) in the probability representation and give examples of the spin-1/2 (qubit) states and the spin observables. We present quantum channels for qubits in the probability representation.

The currently used ghost-image schemes traditionally involve two-mode entangled light states or incoherent radiation. Here, we consider the application of four-mode entangled light states and show that multiplexed ghost images (MGI) formed by four-mode entangled quantum light states have mutual spatial correlations determined by the eighth-order field correlation functions. We develop a special algorithm to calculate high-order correlations of Bose operators. We also demonstrate that accounting of the MGI correlations allows us to improve the quality of the restored image of an object while processing the MGI by the measurement reduction method. We carry out computer modeling of the image recovery from the MGI. We establish that in the considered example the signal-to-noise ratio of the reduced ghost image is 4.6 times higher than the best signal-to-noise ratio for the ghost images themselves.

Quantum key distribution (QKD) offers a practical solution for secure communication between two distinct parties via a quantum channel and an authentic public channel. In this work, we consider different approaches to the quantum bit error rate (QBER) estimation at the information reconciliation stage of the post-processing procedure. For reconciliation schemes employing low-density parity-check (LDPC) codes, we develop a novel syndrome-based QBER estimation algorithm. The algorithm suggested is suitable for irregular LDPC codes and takes into account punctured and shortened bits. Testing our approach in a real QKD setup, we show that an approach combining the proposed algorithm with conventional QBER estimation techniques allows one to improve the accuracy of the QBER estimation.

We construct the quantum density matrix of a spin-1/2 state for three given probability distributions describing positions of three classical coins and associate its matrix elements with the Triada of Malevich's squares. We present the superposition principle of spin-1/2 states in the form of a nonlinear addition rule for these classical coin probabilities. We illustrate the obtained formulas by the statement "God does not play dice - God plays coins.".

On the spin-1/2 example, we demonstrate that quantum states can be described by standard probability distributions, which contain the same information that the wave function and the density matrix do. Within the framework of this approach, called for spin-1/2 the quantum suprematism representation, the probability distributions are illustrated by simplex or triangle geometry or by the Triada of Malevich’s Squares (black, red, and white) associated with the triangles, and new quantum relations for areas of the squares are obtained. The superposition principle for spin states and quantum interference phenomenon are expressed as an explicit new nonlinear addition rule for the probability distributions describing the quantum states and illustrated as the addition of two Triadas of Malevich’s squares. We discuss some analogy of the triangle geometry of spin-1/2 states related to the O(3) symmetry group and the pyramide geometry related to the hydrogen-atom dynamical symmetry O(2, 4).

We demonstrate a diode-pumped acousto-optically (A-O) Q-switched 1,066 nm laser with a novel Nd:Gd0.69Y0.3TaO4 mixed crystal. At a modulation repetition rate of 10 kHz and an absorbed pump power of 15 W, the pulsed laser output characteristics are: average output power 2.41 W, pulse width 27.2 ns, pulse energy 241 μJ, and pulse peak power 8.86 kW. At a modulation repetition rate of 20 kHz and an absorbed pump power of 15 W, the characteristics are: average output power 2.68 W, pulse width 30.1 ns, pulse energy 134 μJ, and pulse peak power 4.45 kW.

We demonstrate experimentally a passive Q-switched (PQS) operation of 2 μm c-cut Tm,Ho:LuVO4 laser with a WSe2 saturable absorber (SA) mirror. We obtain an average output power of 500 mW with a pulse width of 3.5 μs at 116.6 kHz and a pump power of 13.03 W. We measure the output wavelengths of the Tm,Ho:LuVO4 laser to be 2,075.8 nm at the continuous-wave (CW) mode operation and 2,056.9 nm at the PQS mode operation. The beam quality factors M x 2 $$ {M}_x^2 $$ = 1.11 and M y 2 $$ {M}_y^2 $$ = 1.06 are obtained in the PQS Tm,Ho:LuVO4 laser.

We study features of point emitters in ZnSe-based quantum wells using laser-based spectroscopy with a high spatial resolution. We determine the experimental conditions under which quantum emitters formed by donor-acceptor pairs are observed. We discuss the possibility of studying individual defects forming a donor–acceptor pair using the spectrum of the corresponding quantum emitters.

We studied numerically the frequency doubling of femtosecond laser pulses (up to ∼5 fs) in nonlinear photonic crystals. We investigated the impact of a number of factors (phase modulation of the primary emission pulse and spatial modulation of nonlinear susceptibility, up to the third order dispersion of a nonlinear photonic crystal) on the efficiency of nonlinear conversion and the duration of the secondharmonic pulse. We show that the efficiency of second-harmonic generation reaches η 2ω ∼ 45% when ultrashort (down to 5 fs) laser pulses are used.

We investigate the conductivity of a composite of PANI-PAMPSA and PANI-PAMPSA with carbon particles by impedance spectroscopy. We establish that the addition of carbon particles to the PANI-PAMPSA composite leads to a significant rise in the conductivity over the entire range of frequencies studied (from 10 Hz to 5 MHz). Such increase in conductivity is very important for using the PANI-PAMPSA composite in solar energy. We propose an equivalent electrical circuit of the samples investigated. At low frequencies, the conductivity dependence on frequency indicates the hopping mechanism of the charge carrier transport.

We study theoretically coherent dynamics of cold atoms in the near-resonant 2D optical lattice with orthogonal polarizations, taking into account a coupling between the atomic internal (electronic) and external (translational) degrees of freedom. We show that in the semiclassical approximation this dynamics may be regular or chaotic in dependence on the values of the detuning between the electric-dipole transition and the laser field frequencies. Chaos manifests itself both in the Rabi oscillations and in the translational motion at comparatively small absolute values of the detuning. The center-of-mass motion in the chaotic regime resembles the random walk of atoms in a 2D lattice which is an absolutely rigid one. Chaos is quantified by the values of the maximal Lyapunov exponent and is shown to be weaker as compared with the case of cold atoms in a 1D lattice. In fact, chaos appears at the time moments when the atom crosses 1D or 2D nodes of the lattice potential when its induced electric dipole moment changes suddenly in a random-like manner.

Quantum key distribution (QKD), ensuring the unconditional security of information, attracts a significant deal of interest. An important task is to design QKD systems as a platform for education as well as for research and development applications and fast prototyping new QKD protocols. Here, we present a modular QKD setup driven by National Instruments (NI) cards with open source LabView code, open source Python code for post-processing procedures, and open source protocol for external applications. An important feature of the apparatus developed is its flexibility offering the possibility to modify optical schemes, as well as prototype, and verify novel QKD protocols. The other distinctive feature of the setup developed is the implementation of the decoy-state protocol, which is a standard tool for secure long-distance quantum communications. By testing the plug-and-play scheme realizing BB84 and decoy-state BB84 QKD protocols, we show that the developed QKD setup shows a high degree of robustness beyond laboratory conditions. We demonstrate the results of the use of the developed modular setup for QKD experiments in the urban environment.

We designed a compact and efficient high-power laser that generates chaotic green light by perturbing a 520 nm laser diode with optical feedback. This achieved a continuous output power of 30 mW with a bandwidth of more than 1 GHz and an electrical-to-green conversion efficiency of 6.25%. All possible chaotic states were demonstrated for different diode-driving currents and feedback intensities. The FWHM and the peak side-lobe level (PSL) are affected by the injection current and feedback intensity. The FWHM increases with increasing feedback intensity, and the PSL increases with both injection current and feedback intensity. The compact chaotic laser could be used in underwater detection to suppress backscattering because of its intrinsic intensity modulation at high frequency. We finally found the optimum range of chaotic states for underwater detection, for which the ratio of input current to threshold current is 1.5 : 2.1.

In this work, we discuss a far-off resonance optical dipole trap for cold lithium-7 atoms. A single optical trapping beam was produced by a continuous-wave fiber laser. Using our experimental data, we obtained important parameters of the trap, such as size of the cold atomic cloud, and evaluate the dipole potential and the rate of the trap losses. Information on temperature is acquired from observation of parametric resonances. We investigate the parametric resonances obtained with strong modulation of the trap potential and record superharmonics. We plan to prepare ultra-cold gas of highly-excited lithium atoms and study interactions between Rydberg atoms.

We produce composite ceramic laser elements Nd3+:YAG/Cr4+:YAG using two different methods, i.e., (1) layer-by-layer uniaxial pressing in a metallic mold with sequential addition of appropriate portions of the powder; (2) stacking and pressing of previously uniaxially pressed tablets in a cold isostatic press. In lasers where composite ceramic Nd3+:YAG/Cr4+:YAG elements are used together with longitudinal diode pumping, we obtain the lasing regime for both types of active elements at a wavelength of 1,064 nm in the Q-switch mode. Lasers with ceramic composites produced by stacking and cold isostatic pressing of the previously pressed tablets demonstrate a slope efficiency (~30–33%) even higher than lasers with crystalline saturable absorber (~21%). Lasers based on ceramic composites manufactured by layer-by-layer pressing of powders have an efficiency of ~11–19%. Composites made by stacking and cold isostatic pressing of the previously pressed tablets demonstrate a generation threshold close to those in lasers with crystalline saturable absorbers.

In this paper, we demonstrate the self-Q-switched (SQS) operation of a Nd:GdTaO4 laser with different output couplers. The SQS pulse trains are obtained by slightly adjusting the output coupler (OC) under stable continuous wave (CW) operation. In the SQS operation, we observe an output laser with the laser line at 1,066 nm. Under a pump power of 3.8 W, the laser designed shows the following results: (1) with an OC of 5%, the average output power is 0.249 W, the pulse width is 2.83 μs, and the repetition rate is 48.9 kHz, (2) with an OC of 10%, the average output power is 0.299 W, the pulse width is 3.62 μs, and the repetition rate is 53.0 kHz. In a word, the laser system designed is quite stable and suitable for generation of subnanosecond pulses.