Anton Kozubov

This work examines a non-stationary phase-modulation process characterized by a time-dependent interaction coefficient, confined to the non-degenerate case. We derive an exact non-adiabatic solution to the time-dependent Schro¨ dinger equation. We then establish the criterion under which adiabatic considerations become valid-specifically, the condition permitting a stationary-state solution to approximate the non-stationary problem. Our results hold for an arbitrary number of interacting modes and an arbitrary photon number.

In this work, we investigate an alternative approach to phase modulation for information encoding and decoding in quantum key distribution systems. Specifically, we propose pulsed phase modulation, i.e. it has an envelope in addition to previously considered only harmonic temporal behavior of modulation index (depth), as a replacement for the conventional use of both intensity and phase modulators, which enables a more compact and cost-effective optical design. We further analyze how the principal parameters of this scheme affect the interference observed at the receiver and, consequently, their contribution to the quantum bit error rate in a subcarrier-wave quantum key distribution implementation.

Phase-randomized coherent states are widely used in various applications of quantum optics. They are best known to be the core part of decoy-state quantum key distribution protocols with phase-coding. From the perspective of future development of quantum protocol architecture, it is important to determine whether phase randomization can be applied at an arbitrary stage of an optical scheme without affecting the informational properties of the quantum system. In this paper, using the superoperator formalism, we have shown that phase randomization of a two-mode coherent state commutes with linear optical transformations. This implies that phase randomization can be applied virtually at any point within the optical setup. We further demonstrate that the Holevo bound for such a state coincides with that of regular coherent states, bearing in mind that the Holevo bound quantifies only the maximum amount of information accessible to an eavesdropper. Advantages of phase-randomized coherent states compare to regular ones in particular cases of eavesdropper’s strategies should be considered separately. Also, these findings indicate that phase randomization can be directly applied to a subcarrier wave quantum key distribution type of systems, opening prospects for its future development.

We theoretically study the dynamical regimes of the observables that govern the operating conditions of continuous-variable (CV) quantum-key-distribution (QKD) systems, depending on quantum-channel-induced intermode interactions. In contrast to the widely used approach in which losses and thermal broadening are introduced through a beam-splitter transformation, our analysis uses the exactly solvable quantum channel model describing the Lindblad dynamics of multimode bosonic systems interacting with a heat bath and additionally takes into account imperfections of the homodyne-detection scheme. The analytical results for the photon-count difference and the quadrature probability distributions are used to derive the expression for the mutual information between legitimate parties, which explicitly links the information properties of CV QKD and the parameters of the channel. For the important special case of a two-mode photonic system propagating in a fiber channel, the latter can be conveniently parameterized using the frequency and the relaxation-rate vectors that characterize the coherent (dynamical) intermode couplings and the incoherent (environment mediated) interaction between the bosonic modes, respectively. It turns out that these vectors determine four qualitatively different dynamical regimes of the mutual information and the phase difference between the signal and the local oscillator that may significantly affect the operation of CV QKD.

Subject of study. The spatial distribution of polarization properties for objects with linear dichroism is studied. Aim of study. A ghost polarimetry system using a spatial light modulator to measure the optical fields illuminating an object with linear dichroism is developed. Method. Ghost polarimetry is used to measure the integral intensity of radiation passing through an object. The intensity correlation functions are numerically calculated for two orthogonally polarized sets of optical fields for subsequent calculation of the modulus of the anisotropy azimuth for an object with linear dichroism. Main results. A ghost polarimetry device using a spatial light modulator is developed to measure the characteristics of structured optical fields. Images showing the polarization properties of three objects with linear dichroism are obtained. Practical significance. Ghost polarimetry has several advantages over traditional methods of obtaining polarization images when imaging objects with low radiation intensity, when imaging in high-turbulence environments, or when imaging in spectral ranges for which traditional measuring instruments are expensive or unavailable. The inclusion of a spatial light modulator in the ghost polarimetry device means it is no longer necessary to record the fields generated. This modification will enable the characteristics of optical fields for specific objects to be varied for microbiological and medical research.

A mathematical model of quantum random number generator based on vacuum fluctuations is considered. A mathematical justification for the “randomness” of the generated sequence is given under the assumption of the truth of the basic postulates of quantum theory and the validity of the Poisson probability distribution for the photon flux. The results of experiments and the obtained estimates of the minimum entropy are presented.

Besides true quantum repeaters, a trusted node paradigm seems to be inevitable for practical implementations (at least in the short term), and one should consider configurations of trusted nodes as the basis for global quantum networks. In this paper, we estimate how the introduction of additional connections between trusted nodes through one, two, etc., nodes (i.e., connection density) to a quantum network with serial connections of trusted nodes affects its security. We provide proper scaling of the failure probability of authentication and quantum key distribution protocols to the level of the whole quantum network. Expressions of the failure probability dependent on the total number of connected nodes between users and the connection density for the given mean failure probability of each element are derived. The result provides an explicit trade-off between an increase of key transport security and a consequent increase of spent resources. We believe that the obtained result may be useful for both the design of future networks and optimization of existing ones.