arXiv subjects

Alexei D. Kiselev

Publications and source records attributed to Alexei D. Kiselev.

At least 19 recordsLinked to original sources

arXiv2026

Performance of the subcarrier-wave quantum key distribution in the presence of spontaneous Raman scattering noise generated by classical DWDM channels

In this paper we study performance of the subcarrier-wave quantum key distribution system (SCW QKD) in the presence of spontaneous Raman scattering (SpRS) noise generated by classical channels of dense wavelength division multiplexing (DWDM) network within a single-mode optical fiber. We present the mathematical model for evaluation of the quantum bit error rate (QBER) and the secure key generation rate with the SpRS noise taken into account. We consider two regimes of the SCW QKD system: the continuous wave regime that uses continuous wave laser and the pulsed regime. For these regimes, performance of the system is analyzed depending on receiver sensitivity of classical DWDM. It is found that the pulsed regime outperforms the continuous wave regime in both the secure key generation rate and the maximum achievable distance.

quant-ph↗

arXiv2026

Iterative $C_Z$-gate-based protocol for squeezed Schr\"odinger cat state engineering

Squeezed optical Schr\"odinger cat states constitute a key resource for both fundamental tests of quantum theory and up-to-date quantum technologies. We propose a measurement-assisted gate for the generation and manipulation of the cat states. In this scheme, an ancilla in the non-Gaussian small-amplitude (in general, squeezed) Schr\"odinger cat state and the target oscillator initially prepared in a squeezed vacuum (or coherent) state are subjected to a quantum nondemolition (QND) entangling operation followed by projective homodyne measurement. The proposed gate enables generation of high-fidelity squeezed Schr\"odinger cat states with controllable size and squeezing with tunable fidelity/success-probability trade-off. We also introduce an iterative, homodyne-conditioned $C_Z$-based protocol for cat-state amplification. The parameter regimes required to achieve the desired fidelity and the success probability are analyzed. The approach is well suited for applications in measurement-based quantum computing and hybrid quantum networks where non-Gaussian resources enhance computational and communication capabilities.

quant-ph↗

arXiv2025

Asymmetry effects in homodyne and heterodyne measurements: Positive operator-valued measures and asymptotic security of Gaussian-continuous-variable quantum key distribution

We use the Gaussian approximation describing photocount statistics for both the homodyne and the double homodyne (heterodyne) measurements to study asymmetry effects arising from imbalance of the beam splitters and variations in quantum efficiencies of the photodetectors. After computing the $Q$ symbols of the positive operator-valued measures (POVMs) of noisy measurements that take into account the asymmetry effects, the operator representations for the POVMs are obtained in the form that assumes applying the additive noise quantum channel to the POVMs of noiseless (ideal) measurements. For double homodyne detection, it was found that the noiseless measurements should generally be expressed in terms of the projectors onto squeezed-states and the corresponding squeezed-state operator representation of POVM along with the measurement noise channel depend on the squeezing parameter that lies in the interval dictated by the condition for the excess noise covariance matrix to be positive semi-definite. The analytical results are used to perform analysis of the asymptotic security of the Gaussian-modulated continuous variable quantum key distribution (CV-QKD) protocol in the untrusted-noise scenario where the measurement noise is assumed to be accessible to an adversary. The inherent non-uniqueness of the operator representation for the double-homodyne POVM manifests itself in the squeezing dependent Holevo information that needs to be additionally optimized. For both types of the measurements, the mutual information, the Holevo information and the asymptotic secret fraction are sensitive to asymmetry effects leading to degraded performance of the protocol.

quant-ph↗

arXiv2025

Controlling Hong-Ou-Mandel antibunching via parity governed local spectral shaping of biphoton states

We investigate into experimentally detectable effects such as the Hong-Ou-Mandel (HOM) bunching and antibunching. These regimes can be characterized using the symmetry degree parameter $D_S$ that enters the two-photon coincidence probability $P_{2c}=(1-D_S)/2$. In the case of HOM bunching (antibunching), $D_S$ is positive (negative). Though the symmetry degree can generally be expressed in terms of the difference between the contributions coming from the symmetric and antisymmetric parts of the biphoton joint spectral amplitude (JSA), $\psi(\omega_1,\omega_2)$, for a certain physically realizable class of the JSA, where $\psi(\omega_1,\omega_2)$ is proportional to the product of amplitudes $\varphi_1(\omega_1)\varphi_2(\omega_2)$ multiplied by a Gaussian shaped entangling factor, we find the sign of $D_S$ is primarily governed by the parity properties of the spectral function, $\varphi_{12}(\omega)=\varphi_1(\omega)\varphi_2^*(\omega)$. It is the even (odd) part of $\varphi_{12}=\varphi_{12}^{(+)}+\varphi_{12}^{(-)}$ that meets the parity condition $\varphi_{12}^{(+)}(\omega-\Omega)=\varphi_{12}^{(+)}(\Omega-\omega)$ ($\varphi_{12}^{(-)}(\omega-\Omega)=- \varphi_{12}^{(-)}(\Omega-\omega)$) to yield the positive (negative) contribution, $D_S^{(+)}$ ($-D_S^{(-)}$), to the symmetry degree parameter: $D_S=D_S^{(+)}-D_S^{(-)}$. We have shown that switching between the bunching and antibunching regimes can be realized using the experimentally accessible family of modulated biphoton states produced using the spectral phase modulation fine-tuned via the sub-nanometer scale variation of the path length. For this class of modulated states, the Schmidt number has been computed as a function of the modulation parameter. This dependence reveals the structure of narrow resonance peaks strongly correlated with the corresponding narrow dips of the symmetry degree where the HOM antibunching occurs.

quant-ph↗

arXiv2025

Vulnerabilities of quantum key distribution systems in visible range

In this paper we investigate spectral vulnerabilities in quantum key distribution systems arising from the use of shorter-wavelength radiation in the 400-800 nm range, with particular focus on the induced photorefraction attack (IPA). Crucial elements influenced by IPA include various types of modulators, both phase and intensity modulators. In the following paper, we consider different scenarios and their implications. Through combined theoretical and experimental analysis, we demonstrate that optical components commonly used as countermeasures in the telecom band (1000-2100 nm) exhibit significantly reduced effectiveness at shorter wavelengths. The efficiency of IPA is shown to increase as the wavelength decreases, posing a substantial threat to phase-modulation-based QKD protocols. We analyze the impact of IPA across different QKD architectures and assess the feasibility of potential countermeasures under realistic implementation scenarios. Our results highlight the necessity of broadband security evaluations and wavelength-aware component design in future QKD systems.

quant-ph↗

arXiv2025

Creation and manipulation of Schrödinger cat states based on semiclassical predictions

We consider the generation of Schr{ö}dinger cat states using a quantum measurement-induced logical gate where entanglement between the input state of the target oscillator and the Fock state of the ancillary system produced by the quantum non-demolition entangling $\hat{C}_Z$ operation is combined with the homodyne measurement. We utilize the semiclassical approach to construct both the input-output mapping of the field variables in the phase space and the wave function of the output state. This approach is found to predict that the state at the gate output can be represented by a minimally disturbed cat-like state which is a superposition of two copies of the initial state symmetrically displaced by momentum variable. For the target oscillator prepared in the coherent state, we show that the fidelity between the exact solution for the gate output state and the ``perfect'' Schr{ö}dinger cat reconstructed from the semiclassical theory can reach high values exceeding 0.99.

quant-ph↗

arXiv2025

Lindblad dynamics of open multi-mode bosonic systems: Algebra of bilinear superoperators, exceptional points and speed of evolution

We develop the algebraic method based on the Lie algebra of quadratic combinations of left and right superoperators associated with matrices to study the Lindblad dynamics of multimode bosonic systems coupled a thermal bath and described by the Liouvillian superoperator that takes into account both dynamical (coherent) and environment mediated (incoherent) interactions between the modes. Our algebraic technique is applied to transform the Liouvillian into the diagonalized form by eliminating jump superoperators and solve the spectral problem. The temperature independent effective non-Hermitian Hamiltonian, $\hat{H}_{eff}$, is found to govern both the diagonalized Liouvillian and the spectral properties. It is shown that the Liouvillian exceptional points are represented by the points in the parameter space where the matrix, $H$, associated with $\hat{H}_{eff}$ is non-diagonalizable. We use our method to derive the low-temperature approximation for the superpropagator and to study the special case of a two mode system representing the photonic polarization modes. For this system, we describe the geometry of exceptional points in the space of frequency and relaxation vectors parameterizing the intermode couplings and, for a single-photon state, evaluate the time dependence of the speed of evolution as a function of the angles characterizing the couplings and the initial state.

quant-ph↗

arXiv2025

Sub-Poissonian light in fluctuating thermal-loss bosonic channels

We study the photon statistics of a single-mode sub-Poissonian light propagating in the temperature-loss bosonic channel with fluctuating transmittance which can be regarded as a temperature-dependent model of turbulent atmosphere. By assuming that the variance of the transmittance can be expressed in terms of the fluctuation strength parameter we show that the photon statistics of the light remains sub-Poissonian provided the averaged transmittance exceeds its critical value. The critical transmittance is analytically computed as a function of the input states parameters, the temperature and the fluctuation strength. The results are applied to study special cases of the one-mode squeezed states and the odd optical Shrödinger cats.

quant-ph↗

arXiv2023

Quantum repeater via entangled phase modulated multimode coherent states

We present a scheme of quantum repeater that uses entangled multimode coherent states which are obtained by electro-optic modulation of symmetric and antisymmetric Schrödinger cat states. In this method subcarrier modes of the phase modulated states generated by the remote parties are sent to a symmetric beam splitter at the central node. The entangled coherent states are heraldedly prepared by photon counting measurements at the output channels of the beam splitter. We study how the effects of decoherence in the quantum channel affect statistics of photocounts and corresponding fidelity. We show how the proposed scheme can be useful for extending range of quantum key distribution with sub carrier wave encoding by exploiting quantum teleportation with the generated entanglement.

quant-ph↗

arXiv2022

Continuous-variable quantum key distribution: security analysis with trusted hardware noise against general attacks

In this paper, using the full security framework for continuous variable quantum key distribution (CV-QKD), we provide a composable security proof for the CV-QKD system in a realistic implementation. We take into account equipment losses and contributions from various components of excess noise and evaluate performance against collective and coherent attacks assuming trusted hardware noise. The calculation showed that the system remains operable at channel losses up to 10.2 dB in the presence of collective attacks and up to 7.5 dB in the presence of coherent ones.

quant-ph↗

arXiv2020

Hysteresis and Freedericksz thresholds for twisted states in chiral nematic liquid crystals: Minimum-energy path approach

We study minimum-energy pathways (MEPs) between the branches of metastable helical structures in chiral nematic liquid crystals (CNLCs)subjected to the electric field applied across the cell. By performing stability analysis we have found that, for the branches with non-vanishing half-turn number, the threshold (critical) voltage of the Freedericksz transition is an increasing function of the free twisting wave number. The curves for the threshold voltage depend on the elastic anisotropy and determine the zero-field critical free twisting number where the director out-of-plane fluctuations destabilize the CNLC helix. For each MEP passing through a first order saddle point we have computed the energy barrier as the energy difference between the saddle-point and the initial structures at different values of the applied field. In our calculations, where the initial approximation for a MEP at the next step was determined by the MEP obtained at the previous step, the electric field dependence of the energy barrier is found to exhibit the hysteresis. This is the hysteresis of electrically driven transition of the saddle-point configuration between the planar and the tilted structures involving out-of-plane director deformations. It turned out that, by contrast to the second-order Freedericksz transition, this transition is first order and we have studied how it depends on the zenithal anchoring energy strength.

cond-mat.soft↗

arXiv2019

Multiple minimum energy paths and scenarios of unwinding transitions in chiral nematic liquid crystals

We apply the minimum energy paths (MEPs) approach to study the helix unwinding transition in chiral nematic liquid crystals. A mechanism of the transition is determined by a MEP passing through a first order saddle point on the free energy surface. The energy difference between the saddle point and the initial state gives the energy barrier of the transition. Two starting approximations for the paths are used to find the MEPs representing different transition scenarios: (a) the director slippage approximation with in-plane helical structures; and (b) the anchoring breaking approximation that involves the structures with profound out-of-plane director deviations. It is shown that, at sufficiently low voltages, the unwinding transition is solely governed by the director slippage mechanism with the planar saddle point structures. When the applied voltage exceeds its critical value below the threshold of the Freedericksz transition, the additional scenario through the anchoring breaking transitions is found to come into play. For these transitions, the saddle point structure is characterized by out-of-plane deformations localized near the bounding surface. The energy barriers for different paths of transitions are computed as a function of the voltage and the anchoring energy strengths.

cond-mat.soft↗

arXiv2019

Polarization resolved radiation angular patterns of orientationally ordered nanorods

We employ the transfer matrix approach combined with the Green's function method to theoretically study polarization resolved far-field angular distributions of photoluminescence from quantum nanorods (NRs) embedded in an anisotropic polymer film. The emission and excitation properties of NRs are described by the emission and excitation anisotropy tensors. These tensors and the solution of the emission problem expressed in terms of the evolution operators are used to derive the orientationally averaged coherency matrix of the emitted wavefield. For the case of in-plane alignment and unpolarized excitation, we estimate the emission anisotropy parameter and compute the angular profiles for the photoluminescence polarization parameter such as the degree of linear polarization, the Stokes parameter $s_1$, the ellipticity and the polarization azimuth. We show that the alignment order parameter has a profound effect on the angular profiles.

physics.optics↗

arXiv2019

Waveguide propagation of light in polymer porous films filled with nematic liquid crystals

We theoretically analyze the waveguide regime of light propagation in a cylindrical pore of a polymer matrix filled with liquid crystals assuming that the effective radial optical anisotropy is biaxial. From numerical analysis of the dispersion relations, the waveguide modes are found to be sensitive to the field-induced changes of the anisotropy. The electro-optic properties of the polymer porous polyethylene terephthalate (PET) films filled with the nematic liquid crystal 5CB are studied experimentally and the experimental results are compared with the results of the theoretical investigation.

physics.optics↗

arXiv2019

Interferometric and Uhlmann phases of mixed polarization states

In our investigation into the effects of the degree of polarization in modulation of partially polarized light we assume general settings of the interferometry of partially polarized lightwaves and perform theoretical analysis of the Uhlmann and the interferometric phases. We introduce the relative Uhlmann phase determined by the Uhlmann holonomies of interfering beams and show that the interferometric phase generalized to the case of nonunitary evolution can, similar to the Uhlmann phase, be cast into the holonomy defined form. By using the technique based on a two-arm Mach-Zehnder interferometer, two different dynamical regimes of light modulation are experimentally studied: (a) modulation of the input light by the rotating quarter-wave plate (QWP); and (b) modulation of the testing beam by a birefringent plate with electrically controlled anisotropy represented by the deformed-helix ferroelectric liquid crystal (DHFLC) cell. In the setup with the rotating QWP, the interferometric phase is found to be equal to the relative Uhlmann phase. Experimental and theoretical results being in excellent agreement both show that this phase is an oscillating function of the QWP angle and increases with the degree of polarization. For modulation by the DHFLC cell, the data derived from our electro-optic measurements are fitted using the theory of the orientational Kerr effect in FLCs. This theory in combination with the results of fitting is used to evaluate electric field dependencies of the interferometric and the Uhlmann phases.

physics.optics↗

arXiv2017

Algebraic approach to electro-optic modulation of light: Exactly solvable multimode quantum model

We theoretically study electro-optic light modulation based on the quantum model where the linear electro-optic effect and the externally applied microwave field result in the interaction between optical cavity modes. The model assumes that the number of interacting modes is finite and effects of the mode overlapping coefficient on the strength of the intermode interaction can be taken into account through dependence of the coupling coefficient on the mode characteristics. We show that, under certain conditions, the model is exactly solvable and, in the semiclassical approximation where the microwave field is treated as a classical mode, can be analyzed using the technique of the Jordan mappings for the su(2) Lie algebra. Analytical results are applied to study effects of light modulation on the frequency dependence of the photon counting rate. We also establish the conditions of validity of the semiclassical approximation by applying the methods of polynomially deformed Lie algebras for analysis of the model with quantized microwave field.

physics.optics↗

arXiv2017

Modulation of unpolarized light in planar aligned subwavelength-pitch deformed-helix ferroelectric liquid crystals

We study the electro-optic properties of subwavelength-pitch deformed-helix ferroelectric liquid crystals (DHFLC) illuminated with unpolarized light. In the experimental setup based on the Mach-Zehnder interferometer, it was observed that the reference and the sample beams being both unpolarized produce the interference pattern which is insensitive to rotation of in-plane optical axes of the DHFLC cell. We find that the field-induced shift of the interference fringes can be described in terms of the electrically dependent Pancharatnam relative phase determined by the averaged phase shift, whereas the visibility of the fringes is solely dictated by the phase retardation.

physics.optics↗

arXiv2016

Optical trapping by Laguerre-Gaussian beams: Symmetries, stability and equilibria

We use the T-matrix formalism in combination with the method of far-field matching to evaluate the optical force exerted by Laguerre-Gaussian (LG) light beams on a spherical (Mie) particle. For both non-vortex and optical vortex LG beams, the theoretical results are used to analyze the optical-force-induced dynamics of the scatterer near the trapping points represented by the equilibrium (zero-force) positions. The regimes of linearized dynamics are described in terms of the stiffness matrix spectrum and the damping constant of the ambient medium. For the purely azimuthal LG beams, the dynamics is found to be locally non-conservative and is characterized by the presence of conditionally stable equilibria (unstable zero-force points that can be stabilized by the ambient damping). The effects related to the Mie resonances that under certain conditions manifest themselves as the points changing the trapping properties of the particles are discussed.

physics.optics↗