arXiv subjects

M. A. Yurischev

Publications and source records attributed to M. A. Yurischev.

At least 19 recordsLinked to original sources

arXiv2025

Quantum correlations versus spin magnitude: Transition to the classical limit

Quantum-classical transitions have long attracted much attention. We study such transitions in quantum spin-($j$,1/2) systems at thermal equilibrium. Unlike the previous work [Phys. Rev. A 73, 064302 (2006)], it is found that the threshold temperature of quantum entanglement decreases with increasing spin $j$ and completely disappears in the limit $j\to\infty$. In the ground state of systems with highly symmetric interactions, the discord-type quantum correlations can exist even for arbitrarily large spin. Such correlations turn out to be unstable and are destroyed by small perturbations that violate the symmetry of the Hamiltonian. The stable quantum correlations gradually degrade as the spin $j$ grows and eventually vanish when the classical limit is reached.

quant-ph↗

arXiv2025

Quantum correlations in general qubit-qudit axially symmetric states

The development and improvement of analytical methods for evaluating nonclassical correlations is one of the most important tasks in quantum information science. In this paper, we investigate a mixed spin-$(1/2,S)$ system with an arbitrary spin $S$, where the interactions satisfy the U(1) axial symmetry. Analytical formulas for the local quantum uncertainty (LQU) and local quantum Fisher information (LQFI) are derived directly from the elements and eigenvalues of the density matrix. These results are then used to conduct a comparative analysis of the discord-like quantum correlations, LQU and LQFI, in the system at thermal equilibrium. The high-temperature asymptotics of both quantum correlations are found explicitly. Despite the destructive role of temperature in general, the calculations show that the quantum correlations can increase with temperature in local intervals. Under certain conditions, temperature even generates quantum correlations from uncorrelated ground states. Further, as the system cools, quantum correlations can undergo a series of abrupt transitions with a smooth temperature change. These phenomena are demonstrated for different choices of coupling parameters and spin lengths $S$.

quant-ph↗

arXiv2025

A comparative study of LQU and LQFI in general qubit-qutrit axially symmetric states

We derive the compact closed forms of local quantum uncertainty (LQU) and local quantum Fisher information (LQFI) for hybrid qubit-qutrit axially symmetric (AS) states. This allows us to study the quantum correlations in detail and present some essentially novel results for spin-(1/2, 1) systems, the Hamiltonian of which contains ten independent types of physically important parameters. As an application of the derived formulas, we study the behavior of these two quantum correlation measures at thermal equilibrium. New features are observed in their behavior that are important for quantum information processing. Specifically, cascades of sudden changes in the behavior of LQU and LQFI are found with a smooth change in temperature or interaction parameters. Interestingly, in some cases, sudden transitions are observed in the behavior of LQU but not in LQFI, and vice versa. Moreover, our compact formulas open a way to apply them to other problems, for instance, when investigating the environmental effects on quantum correlations in open systems.

quant-ph↗

arXiv2024

Quantumness near a Schwarzschild black hole

The merging of quantum information science with the relativity theory presents novel opportunities for understanding the enigmas surrounding the transmission of information in relation to black holes. For this purpose, we study the quantumness near a Schwarzschild black hole in a practical model under decoherence. The scenario we consider in this paper is that a stationary particle in the flat region interacts with its surroundings while another particle experiences free fall in the vicinity of a Schwarzschild black hole's event horizon. We explore the impacts of Hawking radiation and decoherence on the system under investigation and find that these effects can limit the survival of quantum characteristics, but cannot destroy them completely. Hence, the results of this study possess the potential to yield valuable insights into the comprehension of the quantum properties of a real system operating within a curved space-time framework.

gr-qc↗

arXiv2023

Local quantum Fisher information and local quantum uncertainty for general X-states

A two-spin-1/2 Heisenberg XYZ model with Dzyaloshinsky--Moriya (DM) and Kaplan--Shekhtman--Entin-Wohlman--Aharony (KSEA) interactions in the presence of an inhomogeneous external magnetic field is considered at thermal equilibrium. Its density matrix has the general X form for which we derive explicit formulas for the local quantum Fisher information (LQFI) and local quantum uncertainty (LQU) directly in terms of model parameters. This allows us to perform a comparative study for the discord-type quantum correlations LQFI and LQU and to reveal a number of new features in their behavior. In particular, the sudden transitions of quantum correlations with a smooth change in temperature are found. Moreover, it is shown that there may be a sequence of such transitions for certain choices of interaction constants.

quant-ph↗

arXiv2023

Quantum Otto heat engines on XYZ spin working medium with DM and KSEA interactions: Operating modes and efficiency at maximal work output

The magnetic Otto thermal machine based on a two-spin-1/2 XYZ working fluid in the presence of an inhomogeneous magnetic field and antisymmetric Dzyaloshinsky--Moriya (DM) and symmetric Kaplan--Shekhtman--Entin-Wohlman--Aharony (KSEA) interactions is considered. Its possible modes of operation are found and classified. The efficiencies of engines at maximum power are estimated for various choices of model parameters. There are cases when these efficiencies exceed the Novikov value. New additional points of local minima of the total work are revealed and the mechanism of their occurrence is analyzed.

quant-ph↗

arXiv2022

Behavior of quantum discord, local quantum uncertainty, and local quantum Fisher information in two-spin-1/2 Heisenberg chain with DM and KSEA interactions

A two-qubit Heisenberg XYZ model with Dzyaloshinsky-Moriya (DM) and Kaplan-Shekhtman-Entin-Wohlman--Aharony (KSEA) interactions is considered at thermal equilibrium. Analytical formulas are derived for the local quantum uncertainty (LQU) and local quantum Fisher information (LQFI). Using the available expressions for the entropic quantum discord, we perform a comparative study of these measures of nonclassical correlation. Our analysis showed the following: all three measures of quantum correlation have similar qualitative and even quantitative behavior on temperature for different values of system parameters, there are regions in the parameter space which correspondent to a local increase of correlations with increasing temperature, and sudden changes in the behavior of quantum correlations occur at certain values of the interaction parameters.

quant-ph↗

arXiv2021

Quantum entanglement in the anisotropic Heisenberg model with multicomponent DM and KSEA interactions

Using group-theoretical approach we found a family of four nine-parameter quantum states for the two-spin-1/2 Heisenberg system in an external magnetic field and with multiple components of Dzyaloshinsky-Moriya (DM) and Kaplan-Shekhtman-Entin-Wohlman-Aharony (KSEA) interactions. Exact analytical formulas are derived for the entanglement of formation for the quantum states found. The influence of DM and KSEA interactions on the behavior of entanglement and on the shape of disentangled region is studied. A connection between the two-qubit quantum states and the reduced density matrices of many-particle systems is discussed.

quant-ph↗

arXiv2021

On the quantum correlations in two-qubit XYZ spin chains with Dzyaloshinsky-Moriya and Kaplan-Shekhtman-Entin-Wohlman-Aharony interactions

The anisotropic Heisenberg two-spin-1/2 model in an inhomogeneous magnetic field with both antisymmetric Dzyaloshinsky-Moriya and symmetric Kaplan-Shekhtman-Entin-Wohlman-Aharony cross interactions is considered at thermal equilibrium. Using a group-theoretical approach, we find fifteen spin Hamiltonians and as many corresponding Gibbs density matrices (quantum states) whose eigenvalues are expressed only through square radicals. We also found local unitary transformations that connect nine of this fifteen state collection, and one of them is the X quantum state. Since such quantum correlations as quantum entanglement, quantum discord, one-way quantum work deficit, and others are known for the X state, this allows to get the quantum correlations for any member from the nine state family. Further, we show that the remaining six quantum states are separable, that they are also connected by local unitary transformations, but, however, now the case with known correlations beyond entanglement is generally not available.

quant-ph↗

arXiv2019

Temperature-field phase diagrams of one-way quantum work deficit in two-qubit XXZ spin systems

The spin-1/2 XXZ chain in a uniform magnetic field at thermal equilibrium is considered. For this model, we give a complete classification of all qualitatively different phase diagrams for the one-way quantum work (information) deficit. The diagrams can contain regions (phases, fractions) with both stationary and variable (state-dependent) angles of optimal measurement. We found cases of phase diagrams in which the sizes of regions with the variable optimal measurement angle are large and perhaps such regions can be detected experimentally. We also established a relationship between the behavior of optimal measurement angles near the boundaries separated different regions and Landau's theory of phase transitions of the second and first kind.

quant-ph↗

arXiv2018

Phase diagram for the one-way quantum deficit of two-qubit X states

The one-way quantum deficit, a measure of quantum correlation, can exhibit for X quantum states the regions (subdomains) with the phases $Δ_0$ and $Δ_{π/2}$ which are characterized by constant (i.e., universal) optimal measurement angles, correspondingly, zero and $π/2$ with respect to the $z$-axis and a third phase $Δ_\vartheta$ with the variable (state-dependent) optimal measurement angle $\vartheta$. We build the complete phase diagram of one-way quantum deficit for the XXZ subclass of symmetric X states. In contrast to the quantum discord where the region for the phase with variable optimal measurement angle is very tiny (more exactly, it is a very thin layer), the similar region $Δ_\vartheta$ is large and achieves the sizes comparable to those of regions $Δ_0$ and $Δ_{π/2}$. This instils hope to detect the mysterious fraction of quantum correlation with the variable optimal measurement angle experimentally.

quant-ph↗

arXiv2017

Bimodal behavior of post-measured entropy and one-way quantum deficit for two-qubit X states

A method for calculating the one-way quantum deficit is developed. It involves a careful study of post-measured entropy shapes. We discovered that in some regions of X-state space the post-measured entropy $\tilde S$ as a function of measurement angle $θ\in[0,π/2]$ exhibits a bimodal behavior inside the open interval $(0,π/2)$, i.e., it has two interior extrema: one minimum and one maximum. Furthermore, cases are found when the interior minimum of such a bimodal function $\tilde S(θ)$ is less than that one at the endpoint $θ=0$ or $π/2$. This leads to the formation of a boundary between the phases of one-way quantum deficit via {\em finite} jumps of optimal measured angle from the endpoint to the interior minimum. Phase diagram is built up for a two-parameter family of X states. The subregions with variable optimal measured angle are around 1$\%$ of the total region, with their relative linear sizes achieving $17.5\%$, and the fidelity between the states of those subregions can be reduced to $F=0.968$. In addition, a correction to the one-way deficit due to the interior minimum can achieve $2.3\%$. Such conditions are favorable to detect the subregions with variable optimal measured angle of one-way quantum deficit in an experiment.

quant-ph↗

arXiv2017

Extremal properties of conditional entropy and quantum discord for XXZ symmetric quantum states

For the XXZ subclass of symmetric two-qubit X states, we study the behavior of quantum conditional entropy S_{cond} as a function of measurement angle θ\in[0,π/2]. Numerical calculations show that the function S_{cond}(θ) for X states can have at most one local extremum in the open interval from zero to π/2 (unimodality property). If the extremum is a minimum the quantum discord displays region with variable (state-dependent) optimal measurement angle θ^*. Such θ-regions (phases, fractions) are very tiny in the space of X state parameters. We also discover the cases when the conditional entropy has a local maximum inside the interval (0,π/2). It is remarkable that the maxima exist in surprisingly wide regions and the boundaries for such regions are defined by the same bifurcation conditions as for those with a minimum. Moreover, the found maxima can exceed the conditional entropy values at the ends of interval [0,π/2] more than by 1%. This instils hope in the possibility to detect such maxima in experiment.

quant-ph↗

arXiv2016

Experimental Exploration of Quantum Correlations: Discord versus Entanglement

quant-ph↗

arXiv2016

Exploring Quantum Correlations from Discord to Entanglement

Quantum correlations represent a fundamental tool for studies ranging from basic science to quantum technologies. Different non-classical correlations have been identified and studied, as entanglement and discord. In this Paper we explore experimentally the rich geometry of polarization Bell-diagonal states. By taking advantage of the statistical method of generation, the values of entanglement and discord along different trajectories in the space of the parameters of density matrix have been measured. The effects of sudden death of entanglement and complete "freeze" of discord were investigated in order to detect the domains with different domination of one type of quantum correlation against to other. A geometric interpretation for each considered phenomena is addressed. The observed good agreement between experiment and theory for all investigated trajectories ensures the reliability of this method.

quant-ph↗

arXiv2015

NMR dynamics of quantum discord for spin-carrying gas molecules in a closed nanopore

A local orthogonal transformation that transforms any centrosymmetric (CS) matrix of fourth order to the X form is found. A picewise-analytic-numerical formula Q=min{Q_π/2,Q_θ,Q_0}, where Q_π/2 and Q_0 are analytical expressions and the branch Q_θ is found by numerical searching the optimal measurement angle θ\in(0,π/2), is proposed to calculate the quantum discord Q of general X state. The developed approaches are applied to a quantitative description of recently predicted flickerings (periodic birth and death) of the quantum-information pair correlation between nuclear 1/2 spin of atoms or molecules of a gas (for examples Xe^129) in a finite asymmetric volume in the presence of a strong magnetic field.

quant-ph↗

arXiv2015

On the quantum discord of general X states

Quantum discord Q is a function of density matrix elements. The domain of such a function in the case of two-qubit system with X density matrix may consist of three subdomains at most: two ones where the quantum discord is expressed in closed analytical forms (Q_{π/2} and Q_0) and an intermediate subdomain for which, to extract the quantum discord Q_θ, it is required to solve in general numerically a one-dimensional minimization problem to find the optimal measurement angle θ\in(0,π/2). Hence the quantum discord is given by a piecewise-analytic-numerical formula Q=\min{Q_{π/2}, Q_θ, Q_0}. Equations for determining the boundaries between these subdomains are obtained. The boundaries consist of bifurcation points. The Q_θ subdomains are discovered in the generalized Horodecki states, in the dynamical phase flip channel model, in the anisotropic spin systems at thermal equilibrium, in the heteronuclear dimers in an external magnetic field. We found that transitions between Q_θ subdomain and Q_{π/2} and Q_0 ones occur suddenly but continuously and smoothly, i.e., nonanalyticity is hidden and can be observed in higher derivatives of discord function.

quant-ph↗

arXiv2014

Quantum discord for general X and CS states: A piecewise-analytic-numerical formula

Quantum discord is a function of density-matrix elements (and through them, e.~g., of temperature, applied fields, time, and so forth). The domain of such a function in the case of two-qubit system with X or centrosymmetric (CS) density matrix can consist at most of three subdomains: two ones, where the quantum discord is expressed in closed analytical forms (Q_0 and Q_{π/2}), and an intermediate subdomain in which for determining the quantum discord Q_θit is required to solve numerically a one-dimensional minimization problem to find the optimal measurement angle θ\in(0,π/2). Exact equations for determining the boundaries between these subdomains are obtained and solved for a number of models. The Q_θsubdomains are discovered in the anisotropic spin dimers in external field. On the other hand, coinciding boundaries and therefore sudden transitions between optimal measurement angles θ=π/2 and θ=0 are observed in dynamics of spin currying particles in closed nanopore and also in phase flip channels. In latter cases the solutions are entirely analytical.

quant-ph↗