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E. I. Kuznetsova

Publications and source records attributed to E. I. Kuznetsova.

17 recordsLinked to original sources
arXiv2026

Teleportation via spin-1/2 chain in solid-state quantum architecture

We propose the protocol for preparing the maximally entangled Bell state between remote qubits at the ends of the spin-1/2 chain governed by the specially engineered nearest-neighbor XX-Hamiltonian with excited central spin as the initial state. This method does not require including optical constituent in the teleportation protocol and can be implemented in the quantum devices with solid-state architecture for teleporting unknown states or organizing quantum gates between remote qubits. A superconducting flux-qubit chain is an example of such devises.

quant-ph↗
arXiv2026

Influence of environment on quantum correlations in two-spin systems with dipole-dipole interactions

An influence of environment on quantum correlations (entanglement and quantum discord) is studied in a two-spin-1/2 system with dipole-dipole interactions on the basis of Lindblad equation. We consider the simplest case when the environment causes only dephasing of system spins. The dependencies of entanglement and the quantum discord on the relaxation rate are obtained. We compare the influence of the environment on entanglement and quantum discord.

quant-ph↗
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↗
arXiv2024

One-excitation spin dynamics in homogeneous closed chain governed by XX-Hamiltonian

We analytically investigate the one-excitation spin dynamics in a homogeneous closed spin-1/2 chain via diagonalization of the one-excitation block of the XX-Hamiltonian, which allows to derive the analytical expressions for probability amplitudes describing state transfers between any two spins of a chain. We analytically investigate the $M$-neighbor approximation ($M\ge 1$) of spin dynamics with arbitrary initial state and analyze its accuracy using special integral characteristics defined in terms of the above probability amplitudes. We find $M$ providing the required accuracy of evolution approximation for chains of different lengths.

quant-ph↗
arXiv2021

Simulation of multiple-quantum NMR dynamics of spin dimer on quantum computer

Dymanics of spin dimers in multiple quantum NMR experiment is studied on the 5-qubit superconducting quantum processor of IBM {Quantum Experience} for the both {pure} ground and thermodynamic equilibrium (mixed) initial states. The work can be considered as a first step towards an application of quantum computers to solving problems of magnetic resonance. This article is dedicated to Prof. Klaus Möbius and Prof. Kev Salikhov on the occasion of their 85th birthdays.

quant-ph↗
arXiv2018

Theoretical investigations of quantum correlations in NMR multiple-pulse spin-locking experiments

Quantum correlations are investigated theoretically in a two-spin system with the dipole-dipole interactions in the NMR multiple-pulse spin-locking experiments. We consider two schemes of the multiple-pulse spin-locking. The first scheme consists of $π/2$-pulses only and the delays between the pulses can differ. The second scheme contains $φ$-pulses ($0<φ<π$) and has equal delays between them. We calculate entanglement for both schemes for an initial separable state. We show that entanglement is absent for the first scheme at equal delays between $π/2$-pulses at arbitraty temperatures. Entanglement emerges after several periods of the pulse sequence in the second scheme at $φ=π/4$ at milliKelvin temperatures. The necessary number of the periods increases with increasing temperature. We demonstrate the dependence of entanglement on the number of the periods of the multiple-pulse sequence. Quantum discord is obtained for the first scheme of the multiple-pulse spin-locking experiment at different temperatures.

quant-ph↗
arXiv2017

Quantum entanglement and quantum discord in dimers in multiple quantum NMR experiments

Quantum correlations in multiple quantum (MQ) NMR experiments are investigated in two-spin systems (dimers). In the initial moment of time one spin is in a pure quantum polarized state and the other spin is in the thermodynamic equilibrium state defined by the temperature of the sample. MQ NMR dynamics of dimers is investigated. It is shown that MQ NMR coherences of only the zeroth and second orders emerge in such a system. The intensities of those coherences are calculated. Entangled states appear in the course of the system evolution in the MQ NMR experiment. The quantum discord is obtained in the high temperature approximation.

quant-ph↗
arXiv2016

Temperature-dependent remote control of polarization and coherence intensity with pure sender's initial state

We study the remote creation of the polarization and intensity of the first-order coherence (or coherence intensity) in long spin-1/2 chains with one qubit sender and receiver. Therewith we use a physically motivated initial condition with the pure state of the sender and the thermodynamical equilibrium state of the other nodes. The main part of the creatable region is a one-to-one map of the initial-state (control) parameters, except the small subregion twice covered by the control parameters, which appears owing to the chosen initial state. The polarization and coherence intensity behave differently in the state creation process. In particular, the coherence intensity can not reach any significant value unless the polarization is large in long chains (unlike the short ones), but the opposite is not true. The coherence intensity vanishes with an increase in the chain length, while the polarization (by absolute value) is not sensitive to this parameter. We represent several characteristics of the creatable polarization and coherence intensity and describe their relation to the parameters of the initial state. The link to the eigenvalue-eigenvector parametrization of the receiver's state-space is given.

quant-ph↗
arXiv2015

Contributions of different parts of spin-spin interactions to quantum correlations in a spin ring model in an external magnetic field

We study quantum correlations in a bipartite heteronuclear $(N-1)\times1$ system in an external magnetic field. The system consists of a spin ring with an arbitrary number $N-1$ of spins on the ring and one spin in its center. The spins on the ring are connected by secular dipole-dipole interactions and interact with the central spin through the Heisenberg $zz$-interaction. We show that the quantum discord, describing quantum correlations between the ring and the central spin, can be obtained analytically for this model in the high temperature approximation. The model allows us to find contributions of different parts of the spin-spin interactions to quantum correlations. We also investigate the evolution of quantum and classical correlations at different numbers of spins.

quant-ph↗
arXiv2013

Mechanism of formation and peculiarities of structure of bulk MgB2 compound specimens

The existence of two phases within one and the same hexagonal lattice of MgB2 compound, differing in Mg and B (in the homogeneity region) and especially in impurity oxygen content, as well as in microstructure, is demonstrated by various techniques. The regions corresponding to these two phases of MgB2 have the sizes of 100-500 μm, and they fill the whole bulk of specimens, alternating with each other. It is suggested that the two-phase state of MgB2 compound is caused by specific features of its formation mechanism (as a result of synthesis at 800-1000°C), including the stages of Mg melting, dissolution of solid boron in it up to the composition of MgB2 and further crystallization of the MgB2 compound from the melt with the formation of dendrite-like structure with corresponding redistribution of main components and impurities.

cond-mat.supr-con↗
arXiv2013

Displacement Waves of Oxygen Atoms in Bi,Pb-2223 Lattice of Composites Annealed in Oxygen Reduced (O2+N2) Atmosphere

Annealing of Bi,Pb-2223/Ag composites in (O2+N2) atmosphere at 820-780C is believed to reduce the number of the accompanying phases, to make contacts between crystallites closer and to increase the critical current. The goals of this study were to reveal the changes in the 2223 lattice at annealing in the reduced oxygen atmosphere, to elucidate the reasons of these changes and to discuss their effect on the ceramics superconductivity. After the annealing the transversely-polarized displacement waves of oxygen atoms in [010]2223 direction have been found in the 2223 phase by electron diffraction analysis. These waves could appear due to the lack of oxygen in the 2223 lattice or to the nitrogen penetration in it. As demonstrated by the X-ray photo-electron spectroscopy and nuclear microanalysis, nitrogen does not interact with the 2223 lattice, and the oxygen index decreases to 9.67, which is lower than the stoichiometric. Thus, the atomic displacement waves result from the lack of oxygen in Bi-O bilayers.

cond-mat.supr-con↗
arXiv2013

Quantum discord in spin systems with dipole-dipole interaction

The behavior of total purely quantum correlation (discord) in dimers consisting of dipolar-coupled spins 1/2 is studied. We found that the discord Q=0 at the absolute zero temperature. With increasing the temperature $T$, at first the quantum correlations in the system increase, smoothly reach the maximum, and then turn again into zero according to the asymptotic law $T^{-2}$. It is also shown that in absence of external magnetic field $B$, the classical correlation $C$ at $T\to0$ is, vice versa, maximal. Our calculations predict that in crystalline gypsum ${\rm CaSO_4\cdot2H_2O}$ the value of natural (B=0) quantum discord between nuclear spins of hydrogen atoms is maximal at the temperature of 0.644 $μ$K, and for 1,2-dichloroethane ${\rm H_2ClC-CH_2Cl}$ the discord achieves the largest value at $T=0.517 μ$K. In both cases, the discord equals $Q\approx0.083$ bit/dimer what is 8.3% of its upper limit in two-qubit systems. We estimate also that for gypsum at room temperature $Q\sim10^{-18}$ bit/dimer, and for 1,2-dichloroethane at T=90 K the discord is $Q\sim10^{-17}$ bit per a dimer.

quant-ph↗
arXiv2012

Quantum correlations in a system of nuclear s=1/2 spins in a strong magnetic field

Entanglement and quantum discord for a pair of nuclear spins $s=1/2$ in a nanopore filled with a gas of spin-carrying molecules (atoms) are studied. The correlation functions describing dynamics of dipolar coupled spins in a nanopore are found. The dependence of spin-pair entanglement on the temperature and the number of spins is obtained from the reduced density matrix, which is centrosymmetric (CS). An analytic expression for the concurrence is obtained for an arbitrary CS density matrix. It is shown that the quantum discord as a measure of quantum correlations attains a significant value at low temperatures. It is shown also that the discord in the considered model has "flickering" character and disappears periodically in the course of the time evolution of the system. The geometric discord is studied for arbitrary $4\times 4$ CS density matrices.

quant-ph↗
arXiv2012

Quantum discord versus second-order MQ NMR coherence intensity in dimers

We consider the quantum discord in a two-particle spin-1/2 system (dimer) governed by the standard multiple quantum (MQ) Hamiltonian and reveal the relation between the discord and the intensity of the second-order MQ coherence. Since the coherence intensities may be measured in experiment, we obtain a method of detecting the quantum discord in experiment. Comparison of discord and concurrence is represented.

quant-ph↗
arXiv2010

High probability state transfer in spin-1/2 chains: Analytical and numerical approaches

This article is devoted to the development of analytical and numerical approaches to the problem of the end-to-end quantum state transfer along the spin-1/2 chain using two methods: (a) a homogeneous spin chain with week end bonds and equal Larmor frequencies and (b) a homogeneous spin chain with end Larmor frequencies different from inner ones. A tridiagonal matrix representation of the XY Hamiltonian with nearest neighbor interactions relevant to the quantum state transfer is exactly diagonalized for a combination of the above two methods. In order to take into account interactions of the remote spins we used numerical simulations of the quantum state transfer in ten-node chains. We compare the state transfer times obtained using the two above methods for chains governed by the both XY and XXZ Hamiltonians and using both nearest neighbor and all node interactions.

quant-ph↗
arXiv2007

Multiple Quantum NMR Dynamics in Dipolar Ordered Spin Systems

We investigate analytically and numerically the Multiple Quantum (MQ) NMR dynamics in systems of nuclear spins 1/2 coupled by the dipole-dipole interactions in the case of the dipolar ordered initial state. We suggest two different methods of MQ NMR. One of them is based on the measurement of the dipolar temperature in the quasi-equilibrium state which establishes after the time of order T2 after the MQ NMR experiment. The other method uses an additional resonance 45^0 -pulse after the preparation period of the standard MQ NMR experiment in solids. Many-spin clusters and correlations are created faster in such experiments than in the usual MQ NMR experiments and can be used for the investigation of many-spin dynamics of nuclear spins in solids.

cond-mat.other↗