arXiv subjects

Alexey N. Rubtsov

Publications and source records attributed to Alexey N. Rubtsov.

34 records · Page 2Linked to original sources

arXiv2017

Relaxation and decoherence of qubits encoded in collective states of engineered magnetic structures

The quantum nature of a microscopic system can only be revealed when it is sufficiently decoupled from surroundings. Interactions with the environment induce relaxation and decoherence that turn the quantum state into a classical mixture. Here, we study the timescales of these processes for a qubit encoded in the collective state of a set of magnetic atoms deposited on a metallic surface. For that, we provide a generalization of the commonly used definitions of $T_{1}$ and $T_{2}$ characterizing relaxation and decoherence rates. We calculate these quantities for several atomic structures, including a collective spin, a setup implementing a decoherence-free subspace, and two examples of spin chains. Our work contributes to the comprehensive understanding of the relaxation and decoherence processes and shows the advantages of the implementation of a decoherence free subspace in these setups.

cond-mat.mes-hall↗

arXiv2017

Critical behavior at dynamical phase transition in the generalized Bose-Anderson model

Critical properties of the dynamical phase transition in the quenched generalized Bose-Anderson impurity model are studied in the mean-field limit of an infinite number of channels. The transition separates the evolution toward ground state and toward the branch of stable excited states. We perform numerically exact simulations of a close vicinity of the critical quench amplitude. The relaxation constant describing the asymptotic evolution toward ground state, as well as asymptotic frequency of persistent phase rotation and number of cloud particles at stable excited state are power functions of the detuning from the critical quench amplitude. The critical evolution (separatrix between the two regimes) shows a non-Lyapunov power-law instability arising after a certain critical time. The observed critical behavior is attributed to the irreversibility of the dynamics of particles leaving the cloud and to memory effects related to the low-energy behavior of the lattice density of states.

cond-mat.quant-gas↗

arXiv2017

High harmonic imaging of ultrafast many-body dynamics in strongly correlated systems

This Letter brings together two topics that, until now, have been the focus of intense but non-overlapping research efforts. The first concerns high harmonic generation in solids, which occurs when intense light field excites highly non-equilibrium electronic response in a semiconductor or a dielectric. The second concerns many-body dynamics in strongly correlated systems such as the Mott insulator. Here we show that high harmonic generation can be used to time-resolve ultrafast many-body dynamics associated with optically driven phase transition, with accuracy far exceeding one cycle of the driving light field. Our work paves the way for time-resolving highly non-equilibrium many body dynamics in strongly correlated systems, with few femtosecond accuracy.

cond-mat.str-el↗

arXiv2016

Role of coherence in transport through engineered atomic spin devices

We give a further step in the quantum mechanical description of engineered atomic spin structures by deriving a master equation of the Redfield type that governs the dynamics of the atomic spin density matrix. By generalizing this approach to charge-specific density matrices, we are able to describe magnetic transport quantities, such as the average inelastic current and the shot noise, accessible by tunneling spectroscopy. Our method suitably describes moderate lead-atom coupling regimes where quantum coherence effects cannot be disregarded. We contrast our approach with the existing descriptions in terms of rate equations and show examples where coherence effects are crucial to understand the physics of spin-polarized tunnel current through spin structures.

cond-mat.mes-hall↗

arXiv2016

Analytically continued physical states in the path-integral: a sign-problem-free Quantum Monte Carlo simulation of Bell states dynamics

The derivation of path integrals is reconsidered. It is shown that the expression for the discretized action is not unique, and the path integration domain can be deformed so that at least Gaussian path integrals become probabillistic. This leads to a practical algorithm of sign-problem-free Monte Carlo sampling from the Gaussian path integrals. Moreover, the dynamical influence of Gaussian quantum system (the bath) on any other quantum system can be exactly represented as interaction with classical non-Markovian noise. We discuss the relation of these findings to the Bell's theorem and the Feynman's conjecture on the exponential complexity of the classical simulation of quantum systems. In Feynman's path integral we have quasiprobability distributions for trajectories, and in analitycally continued path integrals we have probability distributions for quasitrajectories.

quant-ph↗

arXiv2016

Role of qubit-cavity entanglement for switching dynamics of quantum interfaces in superconductor metamaterials

We study quantum effects of strong driving field applied to dissipative hybrid qubit-cavity system which are relevant for a realization of quantum gates in superconducting quantum metamaterials. We demonstrate that effects of strong and non-stationary drivings have significantly quantum nature and can not be treated by means of mean-field approximation. This is shown from a comparison of steady state solution of the standard Maxwell-Bloch equations and numerical solution of Lindblad equation on a density matrix. We show that mean-field approach provides very good agreement with the density matrix solution at not very strong drivings $f f^*$ a growing value of quantum correlations between fluctuations in qubit and photon sectors changes a behavior of the system. We show that in regime of non-adiabatic switching on of the driving such a quantum correlations influence a dynamics of qubit and photons even at weak $f$.

quant-ph↗

arXiv2016

Transient phases and dynamical transitions in the post quench evolution of the generalized Bose-Anderson model

The exact description of the time evolution of open correlated quantum systems remains one of the major challenges of the condensed matter theory, specially for asymptotic long times where most numerical methods fail. Here, the post-quench dynamics of the $N$-component Bose-Anderson impurity model is studied in the $N\to\infty$ limit. The equilibrium phase diagram is similar to that of the Bose-Hubbard model in that it contains local versions of the Mott and Bose phases. Using a numerically exact procedure we are able to study the real time evolution including asymptotic long time regimes. The formation of long-lived transient phases is observed for quench paths crossing foreign phases. For quenches inside the local Bose condensed phase, a dynamical phase transition is reported, that separates the evolution towards a new equilibrium state and a regime characterized at large times by a persistent phase rotation of the order parameter. We explain how such non-decaying mode can exist in the presence of a dissipative bath. We discuss the extension of our results to the experimental relevant finite-$N$ case and their implication for the existence of non-decaying modes in generic quantum systems in the presence of a bath.

cond-mat.quant-gas↗

arXiv2015

Quantum statistical ensemble for emissive correlated systems

Relaxation dynamics of complex quantum systems with strong interactions towards the steady state is a fundamental problem in statistical mechanics. The steady state of subsystems weakly interacting with their environment is described by the canonical ensemble which assumes the probability distribution for energy to be of the Boltzmann form. The emergence of this probability distribution is ensured by the detailed balance of the transitions induced by the interaction with the environment. Here we consider relaxation of an open correlated quantum system brought into contact with a reservoir in the vacuum state. We refer to such a system as emissive since particles irreversibly evaporate into the vacuum. The steady state of the system is a statistical mixture of the stable eigenstates arising due to the binding energy. We found that, despite the absence of the detailed balance, the stationary probability distribution over these eigenstates is of the Boltzmann form in each $N$-particle sector. A quantum statistical ensemble corresponding to the steady state is characterized by different temperatures in the different sectors, in a contrast to the Gibbs ensemble. We investigate the transition rates between the eigenstates to understand the emergence of the Boltzmann distribution and find their exponential dependence on the transition energy. We argue that this property of transition rates is generic for a wide class of emissive quantum many-body systems.

cond-mat.stat-mech↗

arXiv2015

Synchronization of qubit ensemble under optimized $π$-pulse driving

We propose technique of simultaneous excitation of disordered qubits providing an effective suppression of inhomogeneous broadening in their spectral density. The technique is based on applying of optimally chosen $π$-pulse with non-rectangular smooth shape. We study excitation dynamics of an off-resonant qubit subjected to strong classical electromagnetic driving field with the fast reference frequency and slow envelope. Within this solution we optimize the envelope to achieve a preassigned accuracy in qubit synchronization.

cond-mat.dis-nn↗

arXiv2015

Polaronic mass renormalization of impurities in BEC: correlated Gaussian wavefunction approach

We propose a class of variational Gaussian wavefunctions to describe Fröhlich polarons at finite momenta. Our wavefunctions give polaron energies that are in excellent agreement with the existing Monte Carlo results for a broad range of interactions. We calculate the effective mass of polarons and find smooth crossover between weak and intermediate impurity-bosons coupling. Effective masses that we obtain are considerably larger than those predicted by the mean-field method. A novel prediction based on our variational wavefunctions is a special pattern of correlations between host atoms that can be measured in time-of-flight experiments. We discuss atomic mixtures in systems of ultracold atoms in which our results can be tested with current experimental technology.

cond-mat.quant-gas↗

arXiv2015

Dispersive Response of a Disordered Superconducting Quantum Metamaterial

We consider a disordered quantum metamaterial formed by an array of superconducting flux qubits coupled to microwave photons in a cavity. We map the system on the Tavis-Cummings model accounting for the disorder in frequencies of the qubits. The complex transmittance is calculated with the parameters taken from state-of-the-art experiments. We demonstrate that photon phase shift measurements allow to distinguish individual resonances in the metamaterial with up to 100 qubits, in spite of the decoherence spectral width being remarkably larger than the effective coupling constant. Our simulations are in agreement with the results of the recently reported experiment.

cond-mat.supr-con↗

arXiv2015

Modeling the metastable dynamics of correlated structures

Metastable quantum dynamics of an asymmetric triangular cluster that is coupled to a reservoir is investigated. The dynamics is governed by bath-mediated transitions, which in part require a thermal activation process. The decay rate is controlled by tuning the excitation spectrum of the frustrated cluster. We use the master equation approach and construct transition operators in terms of many-body states. We analyze dynamics of observables and reveal metastability of an excited state and of a magnetically polarized ground state.

cond-mat.str-el↗

arXiv2014

Mott insulator breakdown through pattern formation

We study the breakdown of a Mott insulator with the thermodynamic imbalance induced by an applied bias voltage. By analyzing the instabilities of the magnetic susceptibility, we describe a rich non-equilibrium phase diagram, obtained for different applied voltages, that exhibits phases with a spatially patterned charge gap. For a finite voltage, smaller than the value of the equilibrium Mott gap, the formation of patterns coincides with the emergence of mid-gap states contributing to a finite steady-state conductance. We discuss the experimental implications of this new scenario of Mott breakdown.

cond-mat.str-el↗

arXiv2014

Magnetostriction and ferroelectric state in AgCrS$_2$

The band structure calculations in the GGA+U approximation show the presence of additional lattice distortions in the magnetically ordered phase of AgCrS$_2$. The magnetostriction leads to formation of the long and short Cr-Cr bonds in the case when respective Cr ions have the same or opposite spin projections. These changes of the Cr lattice are accompanied by distortions of the CrS$_6$ octahedra, which in its turn leads to development of the spontaneous electric polarization.

cond-mat.str-el↗

arXiv2012

Competing phases of the Hubbard model on a triangular lattice -- insights from the entropy

Based on the ladder dual-fermion approach, we present a comprehensive study of the phases of the isotropic Hubbard model on the triangular lattice. We find a rich phase diagram containing most of the phases that have already been experimentally observed in systems where the interplay between geometric frustration and electronic correlations is important: paramagnetic metal, paramagnetic insulator, Mott-insulator with $120^{\circ}$ antiferromagnetic and a non-magnetic insulating state, i.e. possibly a spin liquid state. This establishes that the Hubbard model on frustrated lattices can serve as a minimal model to address the intricate interplay of frustration and correlation. We also show that entropic considerations can be successfully used for understanding many striking features of the triangular systems, such as the large thermopower found in Na$_{x}$CoO$_{2}\cdot$$y$H$_{2}$O.

cond-mat.str-el↗

arXiv2011

Continuous-time Monte Carlo methods for quantum impurity models

Quantum impurity models describe an atom or molecule embedded in a host material with which it can exchange electrons. They are basic to nanoscience as representations of quantum dots and molecular conductors and play an increasingly important role in the theory of "correlated electron" materials as auxiliary problems whose solution gives the "dynamical mean field" approximation to the self energy and local correlation functions. These applications require a method of solution which provides access to both high and low energy scales and is effective for wide classes of physically realistic models. The continuous-time quantum Monte Carlo algorithms reviewed in this article meet this challenge. We present derivations and descriptions of the algorithms in enough detail to allow other workers to write their own implementations, discuss the strengths and weaknesses of the methods, summarize the problems to which the new methods have been successfully applied and outline prospects for future applications.

cond-mat.str-el↗