arXiv subjects

I. Vakulchyk

Publications and source records attributed to I. Vakulchyk.

7 recordsLinked to original sources

arXiv2022

Back action in quantum electro-optic sampling of electromagnetic vacuum fluctuations

The influence of measurement back action on electro-optic sampling of electromagnetic quantum fluctuations is investigated. Based on a cascaded treatment of the nonlinear interaction between a near-infrared coherent probe and the mid-infrared vacuum, we account for the generated electric-field contributions that lead to detectable back action. Specifically, we theoretically address two realistic setups, exploiting one or two probe beams for the nonlinear interaction with the quantum vacuum, respectively. The setup parameters at which back action starts to considerably contaminate the measured noise profiles are determined. Due to the vacuum fluctuations entering at the beam splitter, the shot noise of two incoming probe pulses in different channels is uncorrelated. This leads to the absence of the base-level shot noise in the correlation, while further contributions due to nonlinear shot-noise enhancement are still present. Ultimately, the regime in which electro-optic sampling of quantum fields can be considered as effectively back-action free is found.

quant-ph↗

arXiv2021

Heat percolation in many-body flatband localizing systems

Translationally invariant finetuned single-particle lattice Hamiltonians host flat bands only. Suitable short-range many-body interactions result in complete suppression of particle transport due to local constraints and Many-Body Flatband Localization. Heat can still flow between spatially locked charges. We show that heat transport is forbidden in dimension one. In higher dimensions heat transport can be unlocked by tuning filling fractions across a percolation transition for suitable lattice geometries. Transport in percolation clusters is additionally affected by effective bulk disorder and edge scattering induced by the local constraints, which work in favor of arresting the heat flow. We discuss explicit examples in one and two dimensions.

cond-mat.dis-nn↗

arXiv2021

Nonequilibrium Theory of Photoinduced Valley Hall Effect

A recent scientific debate has arisen: Which processes underlie the actual ground of the valley Hall effect (VHE) in two-dimensional materials? The original VHE emerges in samples with ballistic transport of electrons due to the anomalous velocity terms resulting from the Berry phase effect. In disordered samples though, alternative mechanisms associated with electron scattering off impurities have been suggested: (i) asymmetric electron scattering, called skew scattering, and (ii) a shift of the electron wave packet in real space, called a side jump. It has been claimed that the side jump not only contributes to the VHE but fully offsets the anomalous terms regardless of the drag force for fundamental reasons and, thus, the side-jump together with skew scattering become the dominant mechanisms. However, this claim is based on equilibrium theories without any external valley-selective optical pumping, which makes the results fundamentally interesting but incomplete and impracticable. We develop in this paper a microscopic theory of the photoinduced VHE using the Keldysh nonequilibrium diagrammatic technique and find that the asymmetric skew scattering mechanism is dominant in the vicinity of the interband absorption edge. This allows us to explain the operation of optical transistors based on the VHE.

cond-mat.mes-hall↗

arXiv2020

Time molecules with periodically driven interacting qubits

We provide numerical evidence for a temporal quantum-mechanical interference phenomenon: time molecules (TM). A variety of such stroboscopic states are observed in the dynamics of two interacting qubits subject to a periodic sequence of $π$-pulses with the period $T$. The TMs appear periodically in time and have a large duration, $δt_\mathrm{TM} \gg T$. All TMs demonstrate an almost zero value of the total polarization and a strong enhancement of the entanglement entropy $S$ up to the maximum value $S=\ln 2$ of a corresponding Bell state. The TMs are generated by the commensurability of the Floquet eigenvalues and the presence of maximally entangled Floquet eigenstates. The TMs remain stable with detuned system parameters and with an increased number of qubits. The TMs can be observed in microwave experiments with an array of superconducting qubits.

quant-ph↗

arXiv2018

Almost compact moving breathers with fine-tuned discrete time quantum walks

Discrete time quantum walks are unitary maps defined on the Hilbert space of coupled two-level systems. We study the dynamics of excitations in a nonlinear discrete time quantum walk, whose fine-tuned linear counterpart has a flat band structure. The linear counterpart is, therefore, lacking transport, with exact solutions being compactly localized. A solitary entity of the nonlinear walk moving at velocity $v$ would therefore not suffer from resonances with small amplitude plane waves with identical phase velocity, due to the absence of the latter. That solitary excitation would also have to be localized stronger than exponential, due to the absence of a linear dispersion. We report on the existence of a set of stationary and moving breathers with almost compact superexponential spatial tails. At the limit of the largest velocity $v=1$ the moving breather turns into a completely compact bullet.

nlin.PS↗

arXiv2017

Signatures of many-body localization in steady states of open quantum systems

Many-body localization (MBL) is a result of the balance between interference-based Anderson localization and many-body interactions in an ultra-high dimensional Fock space. It is usually expected that dissipation is blurring interference and destroying that balance so that the asymptotic state of a system with an MBL Hamiltonian does not bear localization signatures. We demonstrate, within the framework of the Lindblad formalism, that the system can be brought into a steady state with non-vanishing MBL signatures. We use a set of dissipative operators acting on pairs of connected sites (or spins), and show that the difference between ergodic and MBL Hamiltonians is encoded in the imbalance, entanglement entropy, and level spacing characteristics of the density operator. An MBL system which is exposed to the combined impact of local dephasing and pairwise dissipation evinces localization signatures hitherto absent in the dephasing-outshaped steady state.

cond-mat.dis-nn↗

arXiv2017

Anderson localization in generalized discrete time quantum walks

We study Anderson localization in a generalized discrete time quantum walk - a unitary map related to a Floquet driven quantum lattice. It is controlled by a quantum coin matrix which depends on four angles with the meaning of potential and kinetic energy, and external and internal synthetic flux. Such quantum coins can be engineered with microwave pulses in qubit chains. The ordered case yields a two-band eigenvalue structure on the unit circle which becomes completely flat in the limit of vanishing kinetic energy. Disorder in the external magnetic field does not impact localization. Disorder in all the remaining angles yields Anderson localization. In particular, kinetic energy disorder leads to logarithmic divergence of the localization length at spectral symmetry points. Strong disorder in potential and internal magnetic field energies allows to obtain analytical expressions for spectrally independent localization length which is highly useful for various applications.

cond-mat.dis-nn↗