arXiv subjects

Sergei Khlebnikov

Publications and source records attributed to Sergei Khlebnikov.

At least 19 recordsLinked to original sources

arXiv2026

Hidden order in dielectrics: string condensation, solitons, and the charge-vortex duality

Description of electrons in a dielectric as solitons of the polarization field requires that the interaction between the solitons (prior to their coupling to electromagnetism) is short-range. We present an analytical study of the mechanism by which this is achieved. The mechanism is unusual in that it enables screening of electrically neutral soliton cores by polarization charges. We also argue that the structure of the solitons allows them to be quantized as either fermions or bosons. At the quantum level, the theory has, in addition to the solitonic electric, elementary magnetic excitations, which give rise to a topological contribution to the magnetic susceptibility.

cond-mat.mes-hall↗

arXiv2026

Entanglement production in the decay of a metastable state

When a metastable state decays into radiation, there must be entanglement between the radiation and the decaying system, as well as between radiation collected at late and early times. We study the interplay between these two types of entanglement in simple Gaussian models in the Markov approximation. We define, via a windowed Fourier transform, multimode quantum states associated with radiation fragments produced at different times and compute the corresponding entanglement entropy increments. On the basis of these results, we argue that such entropy increments are useful entanglement measures, especially in cases, such as Hawking radiation, where one wishes to separate the radiation into ``old'' and ``new.''

quant-ph↗

arXiv2025

Observation of self-oscillating supersonic flow across an acoustic horizon in two dimensions

Understanding the dynamics and stability of transonic flows in quantum fluids, especially for those beyond one spatial dimension, is an outstanding challenge, with applications ranging from nonlinear optics and condensed matter to analogue gravity. One intriguing possibility is that a system with a spatially bounded supersonic flow may evolve into a self-oscillating state that periodically emits solitons, in a process originating from the well-known Landau instability. Here, we report observation of self-oscillating supersonic flows in a two-dimensional atomic superfluid. By imposing a local particle sink with strong loss, we induce a convergent radial flow forming an acoustic analogue of a black-hole horizon and an inner horizon around the sink. The observed superflow appears to be modulated by quasi-periodic bursts of superluminal signals. We measure their frequencies and find agreement with numerical simulations of soliton oscillation frequencies within the black-hole horizon. The presented experiment demonstrates a new method for creating supersonic flows in atomic superfluids, which may find applications in quantum simulations of curved spacetime, supersonic turbulence, and self-oscillating dynamics in dissipative many-body systems.

cond-mat.quant-gas↗

arXiv2025

Entropy flow in a parametric amplifier

Computations of entropy in thermodynamics rely on discreteness of the spectra of the subsystems. We argue that, for cases with continuous spectra (typically, radiation), there is a useful definition of entropy flow based on discretizing the signal into Gabor's "atoms," say, by means of a windowed Fourier transform. In particular, applying this method to a parametric amplifier (paramp) driven by a classical pump and coupled to a zero-temperature Markovian bath, we find that the output entropy flux vanishes at large times, even though the energy and photon number fluxes remain nonzero. This is consistent with the manner in which the paramp is expected to release information about its initial state. We relate the quenching of the entropy flow to development of the off-diagonal coherences in the output and discuss possible relevance of this mechanism to the black-hole information problem. We also propose to use measurements of the off-diagonal coherences as a means of extracting the rates at which high-entropy subsystems release information to linear environments.

quant-ph↗

arXiv2025

Cherenkov emission by a fast-moving uncharged Schwarzschild black hole

We demonstrate that, in the presence of an external magnetic field, an uncharged classical Schwarzschild black hole moving superluminally in a dielectric with permittivity $ε> 1$ produces Cherenkov emission. This is a new physical effect: classical (non-quantum) emission of electromagnetic waves by a completely charge-neutral ``particle.'' The governing equations (involving general relativity, electromagnetism, and the physics of continuous media) have no external electromagnetic source -- it is the distortion of the initial electromagnetic fields by the gravity of the black hole that plays the role of a superluminally moving source. The effect relies on nonzero values of both the magnetic field and the gravitational radius, as well as on the usual Cherenkov condition on the velocity, $v/c > 1/\sqrtε$. Unlike Cherenkov emission by a point charge, the effective source in this case is spatially distributed, with emission generated along the single Cherenkov emission cone. The emitted spectrum is red-dominated, with power $\propto dk_z /|k_z|$ for wave numbers $|k_z| \leq 1/R_G$, where $R_G$ is the Schwarzschild radius. We comment on possible observability of this process during black hole -- neutron star mergers.

hep-th↗

arXiv2024

Acoustic horizon as a phase-slip surface

A recent experiment has demonstrated formation of a supersonic region in a convergent two-dimensional flow of a condensate of cesium atoms. Theoretical description of this effect has made use of stationary solutions to the Gross-Pitaevskii equation with a 3-body dissipative term. Here, we further develop that description, focusing on a new stationary solution, linear stability analysis, and properties of the time-dependent "resistive" state.

cond-mat.quant-gas↗

arXiv2021

Entanglement entropy of a superflow

We consider the theory of $N$ free Dirac fermions with a uniformly winding mass, $m e^{iqx}$, in two spacetime dimensions. This theory (which describes for instance a superconducting current in an $N$-channel wire) has been proposed to have a higher-spin gravity with scalar matter as the large-$N$ dual. To order $m^2$, however, thermodynamic quantities in it can be computed using standard general relativity instead. Here, we consider the question if the same is true for the entanglement entropy (EE). By comparing results obtained on two sides of the duality, we find that general relativity indeed accounts correctly for the EE of an interval to order $m^2$ (and all orders in $q$).

hep-th↗

arXiv2021

Observation of quasiparticle pair-production and quantum entanglement in atomic quantum gases quenched to an attractive interaction

We report observation of quasiparticle pair-production and characterize quantum entanglement created by a modulational instability in an atomic superfluid. By quenching the atomic interaction to attractive and then back to weakly repulsive, we produce correlated quasiparticles and monitor their evolution in a superfluid through evaluating the in situ density noise power spectrum, which essentially measures a 'homodyne' interference between ground state atoms and quasiparticles of opposite momenta. We observe large amplitude growth in the power spectrum and subsequent coherent oscillations in a wide spatial frequency band within our resolution limit, demonstrating coherent quasiparticle generation and evolution. The spectrum is observed to oscillate below a quantum limit set by the Peres-Horodecki separability criterion of continuous-variable states, thereby confirming quantum entanglement between interaction quench-induced quasiparticles.

cond-mat.quant-gas↗

arXiv2020

A universal holographic prediction for quantum-critical dynamics

We consider decay of an initial density or current perturbation at finite temperature $T$ near a quantum critical point with emergent Lorentz invariance. We argue that decay of perturbations with wavenumbers $k \gg T$ (in natural units) is a good testing ground for holography---existence of a dual gravitational description---in experimentally accessible systems. The reason is that, computed holographically, the decay rate at large $k$ depends only on the leading correction to the metric near the boundary, and that is quite universal. In the limit of zero detuning (when the temperature is the only dimensionful parameter), the result is a scaling law for the decay rate, with the exponent that depends only on the dimensionality. We show that this follows from an analytical argument and is borne out by a numerical study of quasinormal modes.

hep-th↗

arXiv2019

The first heat: production of entanglement entropy in the early universe

Entanglement entropy (EE) of a spatial region quantifies correlations between the region and its surroundings. For a free scalar in the adiabatic vacuum in de Sitter space the EE is known to remain low, scaling as the surface area of the region. Here, we study the evolution of entanglement after the universe transitions from de Sitter to flat space. We concentrate on the case of a massless minimally coupled scalar. We find numerically that, after the de Sitter stage ends, the EE and the Rényi entropy rapidly grow and saturate at values obeying the volume law. The final state of the subsystem (region) is a partially thermalized state reminiscent of a generalized Gibbs ensemble. We comment on application of our results to the question of when and how cosmological perturbations decohere.

hep-th↗

arXiv2019

Highly skewed current-phase relation in superconductor-topological insulator-superconductor Josephson junctions

Three-dimensional topological insulators (TI's) in proximity with superconductors are expected to exhibit exotic phenomena such as topological superconductivity (TSC) and Majorana bound states (MBS), which may have applications in topological quantum computation. In superconductor-TI-superconductor Josephson junctions, the supercurrent versus the phase difference between the superconductors, referred to as the current-phase relation (CPR), reveals important information including the nature of the superconducting transport. Here, we study the induced superconductivity in gate-tunable Josephson junctions (JJs) made from topological insulator BiSbTeSe2 with superconducting Nb electrodes. We observe highly skewed (non-sinusoidal) CPR in these junctions. The critical current, or the magnitude of the CPR, increases with decreasing temperature down to the lowest accessible temperature (T ~ 20 mK), revealing the existence of low-energy modes in our junctions. The gate dependence shows that close to the Dirac point the CPR becomes less skewed, indicating the transport is more diffusive, most likely due to the presence of electron/hole puddles and charge inhomogeneity. Our experiments provide strong evidence that superconductivity is induced in the highly ballistic topological surface states (TSS) in our gate-tunable TI- based JJs. Furthermore, the measured CPR is in good agreement with the prediction of a model which calculates the phase dependent eigenstate energies in our system, considering the finite width of the electrodes as well as the TSS wave functions extending over the entire circumference of the TI.

cond-mat.supr-con↗

arXiv2018

Local models and global constraints for degeneracies and band crossings

We study topological properties of families of Hamiltonians which may contain degenerate energy levels aka. band crossings. The primary tool are Chern classes, Berry phases and slicing by surfaces. To analyse the degenerate locus, we study local models. These give information about the Chern classes and Berry phases. We then give global constraints for the topological invariants. This is an hitherto relatively unexplored subject. The global constraints are more strict when incorporating symmetries such as time reversal symmetries. The results can also be used in the study of deformations. We furthermore use these constraints to analyse examples which include the Gyroid geometry, which exhibits Weyl points and triple crossings and the honeycomb geometry with its two Dirac points.

math-ph↗

arXiv2018

Landau-Zener transitions for Majorana fermions

One-dimensional systems obtained as low-energy limits of hybrid superconductor-topological insulator devices provide means of production, transport, and destruction of Majorana bound states (MBSs) by variations of the magnetic flux. When two or more pairs of MBSs are present in the intermediate state, there is a possibility of a Landau-Zener transition, wherein even a slow variation of the flux leads to production of a quasiparticle pair. We study numerically a version of this process, with four MBSs produced and subsequently destroyed, and find that, quite universally, the probability of quasiparticle production in it is 50%. This implies that the effect may be a limiting factor in applications requiring a high degree of quantum coherence.

cond-mat.supr-con↗

arXiv2017

Metastability and bifurcation in superconducting nanorings

We describe an approach, based on direct numerical solution of the Usadel equation, to finding stationary points of the free energy of superconducting nanorings. We consider both uniform (equilibrium) solutions and the critical droplets that mediate activated transitions between them. For the uniform solutions, we compute the critical current as a function of the temperature, thus obtaining a correction factor to Bardeen's 1962 interpolation formula. For the droplets, we present a metastability chart that shows the activation energy as a function of the temperature and current. A comparison of the activation energy for a ring to experimental results for a wire connected to superconducting leads reveals a discrepancy at large currents. We discuss possible reasons for it. We also discuss the nature of the bifurcation point at which the droplet merges with the uniform solution.

cond-mat.supr-con↗

arXiv2017

Decay of near-critical currents in superconducting nanowires

We consider decay of supercurrent via phase slips in a discrete one-dimensional superconductor (a chain of nodes connected by superconducting links), aiming to explain the experimentally observed power-3/2 scaling of the activation barrier in nanowires at currents close to the critical. We find that, in this discrete model, the power-3/2 scaling holds for both long and short wires even in the presence of bulk superconducting leads, despite the suppression of thermal fluctuations at the ends. We also consider decay via tunneling (quantum phase slips), which becomes important at low temperatures. We find numerically the relevant Euclidean solutions (periodic instantons) and determine the scaling of the tunneling exponent near the critical current. The scaling law is power-5/4, different from that of the thermal activation exponent.

cond-mat.supr-con↗

arXiv2015

Formation of Quantum Phase Slip Pairs in Superconducting Nanowires

Macroscopic quantum tunneling (MQT) is a fundamental phenomenon of quantum mechanics related to the actively debated topic of quantum-to-classical transition. The ability to realize MQT affects implementation of qubit-based quantum computing schemes and their protection against decoherence. Decoherence in qubits can be reduced by means of topological protection, e.g. by exploiting various parity effects. In particular, paired phase slips can provide such protection for superconducting qubits. Here, we report on the direct observation of quantum paired phase slips in thin-wire superconducting loops. We show that in addition to conventional single phase slips that change superconducting order parameter phase by $2π$, there are quantum transitions changing the phase by $4π$. Quantum paired phase slips represent a synchronized occurrence of two macroscopic quantum tunneling events, i.e. cotunneling. We demonstrate the existence of a remarkable regime in which paired phase slips are exponentially more probable than single ones.

cond-mat.supr-con↗

arXiv2014

Gapless superconductivity and string theory

Coexistence of superconducting and normal components in nanowires at currents below the critical (a "mixed" state) would have important consequences for the nature and range of potential applications of these systems. For clean samples, it represents a genuine interaction effect, not seen in the mean-field theory. Here we consider properties of such a state in the gravity dual of a strongly coupled superconductor constructed from D3 and D5 branes. We find numerically uniform gapless solutions containing both components but argue that they are unstable against phase separation, as their free energies are not convex. We speculate on the possible nature of the resulting non-uniform sate ("emulsion") and draw analogies between that state and the familiar mixed state of a type II superconductor in a magnetic field.

hep-th↗

arXiv2014

Thermalization of isolated quantum systems

Understanding the evolution towards thermal equilibrium of an isolated quantum system is at the foundation of statistical mechanics and a subject of interest in such diverse areas as cold atom physics or the quantum mechanics of black holes. Since a pure state can never evolve into a thermal density matrix, the Eigenstate Thermalization Hypothesis (ETH) has been put forward by Deutsch and Srednicki as a way to explain this apparent thermalization, similarly to what the ergodic theorem does in classical mechanics. In this paper this hypothesis is tested numerically. First, it is observed that thermalization happens in a subspace of states (the Krylov subspace) with dimension much smaller than that of the total Hilbert space. We check numerically the validity of ETH in such a subspace, for a system of hard core bosons on a two-dimensional lattice. We then discuss how well the eigenstates of the Hamiltonian projected on the Krylov subspace represent the true eigenstates. This discussion is aided by bringing the projected Hamiltonian to the tridiagonal form and interpreting it as an Anderson localization problem for a finite one-dimensional chain. We also consider thermalization of a subsystem and argue that generation of a large entanglement entropy can lead to a thermal density matrix for the subsystem well before the whole system thermalizes. Finally, we comment on possible implications of ETH in quantum gravity.

cond-mat.stat-mech↗