arXiv subjects

Gordey B. Lesovik

Publications and source records attributed to Gordey B. Lesovik.

14 recordsLinked to original sources

arXiv2019

Hanbury Brown and Twiss Exchange Correlations in Graphene Box

Quadratic detection in linear mesoscopic transport systems produces cross terms that can be viewed as interference signals reflecting statistical properties of charge carriers. In electronic systems these cross term interferences arise from exchange effects due to Pauli principle. Here we demonstrate fermionic Hanbury Brown and Twiss (HBT) exchange phenomena due to indistinguishability of charge carriers in a diffusive graphene system. These exchange effects are verified using current-current cross correlations in combination with regular shot noise (autocorrelation) experiments at microwave frequencies. Our results can be modeled using semiclassical analysis for a square-shaped metallic diffusive conductor, including contributions from contact transparency. The experimentally determined HBT exchange factor values lie between the calculated ones for coherent and hot electron transport.

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arXiv2016

Time correlators from deferred measurements

Repeated measurements as typically occurring in two- or multi-time correlators rely on von Neumann's projection postulate, telling how to restart the system after an intermediate measurement. We invoke the principle of deferred measurement to describe an alternative procedure where co-evolving quantum memories extract system information through entanglement, combined with a final readout of the memories described by Born's rule. The new approach to repeated quantum measurements respects the unitary evolution of quantum mechanics during intermediate times, unifies the treatment of strong and weak measurements, and reproduces the projected and (anti-) symmetrized correlators in the two limits. As an illustration, we apply our formalism to the calculation of the electron charge correlator in a mesoscopic physics setting, where single electron pulses assume the role of flying memory qubits. We propose an experimental setup which reduces the measurement of the time correlator to the measurement of currents and noise, exploiting the (pulsed) injection of electrons to cope with the challenge of performing short-time measurements.

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arXiv2014

Projective versus weak measurement of charge in a mesoscopic conductor

We study the charge dynamics of a quantum dot as measured by a nearby quantum point contact probing the dot via individual single-particle wave packets. We contrast the two limiting cases of weak and strong system--detector coupling exerting vanishing and strong back-action on the system and analyze the resulting differences in the charge-charge correlator. Extending the study to multiple projective measurements modelling a continuous strong measurement, we identify a transition from a charge dynamics dominated by the system's properties to a universal dynamics governed by the measurement.

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arXiv2013

Statistics of charge transport and unconventional time ordering

The statistics of charge transport across a tunnel junction with energy-dependent scattering is investigated. A model with quadratic dispersion relation is discussed in general and, independently, in the two limiting cases of a large detector and of a linear dispersion relation. The measurement of charge takes place according to various protocols. It is found that, as a rule, the statistics is expressed by means of time-ordered current correlators, differing however from the conventional ones in the ordering prescription. Nevertheless binomial statistics is confirmed in all cases.

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arXiv2012

Scattering matrix approach to interacting electron transport

We investigate the modification in mesoscopic electronic transport due to electron-electron interactions making use of scattering states. We demonstrate that for a specific (finite range) interaction kernel, the knowledge of the scattering matrix is sufficient to take interaction effects into account. We calculate perturbatively the corrections to the current and current-current correlator; in agreement with previous work, we find that, in linear response, interaction effects can be accounted for by an effective (renormalized) transmission probability. Beyond linear response, simple renormalization of scattering coefficients is not sufficient to describe the current-current correlator, as additional corrections arise due to irreducible two-particle processes. Furthermore, we find that the correlations between opposite-spin currents induced by interaction are enhanced for an asymmetric scatterer, generating a nonzero result already to lowest order in the interaction.

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arXiv2011

Single-Particle Excitations Generated by Voltage Pulses

We analyze properties of excitations due to voltage pulses applied to a 1D noninteracting electron gas, assuming that the integral of the voltage over time is equal to the unit of flux. We show that the average charge transfer due to such pulses does not depend on the pulse shape. For pulses with a Lorentzian profile, we prove the single-particle nature of the electron and the hole excitations.

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arXiv2011

Influence of a Random Telegraph Process on the Transport through a Point Contact

We describe the transport properties of a point contact under the influence of a classical two-level fluctuator. We employ a transfer matrix formalism allowing us to calculate arbitrary correlation functions of the stochastic process by mapping them on matrix products. The result is used to obtain the generating function of the full counting statistics of a classical point contact subject to a classical fluctuator, including extensions to a pair of two-level fluctuators as well as to a quantum point contact. We show that the noise in the quantum point contact is a sum of the (quantum) partitioning noise and the (classical) noise due to the two-level fluctuator. As a side result, we obtain the full counting statistics of a quantum point contact with time-dependent transmission probabilities.

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arXiv2002

Bell inequalities and entanglement in solid state devices

Bell-inequality checks constitute a probe of entanglement -- given a source of entangled particles, their violation are a signature of the non-local nature of quantum mechanics. Here, we study a solid state device producing pairs of entangled electrons, a superconductor emitting Cooper pairs properly split into the two arms of a normal-metallic fork with the help of appropriate filters. We formulate Bell-type inequalities in terms of current-current cross-correlators, the natural quantities measured in mesoscopic physics; their violation provides evidence that this device indeed is a source of entangled electrons.

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arXiv2001

Electronic Entanglement in the Vicinity of a Superconductor

A weakly biased normal-metal-superconductor junction is considered as a potential device injecting entangled pairs of quasi-particles into a normal-metal lead. The two-particle states arise from Cooper pairs decaying into the normal lead and are characterized by entangled spin- and orbital degrees of freedom. The separation of the entangled quasi-particles is achieved with a fork geometry and normal leads containing spin- or energy-selective filters. Measuring the current-current cross-correlator between the two normal leads allows to probe the efficiency of the entanglement.

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arXiv2001

Tunable decoherence in the vicinity of a normal metal-superconducting junction

The decoherence rate of a quantum dot coupled to a fluctuating environment described by a normal-metal superconductor junction is considered. The density-density correlator at low frequencies constitutes the kernel which enters the calculation of the phase coherence time. The density fluctuations are connected to the finite frequency current-current correlations in the point contact via the continuity equation. Below and above the gap, at zero temperature the density correlator contains spatial oscillations at half of the Fermi wave length on the normal side. As the bias crosses the superconducting gap, the opening of new scattering channels enhances the decoherence rate dramatically, suggesting the possibility of tuning the decoherence rate in a controllable manner.

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arXiv2000

Effective charges and statistical signatures in the noise of normal metal--superconductor junctions at arbitrary bias

Shot noise is studied in a single normal metal-superconductor (N-S) junction at finite frequency, and for branched N-S junctions at zero frequency. The noise spectral density displays a singularity at the Josephson frequency ($ω= 2eV/\hbar$) when the applied bias is smaller than gap of the superconductor. Yet, in the limit $eV\gg Δ$, quasiparticle contributions yield a singularity at $ω= eV/\hbar$ analogous to that of a normal metal. The crossover between these two regimes shows new structures in the noise characteristic, pointing out the failure of the effective charge model. As an alternative to a finite frequency measurement, if a sinusoidal external field is superposed to the constant bias (non stationary Aharonov--Bohm effect), the second derivative of the zero frequency noise with respect to the voltage exhibits peaks when the frequency of the perturbation is commensurate with the Josephson frequency. Finally, the statistical aspects of noise are studied with an analog of the Hanbury-Brown and Twiss experiment for fermions: a superconductor connected to two normal leads. Noise correlations are found to be either negative (fermionic) or positive (bosonic), due to the presence of evanescent Cooper pairs in the normal side of the junction, in the latter case.

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arXiv1999

Supercurrent Quantization in Narrow Channel SNS Junctions

We determine the quasi-particle excitation spectrum in the normal region of a narrow ballistic superconductor--normal-metal--superconductor (SNS) Josephson contact. Increasing the effective chemical potential in the contact converts the electronic levels into Andreev-levels carrying supercurrent. The opening of these superchannels leads to a supercurrent quantization which exhibits a non-universal behavior in general and we discuss its dependence on the junction parameters.

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arXiv1999

Josephson Frequency Singularity in the Noise of Normal Metal-Superconductor Junctions

A singularity at the Josephson frequency in the noise spectral density of a disordered normal metal -- superconductor junction is predicted for bias voltages below the superconducting gap. The non-stationary Aharonov-Bohm effect, recently introduced for normal metals, is proposed as a tool for detecting this singularity. In the presence of a harmonic external field, the derivative of the noise with respect to the voltage bias reveals jumps when the applied frequency is commensurate with the Josephson frequency associated with this bias. The height of these jumps is non-monotonic in the amplitude of the periodic field. The superconducting flux quantum enters this dependence. Additional singularities in the frequency dependent noise are predicted above gap.

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arXiv1998

Nonlinear Transport through NS Junctions due to Imperfect Andreev Reflection

We investigate a normal metal -- superconductor (point) contact in the limit where the number of conducting channels in the metallic wire is reduced to few channels. As the effective Fermi energy drops below the gap energy, a conducting band with a width twice the Fermi energy is formed. Depending on the mode of operation, the conduction band can be further squeezed, leading to various non-linear effects in the current-voltage characteristics such as current saturation, a N-shaped negative differential resistance, bistability, and hysteresis.

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