arXiv subjects

Izhar Neder

Publications and source records attributed to Izhar Neder.

13 recordsLinked to original sources

arXiv2025

Efficient calculation of reactor noise via Ito-Langevin Process for correlated fluctuations

We derive an Ito-Langevin stochastic process that captures the time-dependent deviation from Poisson behavior of the noise detected from a general heterogeneous sub-critical neutron system. Using the probability generating function for the actual physical process, we deduce the super-Poisson deviation of the covariance matrix of counts at the detector due to neutron multiplication upon fission. This leads to a general form that coincides with the second moment of an Ito process. This comparison facilitates the formulation of a corresponding effective Langevin equation, which potentially enables simulations that significantly reduce the computational resources required compared to direct simulation of the system's actual noise. This method could assist in designing sub-critical noise experiments for licensing new research reactors, for improving cross-section libraries and for non-destructive assays of spent fuel.

cond-mat.stat-mech↗

arXiv2024

A Future-Input Dependent model for Greenberger-Horne-Zeilinger correlations

It is widely appreciated, due to Bell's theorem, that quantum phenomena are inconsistent with local-realist models. In this context, locality refers to local causality, and there is thus an open possibility for reproducing the quantum predictions with models which internally violate the causal arrow of time, while otherwise adhering to the relevant locality condition. So far, this possibility has been demonstrated only at a toy-model level, and only for systems involving one or two spins (or photons). The present work extends one of these models to quantum correlations between three or more spins which are entangled in the Greenberger-Horne-Zeilinger state.

quant-ph↗

arXiv2023

Bloch Oscillations, Landau-Zener Transition, and Topological Phase Evolution in a Pendula Array

We experimentally and theoretically study the dynamics of a one-dimensional array of pendula with a mild spatial gradient in their self-frequency and where neighboring pendula are connected with weak and alternating coupling. We map their dynamics to the topological Su-Schrieffer-Heeger (SSH) model of charged quantum particles on a lattice with alternating hopping rates in an external electric field. By directly tracking the dynamics of a wavepacket in the bulk of the lattice, we observe Bloch oscillations, Landau-Zener transitions, and coupling between the isospin (i.e. the inner wave function distribution within the unit cell) and the spatial degrees of freedom (the distribution between unit cells). We then use Bloch oscillations in the bulk to directly measure the non-trivial global topological phase winding and local geometric phase of the band. We measure an overall evolution of 3.1 $\pm$ 0.2 radians for the geometrical phase during the Bloch period, consistent with the expected Zak phase of $π$. Our results demonstrate the power of classical analogs of quantum models to directly observe the topological properties of the band structure, and sheds light on the similarities and the differences between quantum and classical topological effects.

cond-mat.mes-hall↗

arXiv2014

Shot-Noise in Fractional Wires: a Universal Fano-Factor Different than the Tunneling Charge

We consider partially gapped one dimensional (1D) conductors connected to normal leads, as realized in fractional helical wires. At certain electron densities, some distinct charge mode develops a gap due to electron interactions, leading to a fractional conductance. For this state we study the current noise caused by tunneling events inside the wire. We find that the noise's Fano-factor is different from the tunneling charge. This fact arises from charge scattering at the wire-leads interfaces. The resulting noise is, however, universal - it depends only on the identification of the gapped mode, and is insensitive to additional interactions in the wire. We further show that the tunneling charge can be deduced from the finite frequency noise, and yet is interaction dependent due to screening effects.

cond-mat.mes-hall↗

arXiv2013

Fractional charges in emergent neutral modes at the integer quantum Hall effect

Charge fractionalization is a possible emergent excitation in a low-dimensional system of interacting electrons. A known example is that of fractional charges in the fractional quantum Hall effect (FQHE) regime, which is a consequence of strong Coulomb interaction among the electrons whose kinetic energy is quenched by the strong magnetic field. Alternatively, the integer QHE (IQHE), with electrons behaving largely as independent particles in Landau levels (LLs), lacks such fractionalization. However, for integer LLs filling v=2, 3,... electrons propagate in copropagating adjacent chiral edge channels, and thus interact and modify the non-interacting LLs. For example, at v=2, an electron injected selectively into a single non-interacting (bare) edge channel is expected to decompose into a 'fast' mode and a 'slow' mode in the region of interaction; each mode carry fractional charges shared between the two bare channels. Here, we report our sensitive shot noise measurement that affirms the presence of such fractionalization in v=2. Injecting partitioned current into a 'hot' edge channel led to low frequency shot noise in the adjacent currentless 'cold' edge channel after it had been partitioned. Controlling the partitioning of the hot and cold channels allowed a determination of the fractional charges in both channels as well as the channels' velocity difference. This approach can be easily extended to study interaction in two-dimensional systems with a topology dictating edge channels transport.

cond-mat.mes-hall↗

arXiv2013

The theory of coherent dynamic nuclear polarization in quantum dots

We consider the dynamic nuclear spin polarization (DNP) using two electrons in a double quantum dot in presence of external magnetic field and spin-orbit interaction, in various schemes of periodically repeated sweeps through the S-T+ avoided crossing. By treating the problem semi-classically, we find that generally the DNP have two distinct contributions - a geometrical polarization and a dynamic polarization, which have different dependence on the control parameters such as the sweep rates and waiting times in each period. Both terms show non-trivial dependence on those control parameter. We find that even for small spin-orbit term, the dynamical polarization dominates the DNP in presence of a long waiting period near the S-T+ avoided crossing, of the order of the nuclear Larmor precession periods. A detailed numerical analysis of a specific control regime can explain the oscillations observed by Foletti et.~al.~in arXiv:0801.3613.

cond-mat.mes-hall↗

arXiv2013

Anomalous response to gate voltage application in mesoscopic LaAlO_3/SrTiO_3 devices

We report on resistivity and Hall measurements performed on a series of narrow mesa devices fabricated from LaAlO_3/SrTiO_3 single interface heterostructure with a bridge width range of 1.5-10 microns. Upon applying back-gate voltage of the order of a few Volts, a strong increase in the sample resistance (up to factor of 35) is observed, suggesting a relatively large capacitance between the Hall-bar and the gate. The high value of this capacitance is due to the device geometry, and can be explained within an electrostatic model using the Thomas Fermi approximation. The Hall coefficient is sometimes a non-monotonic function of the gate voltage. This behavior is inconsistent with a single conduction band model. We show that a theoretical two-band model is consistent with this transport behavior, and indicates a metal to insulator transition in at least one of these bands.

cond-mat.mes-hall↗

arXiv2012

Detecting Non-Abelian Anyons by Charging Spectroscopy

Observation of non-Abelian statistics for the e/4 quasiparticles in the ν=5/2 fractional quantum Hall state remains an outstanding experimental problem. The non-Abelian statistics are linked to the presence of additional low energy states in a system with localised quasiparticles, and hence an additional low-temperature entropy. Recent experiments, which detect changes in the number of quasiparticles trapped in a local potential well as a function of an applied gate voltage, V_G, provide a possibility for measuring this entropy, if carried out over a suitable range of temperatures, T. We present a microscopic model for quasiparticles in a potential well and study the effects of non-Abelian statistics on the charge stability diagram in the V_G-T plane, including broadening at finite temperature. We predict a measurable slope for the first quasiparticle charging line, and an even-odd effect in the diagram, which is a signature of non-Abelian statistics.

cond-mat.mes-hall↗

arXiv2012

Fractionalization noise in edge channels of integer quantum Hall states

A theoretical calculation is presented of current noise which is due charge fractionalization, in two interacting edge channels in the integer quantum Hall state at filling factor $ν=2$. Because of the capacitive coupling between the channels, a tunneling event, in which an electron is transferred from a biased source lead to one of the two channels, generates propagating plasma mode excitations which carry fractional charges on the other edge channel. When these excitations impinge on a quantum point contact, they induce low-frequency current fluctuations with no net average current. A perturbative treatment in the weak tunneling regime yields analytical integral expressions for the noise as a function of the bias on the source. Asymptotic expressions of the noise in the limits of high and low bias are found.

cond-mat.mes-hall↗

arXiv2011

Semi-classical model for the dephasing of a two-electron spin qubit coupled to a coherently evolving nuclear spin bath

We study electron spin decoherence in a two-electron double quantum dot due to the hyperfine interaction, under spin-echo conditions as studied in recent experiments. We develop a semi-classical model for the interaction between the electron and nuclear spins, in which the time-dependent Overhauser fields induced by the nuclear spins are treated as classical vector variables. Comparison of the model with experimentally-obtained echo signals allows us to quantify the contributions of various processes such as coherent Larmor precession and spin diffusion to the nuclear spin evolution.

cond-mat.mes-hall↗

arXiv2010

Theory of the Fabry-Perot Quantum Hall Interferometer

We analyze interference phenomena in the quantum-Hall analog of the Fabry-Perot interferometer, exploring the roles of the Aharonov-Bohm effect, Coulomb interactions, and fractional statistics on the oscillations of the resistance as one varies the magnetic field B and/or the voltage V_G applied to a side gate. Coulomb interactions couple the interfering edge mode to localized quasiparticle states in the bulk, whose occupation is quantized in integer values. For the integer quantum Hall effect, if the bulk-edge coupling is absent, the resistance exhibits an Aharonov-Bohm (AB) periodicity, where the phase is equal to the number of quanta of magnetic flux enclosed by a specified interferometer area. When bulk-edge coupling is present, the actual area of the interferometer oscillates as function of B and V_G, with a combination of a smooth variation and abrupt jumps due to changes in the number of quasi-particles in the bulk of the interferometer. This modulates the Aharonov-Bohm phase and gives rise to additional periodicities in the resistance. In the limit of strong interactions, the amplitude of the AB oscillations becomes negligible, and one sees only the new "Coulomb-dominated" (CD) periodicity. In the limits where either the AB or the CD periodicities dominate, a color map of resistance will show a series of parallel stripes in the B-V_G plane, but the two cases show different stripe spacings and slopes of opposite signs. At intermediate coupling, one sees a superposition of the two patterns. We discuss the dependence of the interference intensities on parameters including the temperature and the backscattering strengths of the individual constrictions. We also discuss how results are modified in a fractional quantized Hall system, and the extent to which the interferometer may demonstrate the fractional statistics of the quasiparticles.

cond-mat.mes-hall↗

arXiv2010

Long coherence of electron spins coupled to a nuclear spin bath

Qubits, the quantum mechanical bits required for quantum computing, must retain their fragile quantum states over long periods of time. In many types of electron spin qubits, the primary source of decoherence is the interaction between the electron spins and nuclear spins of the host lattice. For electrons in gate defined GaAs quantum dots, previous spin echo measurements have revealed coherence times of about 1 $μ$s at low magnetic fields below 100 mT. Here, we show that coherence in such devices can actually survive to much longer times, and provide a detailed understanding of the measured nuclear spin induced decoherence. At fields above a few hundred millitesla, the coherence time measured using a single-pulse spin echo extends to 30 $μ$s. At lower magnetic fields, the echo first collapses, but then revives at later times given by the period of the relative Larmor precession of different nuclear species. This behavior was recently predicted, and as we show can be quantitatively accounted for by a semi-classical model for the electron spin dynamics in the presence of a nuclear spin bath. Using a multiple-pulse Carr-Purcell-Meiboom-Gill echo sequence, the decoherence time can be extended to more than 200 $μ$s, which represents an improvement by two orders of magnitude compared to previous measurements. This demonstration of effective methods to mitigate nuclear spin induced decoherence puts the quantum error correction threshold within reach.

cond-mat.mes-hall↗

arXiv2006

Coherence oscillations in dephasing by non-Gaussian shot noise

A non-perturbative treatment is developed for the dephasing produced by the shot noise of a one- dimensional electron channel. It is applied to two systems: a charge qubit and the electronic Mach-Zehnder interferometer, both of them interacting with an adjacent partitioned electronic channel acting as a detector. We find that the visibility (interference contrast) can display oscillations as a function of detector voltage and interaction time. This is a unique consequence of the non-Gaussian properties of the shot noise, and only occurs in the strong coupling regime, when the phase contributed by a single electron exceeds pi. The resulting formula reproduces the recent surprising experimental observations reported in [I. Neder et al., cond-mat/0610634], and indicates a general explanation for similar visibility oscillations observed earlier in the Mach-Zehnder interferometer at large bias voltage. We explore in detail the full pattern of oscillations as a function of coupling strength, voltage and time, which might be observable in future experiments.

cond-mat.mes-hall↗