Leonid Glazman

Interaction between fermions in one dimension is usually accounted for within the exactly solvable Tomonaga-Luttinger model. The crucial simplification made in this model is the linearization of the fermionic spectrum. That simplification leads to an infinite lifetime of a fermion at the mass shell, i.e., the corresponding Green function G(,k) diverges at ε=xik. We find that inclusion of the curvature of electron spectrum, xik=vFk+k^2/2m, yields a finite decay rate of a fermion, 1/tau(xik)theta(k)k^8/m^3; here for definiteness we consider right-moving particles, and k is measured from the Fermi wave vector. The found finite lifetime allows one to assess the limitations of the Luttinger liquid paradigm.

A practical quantum computer requires qubits to have long enough coherence times to perform many quantum gates. For a superconducting transmon qubit, non-equilibrium quasiparticles are one possible source for decoherence. Although the origin of these non-equilibrium quasiparticles remains unclear, the poisoning effect of quasiparticles is expected to be reduced by lowering the quasiparticle density near the tunnel junctions of the qubits. Potential approaches to reducing the quasiparticle density include using a higher-gap superconductor and fabricating quasiparticle traps near the tunnel junctions. We will present preliminary data of the coherence times measured on a transmon qubit fabricated by engineered Al bandgap profiles and compare them with those obtained on regular transmon qubits.

We study the Kondo effect in a quantum dot subject to an external ac field. The Kondo effect can be probed by measuring the dc current induced by an auxiliary dc bias Vdc applied across the dot. In the absence of ac perturbation, the corresponding differential conductance G(Vdc) is known to exhibit a sharp peak at Vdc=0, which is the

A recent experiment [Nadj-Perge et al, Science 346, 602 (2014)] gives possible evidence for Majorana bound states in chains of magnetic adatoms placed on a superconductor. While many features of the observed end states are naturally interpreted in terms of Majorana states, their strong localization remained puzzling. We consider a linear chain of Anderson impurities on a superconductor as a minimal model and treat it largely analytically within mean-field theory. We explore the phase diagram, the subgap excitation spectrum, and the Majorana wave functions. Owing to a strong velocity renormalization, the latter are localized on a scale which is parametrically small compared to the coherence length of the host superconductor.

One-dimensional (1D) quantum liquids of bosons, fermions, and spins are conventionally described using an effective hydrodynamic approach known as the Luttinger liquid theory. 1���5 This theory predicts the long-range behavior of equal-time correlation functions at zero temperature, which one obtains as a series expansion with power laws controlled by a dimensionless Luttinger liquid parameter K> 0; see Eqs.(1)���(3). While the ���universal��� parameter K is related to thermodynamic properties and can be easily extracted from ...

A practical quantum computer requires qubits with long coherence times in order to perform many quantum gates. For a superconducting qubit, non-equilibrium quasiparticle tunneling is one possible source of decoherence. Spectroscopy measurements of a superconducting transmon qubit can be used to set a bound on the quasiparticle tunneling rate. When operated in the low EJ/EC regime, the transmon qubit transition

We propose a quantitative electrostatic theory of the gate-induced confinement of two-dimensional electron gas (2DEG) in the quantum Hall regime. The self-consistent electrostatic potential in the region occupied by 2DEG changes in a steplike manner due to the formation of ...

A normal metal ring exhibits a persistent electrical current provided its circumference is less than the electron's phase coherence length and its temperature is less than the Thouless temperature. The amplitude of the persistent current is a random function of the ring's disorder configuration. To date, theory and experiments have focused on measuring the variance of the distribution from which the persistent current amplitude is drawn. We have measured persistent currents in arrays of normal-metal rings over a wide range of magnetic fields, or equivalently, over many independent realizations of the disorder potential. This data allows us to produce a histogram of the current amplitude and determine its higher order moments. We find these higher order moments are consistent with a Gaussian distribution for the persistent currents. We also directly confirm that the amplitudes of different harmonics of the currents' Aharonov-Bohm oscillations are uncorrelated.

Over the past several decades, Luttinger-liquid theory has a framework for interacting electrons in one dimension. However, the validity of the theory is strictly limited to low-energy excitations where the electron dispersion is linear. Interacting electrons in one-dimension beyond the ...

Owing to the low-loss propagation of electromagnetic signals in superconductors, Josephson junctions constitute ideal building blocks for quantum memories, amplifiers, detectors and high-speed processing units, operating over a wide band of microwave frequencies. Nevertheless, although transport in superconducting wires is perfectly lossless for direct current, transport of radio-frequency signals can be dissipative in the presence of quasiparticle excitations above the superconducting gap. Moreover, the exact mechanism of this dissipation in Josephson junctions has never been fully resolved experimentally. In particular, Josephson's key theoretical prediction that quasiparticle dissipation should vanish in transport through a junction when the phase difference across the junction is π (ref. 2) has never been observed. This subtle effect can be understood as resulting from the destructive interference of two separate dissipative channels involving electron-like and hole-like quasiparticles. Here we report the experimental observation of this quantum coherent suppression of quasiparticle dissipation across a Josephson junction. As the average phase bias across the junction is swept through π, we measure an increase of more than one order of magnitude in the energy relaxation time of a superconducting artificial atom. This striking suppression of dissipation, despite the presence of lossy quasiparticle excitations above the superconducting gap, provides a powerful tool for minimizing decoherence in quantum electronic systems and could be directly exploited in quantum information experiments with superconducting quantum bits.