158 research outputs found
Universal transport signatures of Majorana fermions in superconductor-Luttinger liquid junctions
One of the most promising proposals for engineering topological
superconductivity and Majorana fermions employs a spin-orbit coupled nanowire
subjected to a magnetic field and proximate to an s-wave superconductor. When
only part of the wire's length contacts to the superconductor, the remaining
conducting portion serves as a natural lead that can be used to probe these
Majorana modes via tunneling. The enhanced role of interactions in one
dimension dictates that this configuration should be viewed as a
superconductor-Luttinger liquid junction. We investigate such junctions between
both helical and spinful Luttinger liquids, and topological as well as
non-topological superconductors. We determine the phase diagram for each case
and show that universal low-energy transport in these systems is governed by
fixed points describing either perfect normal reflection or perfect Andreev
reflection. In addition to capturing (in some instances) the familiar
Majorana-mediated `zero-bias anomaly' in a new framework, we show that
interactions yield dramatic consequences in certain regimes. Indeed, we
establish that strong repulsion removes this conductance anomaly altogether
while strong attraction produces dynamically generated effective Majorana modes
even in a junction with a trivial superconductor. Interactions further lead to
striking signatures in the local density of states and the line-shape of the
conductance peak at finite voltage, and also are essential for establishing
smoking-gun transport signatures of Majorana fermions in spinful Luttinger
liquid junctions.Comment: 25 pages, 6 figures, v
Dynamics of parametric fluctuations induced by quasiparticle tunneling in superconducting flux qubits
We present experiments on the dynamics of a two-state parametric fluctuator
in a superconducting flux qubit. In spectroscopic measurements, the fluctuator
manifests itself as a doublet line. When the qubit is excited in resonance with
one of the two doublet lines, the correlation of readout results exhibits an
exponential time decay which provides a measure of the fluctuator transition
rate. The rate increases with temperature in the interval 40 to 158 mK. Based
on the magnitude of the transition rate and the doublet line splitting we
conclude that the fluctuation is induced by quasiparticle tunneling. These
results demonstrate the importance of considering quasiparticles as a source of
decoherence in flux qubits.Comment: 12 pages, including supplementary informatio
Quasiparticle decay rate of Josephson charge qubit oscillations
We analyze the decay of Rabi oscillations in a charge qubit consisting of a
Cooper pair box connected to a finite-size superconductor by a Josephson
junction. We concentrate on the contribution of quasiparticles in the
superconductors to the decay rate. Passing of a quasiparticle through the
Josephson junction tunes the qubit away from the charge degeneracy, thus
spoiling the Rabi oscillations. We find the temperature dependence of the
quasiparticle contribution to the decay rate for open and isolated systems. The
former case is realized if a normal-state trap is included in the circuit, or
if just one vortex resides in the qubit; the decay rate has an activational
temperature dependence with the activation energy equal to the superconducting
gap . In a superconducting qubit isolated from the environment, the
activation energy equals if the number of electrons is even, while
for an odd number of electrons the decay rate of an excited qubit state remains
finite in the limit of zero temperature. We estimate the decay rate for
realistic parameters of a qubit.Comment: 8 pages, 3 figures, final version as published in PRB, minor change
Kinetics of non-equilibrium quasiparticle tunneling in superconducting charge qubits
We directly observe low-temperature non-equilibrium quasiparticle tunneling
in a pair of charge qubits based on the single Cooper-pair box. We measure
even- and odd-state dwell time distributions as a function of temperature, and
interpret these results using a kinetic theory. While the even-state lifetime
is exponentially distributed, the odd-state distribution is more heavily
weighted to short times, implying that odd-to-even tunnel events are not
described by a homogenous Poisson process. The mean odd-state dwell time
increases sharply at low temperature, which is consistent with quasiparticles
tunneling out of the island before reaching thermal equilibrium.Comment: Replaced Figure 1 with color version, corrected more typos. Version
submitted to PR
A quantitative study of quasiparticle traps using the single-Cooper-pair-transistor
We use radio-frequency reflectometry to measure quasiparticle tunneling rates
in the single-Cooper-pair-transistor. Devices with and without quasiparticle
traps in proximity to the island are studied. A to -fold reduction
in the quasiparticle tunneling rate onto the island is observed in the case of
quasiparticle traps. In the quasiparticle trap samples we also measure a
commensurate decrease in quasiparticle tunneling rate off the island.Comment: 4 pages, 4 fig
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