133 research outputs found
Quasiparticle scattering by quantum phase slips in one-dimensional superfluids
Quantum phase slips (QPS) in narrow superfluid channels generate momentum by
unwinding the supercurrent. In a uniform Bose gas, this momentum needs to be
absorbed by quasiparticles (phonons). We show that this requirement results in
an additional exponential suppression of the QPS rate (compared to the rate of
QPS induced by a sharply localized perturbation). In BCS-paired fluids,
momentum can be transferred to fermionic quasiparticles, and we find an
interesting interplay between quasiparticle scattering on QPS and on disorder.Comment: 4 pages, revtex, no figures; to be published in Phys. Rev. Letter
Quantum Zeno effect in the Cooper-pair transport through a double-island Josephson system
Motivated by recent experiments, we analyze transport of Cooper pairs through
a double-island Josephson qubit. At low bias in a certain range of gate
voltages coherent superpositions of charge states play a crucial role. Analysis
of the evolution of the density matrix allows us to cover a wide range of
parameters, incl. situations with degenerate levels, when dissipation strongly
affects the coherent eigenstates. At high noise levels the so-called Zeno
effect can be observed, which slows down the transport. Our analysis explains
certain features of the I-V curves, in particular the visibility and shape of
resonant peaks and lines
Magnus Force in Discrete and Continuous Two-Dimensional Superfluids
Motion of vortices in two-dimensional superfluids in the classical limit is
studied by solving the Gross-Pitaevskii equation numerically on a uniform
lattice. We find that, in the presence of a superflow directed along one of the
main lattice periods, vortices move with the superflow on fine lattices but
perpendicular to it on coarse ones. We interpret this result as a transition
from the full Magnus force in the Galilean-invariant limit to vanishing
effective Magnus force in a discrete system, in agreement with the existing
experiments on vortex motion in Josephson junction arrays.Comment: 6 pages, 7 figures; published in Phys. Rev.
Tunneling in a uniform one-dimensional superfluid: emergence of a complex instanton
In a uniform ring-shaped one-dimensional superfluid, quantum fluctuations
that unwind the order parameter need to transfer momentum to quasiparticles
(phonons). We present a detailed calculation of the leading exponential factor
governing the rate of such phonon-assisted tunneling in a weakly-coupled Bose
gas at a low temperature . We also estimate the preexponent. We find that
for small superfluid velocities the -dependence of the rate is given mainly
by , where is the momentum transfer, and is the
phonon speed. At low , this represents a strong suppression of the rate,
compared to the non-uniform case. As a part of our calculation, we identify a
complex instanton, whose analytical continuation to suitable real-time segments
is real and describes formation and decay of coherent quasiparticle states with
nonzero total momenta.Comment: 15 pages, 3 figures; to be published in Phys. Rev.
Recognizing Small-Circuit Structure in Two-Qubit Operators and Timing Hamiltonians to Compute Controlled-Not Gates
This work proposes numerical tests which determine whether a two-qubit
operator has an atypically simple quantum circuit. Specifically, we describe
formulae, written in terms of matrix coefficients, characterizing operators
implementable with exactly zero, one, or two controlled-not (CNOT) gates and
all other gates being one-qubit. We give an algorithm for synthesizing
two-qubit circuits with optimal number of CNOT gates, and illustrate it on
operators appearing in quantum algorithms by Deutsch-Josza, Shor and Grover. In
another application, our explicit numerical tests allow timing a given
Hamiltonian to compute a CNOT modulo one-qubit gates, when this is possible.Comment: 4 pages, circuit examples, an algorithm and a new application (v3
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