743 research outputs found
Edge Tunneling of Vortices in Superconducting Thin Films
We investigate the phenomenon of the decay of a supercurrent due to the
zero-temperature quantum tunneling of vortices from the edge in a thin
superconducting film in the absence of an external magnetic field. An explicit
formula is derived for the tunneling rate of vortices, which are subject to the
Magnus force induced by the supercurrent, through the Coulomb-like potential
barrier binding them to the film's edge. Our approach ensues from the
non-relativistic version of a Schwinger-type calculation for the decay of the
2D vacuum previously employed for describing vortex-antivortex pair-nucleation
in the bulk of the sample. In the dissipation-dominated limit, our explicit
edge-tunneling formula yields numerical estimates which are compared with those
obtained for bulk-nucleation to show that both mechanisms are possible for the
decay of a supercurrent.Comment: REVTeX file, 15 pages, 1 Postscript figure; to appear in Phys.Rev.
The Sound of Sonoluminescence
We consider an air bubble in water under conditions of single bubble
sonoluminescence (SBSL) and evaluate the emitted sound field nonperturbatively
for subsonic gas-liquid interface motion. Sound emission being the dominant
damping mechanism, we also implement the nonperturbative sound damping in the
Rayleigh-Plesset equation for the interface motion. We evaluate numerically the
sound pulse emitted during bubble collapse and compare the nonperturbative and
perturbative results, showing that the usual perturbative description leads to
an overestimate of the maximal surface velocity and maximal sound pressure. The
radius vs. time relation for a full SBSL cycle remains deceptively unaffected.Comment: 25 pages; LaTex and 6 attached ps figure files. Accepted for
publication in Physical Review
Optimized pulses for the control of uncertain qubits
Constructing high-fidelity control fields that are robust to control, system,
and/or surrounding environment uncertainties is a crucial objective for quantum
information processing. Using the two-state Landau-Zener model for illustrative
simulations of a controlled qubit, we generate optimal controls for \pi/2- and
\pi-pulses, and investigate their inherent robustness to uncertainty in the
magnitude of the drift Hamiltonian. Next, we construct a quantum-control
protocol to improve system-drift robustness by combining environment-decoupling
pulse criteria and optimal control theory for unitary operations. By
perturbatively expanding the unitary time-evolution operator for an open
quantum system, previous analysis of environment-decoupling control pulses has
calculated explicit control-field criteria to suppress environment-induced
errors up to (but not including) third order from \pi/2- and \pi-pulses. We
systematically integrate this criteria with optimal control theory,
incorporating an estimate of the uncertain parameter, to produce improvements
in gate fidelity and robustness, demonstrated via a numerical example based on
double quantum dot qubits. For the qubit model used in this work, post facto
analysis of the resulting controls suggests that realistic control-field
fluctuations and noise may contribute just as significantly to gate errors as
system and environment fluctuations.Comment: 38 pages, 15 figures, RevTeX 4.1, minor modifications to the previous
versio
Comment on "Transverse Force on a Quantized Vortex in a Superfluid"
The result of Thouless, Ao and Niu (TAN), that the mutual friction parameter
, contradicts to the experiments made in rotating 3He-B by
Manchester group. The Manchester group observed that at low
temperature and approaches 1 at high temperature. The reason of the
contradiction is that TAN did not take into account the Iordanskii force on the
vortex and the spectral flow force, which comes from the anomaly related to the
low-energy bound states of fermions in cores of quantized vortices. The
Iordanskii force is responsible for the negative at low
temperature, while due to the spectral flow approaches 1 at high
temperature. Relation of the spectral flow anomaly with the paradoxes of the
linear and angular momenta in gapless superfluids is discussed.Comment: revtex, 2 pages, submitted to Physical Review Letters as "Comment" to
the paper D.J. Thouless, P. Ao and Q. Niu, Phys. Rev. Lett. 76, 3758 (1996
Berry's Phase in the Presence of a Stochastically Evolving Environment: A Geometric Mechanism for Energy-Level Broadening
The generic Berry phase scenario in which a two-level system is coupled to a
second system whose dynamical coordinate is slowly-varying is generalized to
allow for stochastic evolution of the slow system. The stochastic behavior is
produced by coupling the slow system to a heat resevoir which is modeled by a
bath of harmonic oscillators initially in equilibrium at temperature T, and
whose spectral density has a bandwidth which is small compared to the
energy-level spacing of the fast system. The well-known energy-level shifts
produced by Berry's phase in the fast system, in conjunction with the
stochastic motion of the slow system, leads to a broadening of the fast system
energy-levels. In the limit of strong damping and sufficiently low temperature,
we determine the degree of level-broadening analytically, and show that the
slow system dynamics satisfies a Langevin equation in which Lorentz-like and
electric-like forces appear as a consequence of geometrical effects. We also
determine the average energy-level shift produced in the fast system by this
mechanism.Comment: 29 pages, RevTex, submitted to Phys. Rev.
Temporal Interferometry: A Mechanism for Controlling Qubit Transitions During Twisted Rapid Passage with Possible Application to Quantum Computing
In an adiabatic rapid passage experiment, the Bloch vector of a two-level
system (qubit) is inverted by slowly inverting an external field to which it is
coupled, and along which it is initially aligned. In twisted rapid passage, the
external field is allowed to twist around its initial direction with azimuthal
angle at the same time that it is inverted. For polynomial twist:
. We show that for , multiple avoided crossings
can occur during the inversion of the external field, and that these crossings
give rise to strong interference effects in the qubit transition probability.
The transition probability is found to be a function of the twist strength ,
which can be used to control the time-separation of the avoided crossings, and
hence the character of the interference. Constructive and destructive
interference are possible. The interference effects are a consequence of the
temporal phase coherence of the wavefunction. The ability to vary this
coherence by varying the temporal separation of the avoided crossings renders
twisted rapid passage with adjustable twist strength into a temporal
interferometer through which qubit transitions can be greatly enhanced or
suppressed. Possible application of this interference mechanism to construction
of fast fault-tolerant quantum CNOT and NOT gates is discussed.Comment: 29 pages, 16 figures, submitted to Phys. Rev.
Quantum error correction : an introductory guide
Quantum error correction protocols will play a central role in the realisation of quantum computing; the choice of error correction code will influence the full quantum computing stack, from the layout of qubits at the physical level to gate compilation strategies at the software level. As such, familiarity with quantum coding is an essential prerequisite for the understanding of current and future quantum computing architectures. In this review, we provide an introductory guide to the theory and implementation of quantum error correction codes. Where possible, fundamental concepts are described using the simplest examples of detection and correction codes, the working of which can be verified by hand. We outline the construction and operation of the surface code, the most widely pursued error correction protocol for experiment. Finally, we discuss issues that arise in the practical implementation of the surface code and other quantum error correction codes
Selfdual Spin 2 Theory in a 2+1 Dimensional Constant Curvature Space-Time
The Lagrangian constraint analysis of the selfdual massive spin 2 theory in a
2+1 dimensional flat space-time and its extension to a curved one, are
performed. Demanding consistence of degrees of freedom in the model with
gravitational interaction, gives rise to physical restrictions on non minimal
coupling terms and background. Finally, a constant curvature scenario is
explored, showing the existence of forbidden mass values. Causality in these
spaces is discussed. Aspects related with the construction of the reduced
action and the one-particle exchange amplitude, are noted.Comment: 20 pages, references added, little modifications performe
PESSTO monitoring of SN 2012hn: further heterogeneity among faint type I supernovae
We present optical and infrared monitoring data of SN 2012hn collected by the
Public ESO Spectroscopic Survey for Transient Objects (PESSTO). We show that SN
2012hn has a faint peak magnitude (MR ~ -15.7) and shows no hydrogen and no
clear evidence for helium in its spectral evolution. Instead, we detect
prominent Ca II lines at all epochs, which relates this transient to previously
described 'Ca-rich' or 'gap' transients. However, the photospheric spectra
(from -3 to +32 d with respect to peak) of SN 2012hn show a series of
absorption lines which are unique, and a red continuum that is likely intrinsic
rather than due to extinction. Lines of Ti II and Cr II are visible. This may
be a temperature effect, which could also explain the red photospheric colour.
A nebular spectrum at +150d shows prominent CaII, OI, CI and possibly MgI lines
which appear similar in strength to those displayed by core-collapse SNe. To
add to the puzzle, SN 2012hn is located at a projected distance of 6 kpc from
an E/S0 host and is not close to any obvious starforming region. Overall SN
2012hn resembles a group of faint H-poor SNe that have been discovered recently
and for which a convincing and consistent physical explanation is still
missing. They all appear to explode preferentially in remote locations offset
from a massive host galaxy with deep limits on any dwarf host galaxies,
favouring old progenitor systems. SN 2012hn adds heterogeneity to this sample
of objects. We discuss potential explosion channels including He-shell
detonations and double detonations of white dwarfs as well as peculiar
core-collapse SNe.Comment: 14 pages, 14 figures, accepted to MNRAS on 14/10/201
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