39,922 research outputs found
Reversal of the circulation of a vortex by quantum tunneling in trapped Bose systems
We study the quantum dynamics of a model for a vortex in a Bose gas with
repulsive interactions in an anisotropic, harmonic trap. By solving the
Schr\"odinger equation numerically, we show that the circulation of the vortex
can undergo periodic reversals by quantum-mechanical tunneling. With increasing
interaction strength or particle number, vortices become increasingly stable,
and the period for reversals increases. Tunneling between vortex and antivortex
states is shown to be described to a good approximation by a superposition of
vortex and antivortex states (a Schr\"odinger cat state), rather than the
mean-field state, and we derive an analytical expression for the oscillation
period. The problem is shown to be equivalent to that of the two-site Bose
Hubbard model with attractive interactions.Comment: 5 pages, 5 figures; published in Phys. Rev. A, Rapid Communication
Superfluid Density of Neutrons in the Inner Crust of Neutron Stars: New Life for Pulsar Glitch Models
Calculations of the effects of band structure on the neutron superfluid
density in the crust of neutron stars made under the assumption that the
effects of pairing are small [N. Chamel, Phys. Rev. C 85, 035801 (2012)] lead
to moments of inertia of superfluid neutrons so small that the crust alone is
insufficient to account for the magnitude of neutron star glitches. Inspired by
earlier work on ultracold atomic gases in an optical lattice, we investigate
fermions with attractive interactions in a periodic lattice in the mean-field
approximation. The effects of band structure are suppressed when the pairing
gap is of order or greater than the strength of the lattice potential. By
applying the results to the inner crust of neutron stars, we conclude that the
reduction of the neutron superfluid density is considerably less than
previously estimated and, consequently, it is premature to rule out models of
glitches based on neutron superfluidity in the crust.Comment: 5 pages, 3 figure
Quantum multiparty key distribution protocol without use of entanglement
We propose a quantum key distribution (QKD) protocol that enables three
parties agree at once on a shared common random bit string in presence of an
eavesdropper without use of entanglement. We prove its unconditional security
and analyze the key rate.Comment: 8 pages, no figur
Raman spectroscopy on mechanically exfoliated pristine graphene ribbons
We present Raman spectroscopy measurements of non-etched graphene
nanoribbons, with widths ranging from 15 to 160 nm, where the D-line intensity
is strongly dependent on the polarization direction of the incident light. The
extracted edge disorder correlation length is approximately one order of
magnitude larger than on previously reported graphene ribbons fabricated by
reactive ion etching techniques. This suggests a more regular crystallographic
orientation of the non-etched graphene ribbons here presented. We further
report on the ribbons width dependence of the line-width and frequency of the
long-wavelength optical phonon mode (G-line) and the 2D-line of the studied
graphene ribbons
Floquet analysis of the modulated two-mode Bose-Hubbard model
We study the tunneling dynamics in a time-periodically modulated two-mode
Bose-Hubbard model using Floquet theory. We consider situations where the
system is in the self-trapping regime and either the tunneling amplitude, the
interaction strength, or the energy difference between the modes is modulated.
In the former two cases, the tunneling is enhanced in a wide range of
modulation frequencies, while in the latter case the resonance is narrow. We
explain this difference with the help of Floquet analysis. If the modulation
amplitude is weak, the locations of the resonances can be found using the
spectrum of the non-modulated Hamiltonian. Furthermore, we use Floquet analysis
to explain the coherent destruction of tunneling (CDT) occurring in a
large-amplitude modulated system. Finally, we present two ways to create a NOON
state (a superposition of particles in mode 1 with zero particles in mode 2
and vice versa). One is based on a coherent oscillation caused by detuning from
a partial CDT. The other makes use of an adiabatic variation of the modulation
frequency. This results in a Landau-Zener type of transition between the ground
state and a NOON-like state.Comment: 16 pages, 11 figures; published in Phys. Rev.
Non-uniform vortex lattices in inhomogeneous rotating Bose-Einstein condensates
We derive a general framework, in terms of elastic theory, for describing the
distortion of the vortex lattice in a rotating Bose-Einstein condensate at
arbitrary rotation speed and determining the dependence of the distortion on
the density inhomogeneity of the system. In the rapidly rotating limit, we
derive the energetics in terms of Landau levels, including excitation to higher
levels; the distortion depends on the excitation of higher levels as well as on
the density gradient. As we show, the dominant effect of higher Landau levels
in a distorted lattice in equilibrium is simply to renormalize the frequency
entering the lowest Landau level condensate wave function -- from the
transverse trap frequency, , to the rotational frequency, , of
the system. Finally, we show how the equilibrium lattice distortion emerges
from elastohydrodynamic theory for inhomogeneous systems.Comment: 6 pages, no figure
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