13,632 research outputs found
Optimal Measurements of Magnetic Flux in Superconducting Circuits and Macroscopic Quantum Mechanics
A model of repeated quantum measurements of magnetic flux in superconducting
circuits manifesting tunneling is discussed. The perturbation due to the
previous measurements of magnetic flux is always present unless quantum
nondemolition measurements are performed. By replacing the classical notion of
noninvasivity with this condition, temporal Bell-like inequalities allows one
to test the observability at the macroscopic level of the conflict between
realism and quantum theory.Comment: 11 pages (no figures), to be published in Physics Letters
Temperature of a Decoherent Oscillator with Strong Coupling
The temperature of an oscillator coupled to the vacuum state of a heat bath
via ohmic coupling is non-zero, as measured by the reduced density matrix of
the oscillator. This paper shows that the actual temperature, as measured by a
thermometer is still zero (or in the thermal state of the bath, the temperature
of the bath). The decoherence temperature is due to "false-decoherence", with
the heat bath state being dragged along with the oscillator.Comment: 6 page
Testing temporal Bell inequalities through repeated measurements in rf-SQUIDs
Temporal Bell-like inequalities are derived taking into account the influence
of the measurement apparatus on the observed magnetic flux in a rf-SQUID.
Quantum measurement theory is shown to predict violations of these inequalities
only when the flux states corresponding to opposite current senses are not
distinguishable. Thus rf-SQUIDs cannot help to discriminate realism and quantum
mechanics at the macroscopic level.Comment: 12 pages, 4 Postscript figures in an uuencoded file figures.uu, uses
revtex.st
Transition from Band insulator to Bose-Einstein Condensate superfluid and Mott State of Cold Fermi Gases with Multiband Effects in Optical Lattices
We study two models realized by two-component Fermi gases loaded in optical
lattices. We clarify that multi-band effects inevitably caused by the optical
lattices generate a rich structure, when the systems crossover from the region
of weakly bound molecular bosons to the region of strongly bound atomic bosons.
Here the crossover can be controlled by attractive fermion interaction. One of
the present models is a case with attractive fermion interaction, where an
insulator-superfluid transition takes place. The transition is characterized as
the transition between a band insulator and a Bose-Einstein condensate (BEC)
superfluid state. Differing from the conventional BCS superfluid transition,
this transition shows unconventional properties. In contrast to the one
particle excitation gap scaled by the superfluid order parameter in the
conventional BCS transition, because of the multi-band effects, a large gap of
one-particle density of states is retained all through the transition although
the superfluid order grows continuously from zero. A reentrant transition with
lowering temperature is another unconventionality. The other model is the case
with coexisting attractive and repulsive interactions. Within a mean field
treatment, we find a new insulating state, an orbital ordered insulator. This
insulator is one candidate for the Mott insulator of molecular bosons and is
the first example that the orbital internal degrees of freedom of molecular
bosons appears explicitly. Besides the emergence of a new phase, a coexisting
phase also appears where superfluidity and an orbital order coexist just by
doping holes or particles. The insulating and superfluid particles show
differentiation in momentum space as in the high-Tc cuprate superconductors.Comment: 13 pages, 10 figure
Universal Properties of the Ultra-Cold Fermi Gas
We present some general considerations on the properties of a two-component
ultra-cold Fermi gas along the BEC-BCS crossover. It is shown that the
interaction energy and the ground state energy can be written in terms of a
single dimensionless function , where and
. The function incorporates all the many-body physics
and naturally occurs in other physical quantities as well. In particular, we
show that the RF-spectroscopy shift \bar{\d\o}(\xi,\tau) and the molecular
fraction in the closed channel can be expressed in terms of
and thus have identical temperature dependence. The conclusions
should have testable consequences in future experiments
Energetics of a strongly correlated Fermi gas
The energy of the two-component Fermi gas with the s-wave contact interaction
is a simple linear functional of its momentum distribution:
E_\text{internal}=\hbar^2\Omega C/4\pi am+\sum_{\vect k\sigma}(\hbar^2
k^2/2m)(n_{\vect k\sigma}-C/k^4) where the external potential energy is not
included, is the scattering length, is the volume, n_{\vect
k\sigma} is the average number of fermions with wave vector \vect k and spin
, and C\equiv\lim_{\vect k\to\infty} k^4 n_{\vect k\up} =\lim_{\vect
k\to\infty} k^4 n_{\vect k\down}. This result is a \textit{universal
identity}. Its proof is facilitated by a novel mathematical idea, which might
be of utility in dealing with ultraviolet divergences in quantum field
theories. Other properties of this Fermi system, including the short-range
structure of the one-body reduced density matrix and the pair correlation
function, and the dimer-fermion scattering length, are also studied.Comment: 28 pages, 1 figur
Natural vacuum electronics
The ambient natural vacuum of space is proposed as a basis for electron valves. Each valve is an electron controlling structure similiar to a vacuum tube that is operated without a vacuum sustaining envelope. The natural vacuum electron valves discussed offer a viable substitute for solid state devices. The natural vacuum valve is highly resistant to ionizing radiation, system generated electromagnetic pulse, current transients, and direct exposure to space conditions
Impact of HE-level work-based learning: an overview of stakeholder perspectives and a proposed model for evaluation of impact
- âŠ