1,030 research outputs found
Signal velocity, causality, and quantum noise in superluminal light pulse propagation
We consider pulse propagation in a linear anomalously dispersive medium where
the group velocity exceeds the speed of light in vacuum (c) or even becomes
negative. A signal velocity is defined operationally based on the optical
signal-to-noise ratio, and is computed for cases appropriate to the recent
experiment where such a negative group velocity was observed. It is found that
quantum fluctuations limit the signal velocity to values less than c.Comment: 4 Journal pages, 3 figure
Theory of vortex lattice effects on STM spectra in d-wave superconductors
Theory of scanning tunneling spectroscopy of low energy quasiparticle (QP)
states in vortex lattices of d-wave superconductors is developed taking account
of the effects caused by an extremely large extension of QP wavefunctions in
the nodal directions and the band structure in the QP spectrum. The oscillatory
structures in STM spectra, which correspond to van Hove singularities are
analysed. Theoretical calculations carried out for finite temperatures and
scattering rates are compared with recent experimental data for high
temperature cuprates.Comment: 4 pages, 3 eps figures, M2S-HTSC-VI conference paper, using Elsevier
style espcrc2.st
Relativistic quantum mechanics and the Bohmian interpretation
Conventional relativistic quantum mechanics, based on the Klein-Gordon
equation, does not possess a natural probabilistic interpretation in
configuration space. The Bohmian interpretation, in which probabilities play a
secondary role, provides a viable interpretation of relativistic quantum
mechanics. We formulate the Bohmian interpretation of many-particle wave
functions in a Lorentz-covariant way. In contrast with the nonrelativistic
case, the relativistic Bohmian interpretation may lead to measurable
predictions on particle positions even when the conventional interpretation
does not lead to such predictions.Comment: 10 pages, revised, to appear in Found. Phys. Let
Quantum incompressibility of a falling Rydberg atom, and a gravitationally-induced charge separation effect in superconducting systems
Freely falling point-like objects converge towards the center of the Earth.
Hence the gravitational field of the Earth is inhomogeneous, and possesses a
tidal component. The free fall of an extended quantum object such as a hydrogen
atom prepared in a high principal-quantum-number stretch state, i.e., a
circular Rydberg atom, is predicted to fall more slowly that a classical
point-like object, when both objects are dropped from the same height from
above the Earth. This indicates that, apart from "quantum jumps," the atom
exhibits a kind of "quantum incompressibility" during free fall in
inhomogeneous, tidal gravitational fields like those of the Earth. A
superconducting ring-like system with a persistent current circulating around
it behaves like the circular Rydberg atom during free fall. Like the electronic
wavefunction of the freely falling atom, the Cooper-pair wavefunction is
"quantum incompressible." The ions of the ionic lattice of the superconductor,
however, are not "quantum incompressible," since they do not possess a globally
coherent quantum phase. The resulting difference during free fall in the
response of the nonlocalizable Cooper pairs of electrons and the localizable
ions to inhomogeneous gravitational fields is predicted to lead to a charge
separation effect, which in turn leads to a large repulsive Coulomb force that
opposes the convergence caused by the tidal, attractive gravitational force on
the superconducting system. A "Cavendish-like" experiment is proposed for
observing the charge separation effect induced by inhomogeneous gravitational
fields in a superconducting circuit. This experiment would demonstrate the
existence of a novel coupling between gravity and electricity via
macroscopically coherent quantum matter.Comment: `2nd Vienna Symposium for the Foundations of Modern Physics'
Festschrift MS for Foundations of Physic
Angular Position of Nodes in the Superconducting Gap of Quasi-2D Heavy-Fermion Superconductor CeCoIn_5
The thermal conductivity of the heavy-fermion superconductor CeCoIn_5 has
been studied in a magnetic field rotating within the 2D planes. A clear
fourfold symmetry of the thermal conductivity which is characteristic of a
superconducting gap with nodes along the (+-pi,+-pi)-directions is resolved.
The thermal conductivity measurement also reveals a first order transition at
H_c2, indicating a Pauli limited superconducting state. These results indicate
that the symmetry most likely belongs to d_{x^2-y^2}, implying that the
anisotropic antiferromagnetic fluctuation is relevant to the superconductivity.Comment: 5 Pages, 4 figure
Experimental observation of nonclassical effects on single-photon detection rates
It is often asserted that quantum effects can be observed in coincidence
detection rates or other correlations, but never in the rate of single-photon
detection. We observe nonclassical interference in a singles rate, thanks to
the intrinsic nonlinearity of photon counters. This is due to a dependence of
the effective detection efficiency on the quantum statistics of the light beam.
Such measurements of detector response to photon pairs promise to shed light on
the microscopic aspects of silicon photodetectors, and on general issues of
quantum measurement and decoherence.Comment: 8 pages, 4 figure
Correcting the quantum clock: conditional sojourn times
Can the quantum-mechanical sojourn time be clocked without the clock
affecting the sojourn time? Here we re-examine the previously proposed
non-unitary clock, involving absorption/amplification by an added infinitesimal
imaginary potential(), and find it {\it not} to preserve, in general,
the positivity of the sojourn time, conditional on eventual reflection or
transmission. The sojourn time is found to be affected by the scattering
concomitant with the mismatch, however small, due to the very clock
potential() introduced for the purpose, as also by any prompt
scattering involving partial waves that have not traversed the region of
interest. We propose a formal procedure whereby the sojourn time so clocked can
be corrected for these spurious scattering effects. The resulting conditional
sojourn times are then positive definite for an arbitrary potential, and have
the proper high- and low-energy limits.Comment: Corrected and rewritten, RevTeX, 4 pages, 2 figures (ps files)
include
Dirac Nodes and Quantized Thermal Hall Effect in the Mixed State of d-wave Superconductors
We consider the vortex state of d-wave superconductors in the clean limit.
Within the linearized approximation the quasiparticle bands obtained are found
to posess Dirac cone dispersion (band touchings) at special points in the
Brillouin zone. They are protected by a symmetry of the linearized Hamiltonian
that we call T_Dirac. Moreover, for vortex lattices that posess inversion
symmetry, it is shown that there is always a Dirac cone centered at zero energy
within the linearized theory. On going beyond the linearized approximation and
including the effect of the smaller curvature terms (that break T_Dirac), the
Dirac cone dispersions are found to acquire small gaps (0.5 K/Tesla in YBCO)
that scale linearly with the applied magnetic field. When the chemical
potential for quasiparticles lies within the gap, quantization of the
thermal-Hall conductivity is expected at low temperatures i.e. kappa_{xy}/T =
n[(pi k_B)^2/(3h)] with the integer `n' taking on values n=+2, -2, 0. This
quantization could be seen in low temperature thermal transport measurements of
clean d-wave superconductors with good vortex lattices.Comment: (23 pages in all [7 pages in appendices], 9 figures
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