3,128 research outputs found
PT-Symmetric Quantum Electrodynamics and Unitarity
More than 15 years ago, a new approach to quantum mechanics was suggested, in
which Hermiticity of the Hamiltonian was to be replaced by invariance under a
discrete symmetry, the product of parity and time-reversal symmetry,
. It was shown that if is unbroken, energies were,
in fact, positive, and unitarity was satisifed. Since quantum mechanics is
quantum field theory in 1 dimension, time, it was natural to extend this idea
to higher-dimensional field theory, and in fact an apparently viable version of
-invariant quantum electrodynamics was proposed. However, it has
proved difficult to establish that the unitarity of the scattering matrix, for
example, the K\"all\'en spectral representation for the photon propagator, can
be maintained in this theory. This has led to questions of whether, in fact,
even quantum mechanical systems are consistent with probability conservation
when Green's functions are examined, since the latter have to possess physical
requirements of analyticity. The status of QED will be reviewed
in this report, as well as the general issue of unitarity.Comment: 13 pages, 2 figures. Revised version includes new evidence for the
violation of unitarit
Repulsive Casimir and Casimir-Polder Forces
Casimir and Casimir-Polder repulsion have been known for more than 50 years.
The general "Lifshitz" configuration of parallel semi-infinite dielectric slabs
permits repulsion if they are separated by a dielectric fluid that has a value
of permittivity that is intermediate between those of the dielectric slabs.
This was indirectly confirmed in the 1970s, and more directly by Capasso's
group recently. It has also been known for many years that electrically and
magnetically polarizable bodies can experience a repulsive quantum vacuum
force. More amenable to practical application are situations where repulsion
could be achieved between ordinary conducting and dielectric bodies in vacuum.
The status of the field of Casimir repulsion with emphasis on recent
developments will be surveyed. Here, stress will be placed on analytic
developments, especially of Casimir-Polder (CP) interactions between
anisotropically polarizable atoms, and CP interactions between anisotropic
atoms and bodies that also exhibit anisotropy, either because of anisotropic
constituents, or because of geometry. Repulsion occurs for wedge-shaped and
cylindrical conductors, provided the geometry is sufficiently asymmetric, that
is, either the wedge is sufficiently sharp or the atom is sufficiently far from
the cylinder.Comment: 24 pages, 14 figures, contribution to the special issue of J. Phys. A
honoring Stuart Dowker. This revision corrects typos and adds additional
references and discussio
Exact results for Casimir interactions between dielectric bodies: The weak-coupling or van der Waals Limit
In earlier papers we have applied multiple scattering techniques to calculate
Casimir forces due to scalar fields between different bodies described by delta
function potentials. When the coupling to the potentials became weak,
closed-form results were obtained. We simplify this weak-coupling technique and
apply it to the case of tenuous dielectric bodies, in which case the method
involves the summation of van der Waals (Casimir-Polder) interactions. Once
again exact results for finite bodies can be obtained. We present closed
formulas describing the interaction between spheres and between cylinders, and
between an infinite plate and a retangular slab of finite size. For such a
slab, we consider the torque acting on it, and find non-trivial equilibrium
points can occur.Comment: 4 pages, 3 figure
AC Stark shift noise in QND measurement arising from quantum fluctuations of light polarization
In a recent letter [Auzinsh {\it{et. al.}} (physics/0403097)] we have
analyzed the noise properties of an idealized atomic magnetometer that utilizes
spin squeezing induced by a continuous quantum nondemolition measurement. Such
a magnetometer measures spin precession of atomic spins by detecting
optical rotation of far-detuned probe light. Here we consider maximally
squeezed probe light, and carry out a detailed derivation of the contribution
to the noise in a magnetometric measurement due to the differential AC Stark
shift between Zeeman sublevels arising from quantum fluctuations of the probe
polarization.Comment: This is a companion note to physics/040309
Can a quantum nondemolition measurement improve the sensitivity of an atomic magnetometer?
Noise properties of an idealized atomic magnetometer that utilizes spin
squeezing induced by a continuous quantum nondemolition measurement are
considered. Such a magnetometer measures spin precession of atomic spins by
detecting optical rotation of far-detuned light. Fundamental noise sources
include the quantum projection noise and the photon shot-noise. For measurement
times much shorter than the spin-relaxation time observed in the absence of
light () divided by , the optimal sensitivity of the
magnetometer scales as , so an advantage over the usual sensitivity
scaling as can be achieved. However, at longer measurement times,
the optimized sensitivity scales as , as for a usual shot-noise
limited magnetometer. If strongly squeezed probe light is used, the Heisenberg
uncertainty limit may, in principle, be reached for very short measurement
times. However, if the measurement time exceeds , the
scaling is again restored.Comment: Some details of calculations can be found in a companion note:
physics/040712
Velocity-selective direct frequency-comb spectroscopy of atomic vapors
We present an experimental and theoretical investigation of two-photon direct
frequency-comb spectroscopy performed through velocity-selective excitation. In
particular, we explore the effect of repetition rate on the
two-photon transitions
excited in a rubidium atomic vapor cell. The transitions occur via step-wise
excitation through the states by use of the direct
output of an optical frequency comb. Experiments were performed with two
different frequency combs, one with a repetition rate of MHz and
one with a repetition rate of MHz. The experimental spectra are
compared to each other and to a theoretical model.Comment: 10 pages, 7 figure
- …