9,328 research outputs found
Three-dimensional light-matter interface for collective spin squeezing in atomic ensembles
We study the three-dimensional nature of the quantum interface between an
ensemble of cold, trapped atomic spins and a paraxial laser beam, coupled
through a dispersive interaction. To achieve strong entanglement between the
collective atomic spin and the photons, one must match the spatial mode of the
collective radiation of the ensemble with the mode of the laser beam while
minimizing the effects of decoherence due to optical pumping. For ensembles
coupling to a probe field that varies over the extent of the cloud, the set of
atoms that indistinguishably radiates into a desired mode of the field defines
an inhomogeneous spin wave. Strong coupling of a spin wave to the probe mode is
not characterized by a single parameter, the optical density, but by a
collection of different effective atom numbers that characterize the coherence
and decoherence of the system. To model the dynamics of the system, we develop
a full stochastic master equation, including coherent collective scattering
into paraxial modes, decoherence by local inhomogeneous diffuse scattering, and
backaction due to continuous measurement of the light entangled with the spin
waves. This formalism is used to study the squeezing of a spin wave via
continuous quantum nondemolition (QND) measurement. We find that the greatest
squeezing occurs in parameter regimes where spatial inhomogeneities are
significant, far from the limit in which the interface is well approximated by
a one-dimensional, homogeneous model.Comment: 24 pages, 7 figure
Cooperative spontaneous emission from indistinguishable atoms in arbitrary motional quantum states
We investigate superradiance and subradiance of indistinguishable atoms with
quantized motional states, starting with an initial total state that factorizes
over the internal and external degrees of freedom of the atoms. Due to the
permutational symmetry of the motional state, the cooperative spontaneous
emission, governed by a recently derived master equation [F. Damanet et al.,
Phys. Rev. A 93, 022124 (2016)], depends only on two decay rates and
and a single parameter describing the
dipole-dipole shifts. We solve the dynamics exactly for atoms,
numerically for up to 30 atoms, and obtain the large--limit by amean-field
approach. We find that there is a critical difference that
depends on beyond which superradiance is lost. We show that exact
non-trivial dark states (i.e. states other than the ground state with vanishing
spontaneous emission) only exist for , and that those states
(dark when ) are subradiant when .Comment: 14 pages, 8 figure
High-resolution imaging of kidney vascular corrosion casts with nano-CT
A vascular corrosion cast of an entire mouse kidney was scanned with a modular multiresolution X-ray nanotomography system. Using an isotropic voxel pitch of 0.5 mu m, capillary systems such as the vasa recta, peritubular capillaries and glomeruli were clearly resolved. This represents a considerable improvement over corrosion casts scanned with microcomputed tomography systems. The resolving power of this system was clearly demonstrated by the unique observation of a dense, subcapsular mat of capillaries enveloping the entire outer surface of the cortical region. Resolution of glomerular capillaries was comparable to similar models derived from laser scanning confocal microscopy. The high-resolution, large field of view and the three-dimensional nature of the resulting data opens new possibilities for the use of corrosion casting in research
Stored energies in electric and magnetic current densities for small antennas
Electric and magnetic currents are essential to describe electromagnetic
stored energy, as well as the associated quantities of antenna Q and the
partial directivity to antenna Q-ratio, D/Q, for general structures. The upper
bound of previous D/Q-results for antennas modeled by electric currents is
accurate enough to be predictive, this motivates us here to extend the analysis
to include magnetic currents. In the present paper we investigate antenna Q
bounds and D/Q-bounds for the combination of electric- and magnetic-currents,
in the limit of electrically small antennas. This investigation is both
analytical and numerical, and we illustrate how the bounds depend on the shape
of the antenna. We show that the antenna Q can be associated with the largest
eigenvalue of certain combinations of the electric and magnetic polarizability
tensors. The results are a fully compatible extension of the electric only
currents, which come as a special case. The here proposed method for antenna Q
provides the minimum Q-value, and it also yields families of minimizers for
optimal electric and magnetic currents that can lend insight into the antenna
design.Comment: 27 pages 7 figure
Black Hole Evaporation along Macroscopic Strings
We develop the quantization of a macroscopic string which extends radially
from a Schwarzschild black hole. The Hawking process excites a thermal bath of
string modes that causes the black hole to lose mass. The resulting typical
string configuration is a random walk in the angular coordinates. We show that
the energy flux in string excitations is approximately that of spacetime field
modes.Comment: 26pp, EFI 93-73. (Original claim that string Hawking flux exceeds
spacetime flux is WRONG. It is the same; revised version provides correct
argument and additional comments.
Wilson loops in heavy ion collisions and their calculation in AdS/CFT
Expectation values of Wilson loops define the nonperturbative properties of
the hot medium produced in heavy ion collisions that arise in the analysis of
both radiative parton energy loss and quarkonium suppression. We use the
AdS/CFT correspondence to calculate the expectation values of such Wilson loops
in the strongly coupled plasma of N=4 super Yang-Mills (SYM) theory, allowing
for the possibility that the plasma may be moving with some collective flow
velocity as is the case in heavy ion collisions. We obtain the N=4 SYM values
of the jet quenching parameter , which describes the energy loss of a
hard parton in QCD, and of the velocity-dependence of the quark-antiquark
screening length for a moving dipole as a function of the angle between its
velocity and its orientation. We show that if the quark-gluon plasma is flowing
with velocity v_f at an angle theta with respect to the trajectory of a hard
parton, the jet quenching parameter is modified by a factor
gamma_f(1-v_f cos theta), and show that this result applies in QCD as in N=4
SYM. We discuss the relevance of the lessons we are learning from all these
calculations to heavy ion collisions at RHIC and at the LHC. Furthermore, we
discuss the relation between our results and those obtained in other theories
with gravity duals, showing in particular that the ratio between in
any two conformal theories with gravity duals is the square root of the ratio
of their central charges. This leads us to conjecture that in nonconformal
theories defines a quantity that always decreases along
renormalization group trajectories and allows us to use our calculation of
in N=4 SYM to make a conjecture for its value in QCD.Comment: 61 pages, 8 figures. Note added discussing relation between our work
and that in several papers that have appeared recently. References adde
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