Linearized self-forces for branes

We compute the regularized force density and renormalized action due to fields of external origin coupled to a brane of arbitrary dimension in a spacetime of any dimension. Specifically, we consider forces generated by gravitational, dilatonic and generalized antisymmetric form-fields. The force density is regularized using a recently developed gradient operator. For the case of a Nambu--Goto brane, we show that the regularization leads to a renormalization of the tension, which is seen to be the same in both approaches. We discuss the specific couplings which lead to cancellation of the self-force in this case.Comment: 15 page

Image Reconstruction with a LaBr3-based Rotational Modulator

A rotational modulator (RM) gamma-ray imager is capable of obtaining significantly better angular resolution than the fundamental geometric resolution defined by the ratio of detector diameter to mask-detector separation. An RM imager consisting of a single grid of absorbing slats rotating ahead of an array of a small number of position-insensitive detectors has the advantage of fewer detector elements (i.e., detector plane pixels) than required by a coded aperture imaging system with comparable angular resolution. The RM therefore offers the possibility of a major reduction in instrument complexity, cost, and power. A novel image reconstruction technique makes it possible to deconvolve the raw images, remove sidelobes, reduce the effects of noise, and provide resolving power a factor of 6 - 8 times better than the geometric resolution. A 19-channel prototype RM developed in our laboratory at Louisiana State University features 13.8 deg full-angle field of view, 1.9 deg geometric angular resolution, and the capability of resolving sources to within 35' separation. We describe the technique, demonstrate the measured performance of the prototype instrument, and describe the prospects for applying the technique to either a high-sensitivity standoff gamma-ray imaging detector or a satellite- or balloon-borne gamma-ray astronomy telescope.Comment: submitted to Nuclear Instrument & Methods, special edition: SORMA 2010 on June 16, 201

Dynamics of a large spin with weak dissipation

We investigate the generalization of the spin-boson model to arbitrary spin size. The Born-Markov approximation is employed to derive a master equation in the regime of small coupling strengths to the environment. For spin one half, the master equation transforms into a set of Bloch equations, the solution of which is in good agreement with results of the spin-boson model for weak ohmic dissipation. For larger spins, we find a superradiance-like behavior known from the Dicke model. The influence of the nonresonant bosons of the dissipative environment can lead to the formation of a beat pattern in the dynamics of the $z$-component of the spin. The beat frequency is approximately proportional to the cutoff $\omega_c$ of the spectral function.Comment: 11 pages, 3 figures, to appear in Chemical Physics Special Issue on the Spin-Boson Problem, ed. by H. Grabert and A. Nitza

Enhanced Transmission of Light and Particle Waves through Subwavelength Nanoapertures by Far-Field Interference

Subwavelength aperture arrays in thin metal films can enable enhanced transmission of light and matter (atom) waves. The phenomenon relies on resonant excitation and interference of the plasmon or matter waves on the metal surface. We show a new mechanism that could provide a great resonant and nonresonant transmission enhancement of the light or de Broglie particle waves passed through the apertures not by the surface waves, but by the constructive interference of diffracted waves (beams generated by the apertures) at the detector placed in the far-field zone. In contrast to other models, the mechanism depends neither on the nature (light or matter) of the beams (continuous waves or pulses) nor on material and shape of the multiple-beam source (arrays of 1-D and 2-D subwavelength apertures, fibers, dipoles or atoms). The Wood anomalies in transmission spectra of gratings, a long standing problem in optics, follow naturally from the interference properties of our model. The new point is the prediction of the Wood anomaly in a classical Young-type two-source system. The new mechanism could be interpreted as a non-quantum analog of the superradiance emission of a subwavelength ensemble of atoms (the light power and energy scales as the number of light-sources squared, regardless of periodicity) predicted by the well-known Dicke quantum model.Comment: Revised version of MS presented at the Nanoelectronic Devices for Defense and Security (NANO-DDS) Conference, 18-21 June, 2007, Washington, US

QED for a Fibrillar Medium of Two-Level Atoms

We consider a fibrillar medium with a continuous distribution of two-level atoms coupled to quantized electromagnetic fields. Perturbation theory is developed based on the current algebra satisfied by the atomic operators. The one-loop corrections to the dispersion relation for the polaritons and the dielectric constant are computed. Renormalization group equations are derived which demonstrate a screening of the two-level splitting at higher energies. Our results are compared with known results in the slowly varying envelope and rotating wave approximations. We also discuss the quantum sine-Gordon theory as an approximate theory.Comment: 32 pages, 4 figures, uses harvmac and epsf. In this revised version, infra-red divergences are more properly handle

Theoretical Study of Fast Light with Short sech Pulses in Coherent Gain Media

We investigate theoretically the phenomenon of so-called fast light in an unconventional regime, using pulses sufficiently short that relaxation effects in a gain medium can be ignored completely. We show that previously recognized gain instabilities, including superfluorescence, can be tolerated in achieving a pulse peak advance of one full peak width.Comment: 7 pages, 8 figures; Replaced with revised version accepted by JOSA

Entanglement and spin squeezing in the two-atom Dicke model

We analyze the relation between the entanglement and spin-squeezing parameter in the two-atom Dicke model and identify the source of the discrepancy recently reported by Banerjee and Zhou et al that one can observe entanglement without spin squeezing. Our calculations demonstrate that there are two criteria for entanglement, one associated with the two-photon coherences that create two-photon entangled states, and the other associated with populations of the collective states. We find that the spin-squeezing parameter correctly predicts entanglement in the two-atom Dicke system only if it is associated with two-photon entangled states, but fails to predict entanglement when it is associated with the entangled symmetric state. This explicitly identifies the source of the discrepancy and explains why the system can be entangled without spin-squeezing. We illustrate these findings in three examples of the interaction of the system with thermal, classical squeezed vacuum and quantum squeezed vacuum fields.Comment: 7 pages, 1 figur

The Origin of Structures in Generalized Gravity

In a class of generalized gravity theories with general couplings between the scalar field and the scalar curvature in the Lagrangian, we can describe the quantum generation and the classical evolution of both the scalar and tensor structures in a simple and unified manner. An accelerated expansion phase based on the generalized gravity in the early universe drives microscopic quantum fluctuations inside a causal domain to expand into macroscopic ripples in the spacetime metric on scales larger than the local horizon. Following their generation from quantum fluctuations, the ripples in the metric spend a long period outside the causal domain. During this phase their evolution is characterized by their conserved amplitudes. The evolution of these fluctuations may lead to the observed large scale structures of the universe and anisotropies in the cosmic microwave background radiation.Comment: 5 pages, latex, no figur

Enhancement and suppression of spontaneous emission and light scattering by quantum degeneracy

Quantum degeneracy modifies light scattering and spontaneous emission. For fermions, Pauli blocking leads to a suppression of both processes. In contrast, in a weakly interacting Bose-Einstein condensate, we find spontaneous emission to be enhanced, while light scattering is suppressed. This difference is attributed to many-body effects and quantum interference in a Bose-Einstein condensate.Comment: 4 pages 1 figur