Can the jet steepen the light curves of GRB afterglow?

Beaming of relativistic ejecta in GRBs has been postulated by many authors in order to reduce the total GRB energy, thus it is very important to look for the observational evidence of beaming. Rhoads (1999) has pointed out that the dynamics of the blast wave, which is formed when the beamed ejecta sweeping the external medium, will be significantly modified by the sideways expansion due to the increased swept up matter. He claimed that shortly after the bulk Lorentz factor ($\Gamma$) of the blast wave drops below the inverse of the initial opening angle ($\theta_{0}$) of the beamed ejecta, there will be a sharp break in the afterglow light curves. However, some other authors have performed numerical calculations and shown that the break of the light curve is weaker and much smoother than the one analytically predicted. In this paper we reanalyse the dynamical evolution of the jet blast wave, calculate the jet emission analytically, we find that the sharp break predicted by Rhoads will actually not exist, and for most cases the afterglow light curve will almost not be affected by sideways expansion unless the beaming angle is extremely small. We demonstrate that only when $\theta_{0}<0.1$, the afterglow light curves may be steepened by sideways expansion, and in fact there cannot be two breaks as claimed before. We have also constructed a simple numerical code to verify our conclusion.Comment: 12 pages, 2 figures, accepted by ApJ, added numerical calculation

Generation of nonclassical photon states using a superconducting qubit in a microcavity

Based on the interaction between the radiation field and a superconductor, we propose a way to engineer quantum states using a SQUID charge qubit inside a microcavity. This device can act as a deterministic single photon source as well as generate any Fock states and an arbitrary superposition of Fock states for the cavity field. The controllable interaction between the cavity field and the qubit can be realized by the tunable gate voltage and classical magnetic field applied to the SQUID.Comment: 4 page

Density-Dependent Response of an Ultracold Plasma to Few-Cycle Radio-Frequency Pulses

Ultracold neutral plasmas exhibit a density-dependent resonant response to applied radio-frequency (RF) fields in the frequency range of several MHz to hundreds of MHz for achievable densities. We have conducted measurements where short bursts of RF were applied to these plasmas, with pulse durations as short as two cycles. We still observed a density-dependent resonant response to these short pulses. However, the too rapid timescale of the response, the dependence of the response on the sign of the driving field, the response as the number of pulses was increased, and the difference in plasma response to radial and axially applied RF fields are inconsistent with the plasma response being due to local resonant heating of electrons in the plasma. Instead, our results are consistent with rapid energy transfer from collective motion of the entire electron cloud to electrons in high-energy orbits. In addition to providing a potentially more robust way to measure ultracold neutral plasma densities, these measurements demonstrate the importance of collective motion in the energy transport in these systems.Comment: 5 pages, 4 figure