Profiles of thermal line emission from advection dominated accretion flows

Recently, Narayan & Raymond (1999) proposed that the thermal emission lines from the hot plasma in advection dominated accretion flows (ADAFs) are potentially observable with the next generation of X-ray observatories, with which the physical properties of some X-ray sources can be probed. In ADAFs, the temperature of the ion is so high that the thermal broadening of the line is important. We calculate the profiles of thermal line emission from ADAFs, in which both the thermal and Doppler broadening have been considered. It is found that the double-peaked profiles are present for high inclination angles between the axis of disk and the line of sight. The double-peaked profiles are smeared in low inclination cases, and completely disappear while the inclination angle is less than $15^{\circ}$, where the thermal and turbulent broadening dominated on the line profiles. We also note that the thermal line profile is affected by the location of the transition radius of ADAF. The self-similar height-integrated disk structure and the emissivity with power-law dependence of radius are adopted in our calculations. The results obtained in this work can be used as a diagnosis on the future X-ray observations of the thermal lines. Some important physical quantities of ADAFs could be inferred from future thermal line observations.Comment: 7 page

Quantum spin mixing in a binary mixture of spin-1 atomic condensates

We study quantum spin mixing in a binary mixture of spin-1 condensates including coherent interspecies mixing process, using the familiar spinor condensates of $^{87}$Rb and $^{23}$Na atoms in the ground lower hyperfine F=1 manifolds as prototype examples. Within the single spatial mode approximation for each of the two spinor condensates, the mixing dynamics reduce to that of three coupled nonlinear pendulums with clear physical interpretations. Using suitably prepared initial states, it is possible to determine the interspecies singlet-pairing as well as spin-exchange interactions from the subsequent mixing dynamics.Comment: 6 pages, 3 figure

Creating maximally entangled atomic states in a Bose-Einstein condensate

We propose a protocol to create maximally entangled pairs, triplets, quartiles, and other clusters of Bose condensed atoms starting from a condensate in the Mott insulator state. The essential element is to drive single atom Raman transitions using laser pulses. Our scheme is simple, efficient, and can be readily applied to the recent experimental system as reported by Greiner {\it et al.} [ Nature {\bf 413}, 44 (2002)].Comment: 4 pages, 2 figures. revised version as to be publishe

Scalable quantum computing with Josephson charge qubits

A goal of quantum information technology is to control the quantum state of a system, including its preparation, manipulation, and measurement. However, scalability to many qubits and controlled connectivity between any selected qubits are two of the major stumbling blocks to achieve quantum computing (QC). Here we propose an experimental method, using Josephson charge qubits, to efficiently solve these two central problems. The proposed QC architecture is scalable since any two charge qubits can be effectively coupled by an experimentally accessible inductance. More importantly, we formulate an efficient and realizable QC scheme that requires only one (instead of two or more) two-bit operation to implement conditional gates.Comment: 4 pages, 2 figure

Encoding a qubit with Majorana modes in superconducting circuits

Majorana fermions are long-sought exotic particles that are their own antiparticles. Here we propose to utilize superconducting circuits to construct two superconducting-qubit arrays where Majorana modes can occur. A so-called Majorana qubit is encoded by using the unpaired Majorana modes, which emerge at the left and right ends of the chain in the Majorana-fermion representation. We also show this Majorana qubit in the spin representation and its advantage, over a single superconducting qubit, regarding quantum coherence. Moreover, we propose to use four superconducting qubits as the smallest system to demonstrate the braiding of Majorana modes and show how the states before and after braiding Majoranas can be discriminated.Comment: 10 pages, 3 figures; an enlarged version of arXiv: 1108.3712v