Information gain versus coupling strength in quantum measurements

We investigate the relationship between the information gain and the interaction strength between the quantum system and the measuring device. A strategy is proposed to calculate the information gain of the measuring device as the coupling strength is a variable. For qubit systems, we prove that the information gain increases monotonically with the coupling strength. It is obtained that the information gain of the projective measurement along the x-direction reduces with the increasing of the measurement strength along the z-direction, and a complementarity of information gain in the measurements along those two directions is presented.Comment: 7 pages, 1 figure

Controlled exchange interaction for quantum logic operations with spin qubits in coupled quantum dots

A two-electron system confined in two coupled semiconductor quantum dots is investigated as a candidate for performing quantum logic operations on spin qubits. We study different processes of swapping the electron spins by controlled switching on/off the exchange interaction. The resulting spin swap corresponds to an elementary operation in quantum information processing. We perform a direct time evolution simulations of the time-dependent Schroedinger equation. Our results show that -- in order to obtain the full interchange of spins -- the exchange interaction should change smoothly in time. The presence of jumps and spikes in the corresponding time characteristics leads to a considerable increase of the spin swap time. We propose several mechanisms to modify the exchange interaction by changing the confinement potential profile and discuss their advantages and disadvantages

On the role of a new type of correlated disorder in extended electronic states in the Thue-Morse lattice

A new type of correlated disorder is shown to be responsible for the appearance of extended electronic states in one-dimensional aperiodic systems like the Thue-Morse lattice. Our analysis leads to an understanding of the underlying reason for the extended states in this system, for which only numerical evidence is available in the literature so far. The present work also sheds light on the restrictive conditions under which the extended states are supported by this lattice.Comment: 11 pages, LaTeX V2.09, 1 figure (available on request), to appear in Physical Review Letter

Resonant pairing between Fermions with unequal masses

We study the pairing between Fermions of different masses, especially at the unitary limit. At equal populations, the thermodynamic properties are identical with the equal mass case provided an appropriate rescaling is made. At unequal populations, for sufficiently light majority species, the system does not phase separate. For sufficiently heavy majority species, the phase separated normal phase have a density larger than that of the superfluid. For atoms in harmonic traps, the density profiles for unequal mass Fermions can be drastically different from their equal-mass counterparts.Comment: 10 pages, 4 figure

Discovery of {\gamma}-ray pulsation and X-ray emission from the black widow pulsar PSR J2051-0827

We report the discovery of pulsed {\gamma}-ray emission and X-ray emission from the black widow millisecond pulsar PSR J2051-0827 by using the data from the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope and the Advanced CCD Imaging Spectrometer array (ACIS-S) on the Chandra X-ray Observatory. Using 3 years of LAT data, PSR J2051-0827 is clearly detected in {\gamma}-ray with a signicance of \sim 8{\sigma} in the 0.2 - 20 GeV band. The 200 MeV - 20 GeV {\gamma}-ray spectrum of PSR J2051-0827 can be modeled by a simple power- law with a photon index of 2.46 \pm 0.15. Significant (\sim 5{\sigma}) {\gamma}-ray pulsations at the radio period were detected. PSR J2051-0827 was also detected in soft (0.3-7 keV) X-ray with Chandra. By comparing the observed {\gamma}-rays and X-rays with theoretical models, we suggest that the {\gamma}-ray emission is from the outer gap while the X-rays can be from intra-binary shock and pulsar magnetospheric synchrotron emissions.Comment: 10 pages, 4 figures, accepted by ApJ on Jan 28, 201

Discovery of gamma-ray emission from the supernova remnant Kes 17 with Fermi Large Area Telescope

We report the discovery of GeV emission at the position of supernova remnant Kes 17 by using the data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. Kes 17 can be clearly detected with a significance of ~12 sigma in the 1 - 20 GeV range. Moreover, a number of gamma-ray sources were detected in its vicinity. The gamma-ray spectrum of Kes 17 can be well described by a simple power-law with a photon index of ~ 2.4. Together with the multi-wavelength evidence for its interactions with the nearby molecular cloud, the gamma-ray detection suggests that Kes 17 is a candidate acceleration site for cosmic-rays.Comment: 13 pages, 3 figures, 1 table, accepted for publication in ApJ Lette

Linking entanglement and quantum phase transitions via density functional theory

Density functional theory (DFT) is shown to provide a novel conceptual and computational framework for entanglement in interacting many-body quantum systems. DFT can, in particular, shed light on the intriguing relationship between quantum phase transitions and entanglement. We use DFT concepts to express entanglement measures in terms of the first or second derivative of the ground state energy. We illustrate the versatility of the DFT approach via a variety of analytically solvable models. As a further application we discuss entanglement and quantum phase transitions in the case of mean field approximations for realistic models of many-body systems.Comment: 6 pages, 2 figure

Theoretical modeling of spatial and temperature dependent exciton energy in coupled quantum wells

Motivated by a recent experiment of spatial and temperature dependent average exciton energy distribution in coupled quantum wells [S. Yang \textit{et al.}, Phys. Rev. B \textbf{75}, 033311 (2007)], we investigate the nature of the interactions in indirect excitons. Based on the uncertainty principle, along with a temperature and energy dependent distribution which includes both population and recombination effects, we show that the interplay between an attractive two-body interaction and a repulsive three-body interaction can lead to a natural and good account for the nonmonotonic temperature dependence of the average exciton energy. Moreover, exciton energy maxima are shown to locate at the brightest regions, in agreement with the recent experiments. Our results provide an alternative way for understanding the underlying physics of the exciton dynamics in coupled quantum wells.Comment: 8 pages, 5 figure