12,671 research outputs found
A unified approach to realize universal quantum gates in a coupled two-qubit system with fixed always-on coupling
We demonstrate that in a coupled two-qubit system any single-qubit gate can
be decomposed into two conditional two-qubit gates and that any conditional
two-qubit gate can be implemented by a manipulation analogous to that used for
a controlled two-qubit gate. Based on this we present a unified approach to
implement universal single-qubit and two-qubit gates in a coupled two-qubit
system with fixed always-on coupling. This approach requires neither
supplementary circuit or additional physical qubits to control the coupling nor
extra hardware to adjust the energy level structure. The feasibility of this
approach is demonstrated by numerical simulation of single-qubit gates and
creation of two-qubit Bell states in rf-driven inductively coupled two SQUID
flux qubits with realistic device parameters and constant always-on coupling.Comment: 4 pages, 3 figure
Molecular evidence of genetic diversity of Borrelia burgdorferi sensu lato detected in Ixodes granulatus ticks removed from rodents in Taiwan
Supersymmetry Breaking, R-Symmetry Breaking and Metastable Vacua
Models of spontaneous supersymmetry breaking generically have an R-symmetry,
which is problematic for obtaining gaugino masses and avoiding light R-axions.
The situation is improved in models of metastable supersymmetry breaking, which
generically have only an approximate R-symmetry. Based on this we argue, with
mild assumptions, that metastable supersymmetry breaking is inevitable. We also
illustrate various general issues regarding spontaneous and explicit R-symmetry
breaking, using simple toy models of supersymmetry breaking.Comment: 23 page
Two-photon interference with thermal light
The study of entangled states has greatly improved the basic understanding
about two-photon interferometry. Two-photon interference is not the
interference of two photons but the result of superposition among
indistinguishable two-photon amplitudes. The concept of two-photon amplitude,
however, has generally been restricted to the case of entangled photons. In
this letter we report an experimental study that may extend this concept to the
general case of independent photons. The experiment also shows interesting
practical applications regarding the possibility of obtaining high resolution
interference patterns with thermal sources.Comment: Added reference 1
R-symmetry breaking, runaway directions and global symmetries in O'Raifeartaigh models
We discuss O'Raifeartaigh models with general R-charge assignments,
introduced by Shih to break R-symmetry spontaneously. We argue that most of
these models have runaway directions related to the R-symmetry. In addition, we
study the simplest model with a U(N) global symmetry and show that in a range
of parameters R-symmetry is spontaneously broken in a metastable vacuum.Comment: 16 pages, 1 figur
Exact Black Hole Degeneracies and the Topological String
Motivated by the recent conjecture of Ooguri, Strominger and Vafa, we compute
the semi-canonical partition function of BPS black holes in N=4 and N=8 string
theories, to all orders in perturbation theory. Not only are the black hole
partition functions surprisingly simple; they capture the full topological
string amplitudes, as expected from the OSV conjecture. The agreement is not
perfect, however, as there are differences between the black hole and
topological string partition functions even at the perturbative level. We
propose a minimal modification of the OSV conjecture, in which these
differences are understood as a nontrivial measure factor for the topological
string.Comment: 24 page
Quantum teleportation between moving detectors in a quantum field
We consider the quantum teleportation of continuous variables modeled by
Unruh-DeWitt detectors coupled to a common quantum field initially in the
Minkowski vacuum. An unknown coherent state of an Unruh-DeWitt detector is
teleported from one inertial agent (Alice) to an almost uniformly accelerated
agent (Rob, for relativistic motion), using a detector pair initially entangled
and shared by these two agents. The averaged physical fidelity of quantum
teleportation, which is independent of the observer's frame, always drops below
the best fidelity value from classical teleportation before the detector pair
becomes disentangled with the measure of entanglement evaluated around the
future lightcone of the joint measurement event by Alice. The distortion of the
quantum state of the entangled detector pair from the initial state can
suppress the fidelity significantly even when the detectors are still strongly
entangled around the lightcone. We point out that the dynamics of entanglement
of the detector pair observed in Minkowski frame or in quasi-Rindler frame are
not directly related to the physical fidelity of quantum teleportation in our
setup. These results are useful as a guide to making judicious choices of
states and parameter ranges and estimation of the efficiency of quantum
teleportation in relativistic quantum systems under environmental influences.Comment: 18 pages, 7 figure
Low energy physical properties of high-Tc superconducting Cu oxides: A comparison between the resonating valence bond and experiments
In a recent review by Anderson and coworkers\cite{Vanilla}, it was pointed
out that an early resonating valence bond (RVB) theory is able to explain a
number of unusual properties of high temperature superconducting (SC)
Cu-oxides. Here we extend previous calculations \cite{anderson87,FC
Zhang,Randeria} to study more systematically low energy physical properties of
the plain vanilla d-wave RVB state, and to compare results with the available
experiments. We use a renormalized mean field theory combined with variational
Monte Carlo and power Lanczos methods to study the RVB state of an extended
model in a square lattice with parameters suitable for the hole doped
Cu-oxides. The physical observable quantities we study include the specific
heat, the linear residual thermal conductivity, the in-plane magnetic
penetration depth, the quasiparticle energy at the antinode , the
superconducting energy gap, the quasiparticle spectra and the Drude weight. The
traits of nodes (including , the Fermi velocity and the velocity
along Fermi surface ), as well as the SC order parameter are also
studied. Comparisons of the theory and the experiments in cuprates show an
overall qualitative agreement, especially on their doping dependences.Comment: 12 pages, 14 figures, 1 tabl
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