1,219 research outputs found
Experimental ancilla-assisted qubit transmission against correlated noise using quantum parity checking
We report the experimental demonstration of a transmission scheme of photonic
qubits over unstabilized optical fibers, which has the plug-and-play feature as
well as the ability to transmit any state of a qubit, regardless of whether it
is known, unknown, or entangled to other systems. A high fidelity to the
noiseless quantum channel was achieved by adding an ancilla photon after the
signal photon within the correlation time of the fiber noise and by performing
quantum parity checking. Simplicity, maintenance-free feature and robustness
against path-length mismatches among the nodes make our scheme suitable for
multi-user quantum communication networks.Comment: 8 pages, 4 figures; published in New J. Phys. and selected in IOP
Selec
Spinning cosmic strings: a general class of solutions
In this work, we give a general class of solutions of the spinning cosmic
string in Einstein's theory of gravity. After treating same problem in Einstein
Cartan (EC) theory of gravity, the exact solution satisfying both exterior and
interior space-times representing a spin fluid moving along the symmetry axis
is presented in the EC theory. The existence of closed timelike curves in this
spacetime are also examined
Coupled wake boundary layer model of wind-farms
We present and test the coupled wake boundary layer (CWBL) model that
describes the distribution of the power output in a wind-farm. The model
couples the traditional, industry-standard wake model approach with a
"top-down" model for the overall wind-farm boundary layer structure. This wake
model captures the effect of turbine positioning, while the "top-down" portion
of the model adds the interactions between the wind-turbine wakes and the
atmospheric boundary layer. Each portion of the model requires specification of
a parameter that is not known a-priori. For the wake model, the wake expansion
coefficient is required, while the "top-down" model requires an effective
spanwise turbine spacing within which the model's momentum balance is relevant.
The wake expansion coefficient is obtained by matching the predicted mean
velocity at the turbine from both approaches, while the effective spanwise
turbine spacing depends on turbine positioning and thus can be determined from
the wake model. Coupling of the constitutive components of the CWBL model is
achieved by iterating these parameters until convergence is reached. We
illustrate the performance of the model by applying it to both developing
wind-farms including entrance effects and to fully developed (deep-array)
conditions. Comparisons of the CWBL model predictions with results from a suite
of large eddy simulations (LES) shows that the model closely represents the
results obtained in these high-fidelity numerical simulations. A comparison
with measured power degradation at the Horns Rev and Nysted wind-farms shows
that the model can also be successfully applied to real wind-farms.Comment: 25 pages, 21 figures, submitted to Journal of Renewable and
Sustainable Energy on July 18, 201
Iridium(i) complexes bearing hemilabile coumarin-functionalised N-heterocyclic carbene ligands with application as alkyne hydrosilylation catalysts
A set of iridium(i) complexes of formula IrCl(¿C, ¿2-IRCouR')(cod) or IrCl(¿C, ¿2-BzIRCouR')(cod) (cod = 1, 5-cyclooctadiene; Cou = coumarin; I = imidazolin-2-carbene; BzI = benzimidazolin-2-carbene) have beeen prepared from the corresponding azolium salt and [Ir(µ-OMe)(cod)]2 in THF at room temperature. The crystalline structures of 4b and 5b show a distorted trigonal bipyramidal configuration in the solid state with a coordinated coumarin moiety. In contrast, an equilibrium between this pentacoordinated structure and the related square planar isomer is observed in solution as a consequence of the hemilability of the pyrone ring. Characterization of both species by NMR was achieved at the low and high temperature limits, respectively. In addition, the thermodynamic parameters of the equilibrium, ¿HR and ¿SR, were obtained by VT 1H NMR spectroscopy and fall in the range 22-33 kJ mol-1 and 72-113 J mol-1 K-1, respectively. Carbonylation of IrCl(¿C, ¿2-BzITolCou7, 8-Me2)(cod) resulted in the formation of a bis-CO derivative showing no hemilabile behaviour. The newly synthesised complexes efficiently catalyze the hydrosilylation of alkynes at room temperature with a preference for the ß-(Z) vinylsilane isomer. © The Royal Society of Chemistry
Gravitational shock waves and vacuum fluctuations
We show that the vacuum expectation value of the stress-energy tensor of a
scalar particle on the background of a spherical gravitational shock wave does
not give a finite expression in second order perturbation theory, contrary to
the case seen for the impulsive wave. No infrared divergences appear at this
order. This result shows that there is a qualitative difference between the
shock and impulsive wave solutions which is not exhibited in first order.Comment: Submitted to Class. and Quant. Grav.,7 pages, no figure
One-way quantum computing in a decoherence-free subspace
We introduce a novel scheme for one-way quantum computing (QC) based on the
use of information encoded qubits in an effective cluster state resource. With
the correct encoding structure, we show that it is possible to protect the
entangled resource from phase damping decoherence, where the effective cluster
state can be described as residing in a Decoherence-Free Subspace (DFS) of its
supporting quantum system. One-way QC then requires either single or two-qubit
adaptive measurements. As an example where this proposal can be realized, we
describe an optical lattice setup where the scheme provides robust quantum
information processing. We also outline how one can adapt the model to provide
protection from other types of decoherence.Comment: 9 pages, 4 figures, RevTeX
Shepherding with robots that do not compute
We examine the problem solving capabilities of swarms of computation- and memory-free agents. Each agent has a single line-of-sight sensor providing two bits of information. The agent maps this information directly onto constant motor commands. In previous work, we showed that such simplistic agents can solve tasks requiring them to organize spatially (multi-robot aggregation and circle formation) and manipulate passive objects (clustering). In the present work, we address the shepherding problem, where the computation- and memory-free agents—the shepherds—are tasked to gather and move a group of dynamic agents—the sheep—towards a pre-defined goal. The shepherds and sheep are modelled as e-puck robots using computer simulations. Our findings show that the shepherding problem does not fundamentally require arithmetic computation or memory to be solved. The obtained controller solution is robust with respect to sensory noise, and copes well with changes in the number of sheep
Particle Creation If a Cosmic String Snaps
We calculate the Bogolubov coefficients for a metric which describes the
snapping of a cosmic string. If we insist on a matching condition for all times
{\it and} a particle interpretation, we find no particle creation.Comment: 10 pages, MRC.PH.17/9
- …