6,334 research outputs found
Unified model for vortex-string network evolution
We describe and numerically test the velocity-dependent one-scale (VOS)
string evolution model, a simple analytic approach describing a string network
with the averaged correlation length and velocity. We show that it accurately
reproduces the large-scale behaviour (in particular the scaling laws) of
numerical simulations of both Goto-Nambu and field theory string networks. We
explicitly demonstrate the relation between the high-energy physics approach
and the damped and non-relativistic limits which are relevant for condensed
matter physics. We also reproduce experimental results in this context and show
that the vortex-string density is significantly reduced by loop production, an
effect not included in the usual `coarse-grained' approach.Comment: 5 pages; v2: cosmetic changes, version to appear in PR
Quantum Computation and Spin Physics
A brief review is given of the physical implementation of quantum computation
within spin systems or other two-state quantum systems. The importance of the
controlled-NOT or quantum XOR gate as the fundamental primitive operation of
quantum logic is emphasized. Recent developments in the use of quantum
entanglement to built error-robust quantum states, and the simplest protocol
for quantum error correction, are discussed.Comment: 21 pages, Latex, 3 eps figures, prepared for the Proceedings of the
Annual MMM Meeting, November, 1996, to be published in J. Appl. Phy
Angular distribution of photoluminescence as a probe of Bose Condensation of trapped excitons
Recent experiments on two-dimensional exciton systems have shown the excitons
collect in shallow in-plane traps. We find that Bose condensation in a trap
results in a dramatic change of the exciton photoluminescence (PL) angular
distribution. The long-range coherence of the condensed state gives rise to a
sharply focussed peak of radiation in the direction normal to the plane. By
comparing the PL profile with and without Bose Condensation we provide a simple
diagnostic for the existence of a Bose condensate. The PL peak has strong
temperature dependence due to the thermal order parameter phase fluctuations
across the system. The angular PL distribution can also be used for imaging
vortices in the trapped condensate. Vortex phase spatial variation leads to
destructive interference of PL radiation in certain directions, creating nodes
in the PL distribution that imprint the vortex configuration.Comment: 4 pages, 3 figure
Separability of very noisy mixed states and implications for NMR quantum computing
We give a constructive proof that all mixed states of N qubits in a
sufficiently small neighborhood of the maximally mixed state are separable. The
construction provides an explicit representation of any such state as a mixture
of product states. We give upper and lower bounds on the size of the
neighborhood, which show that its extent decreases exponentially with the
number of qubits. We also discuss the implications of the bounds for NMR
quantum computing.Comment: 4 pages, extensively revised, references adde
ROM-based quantum computation: Experimental explorations using Nuclear Magnetic Resonance, and future prospects
ROM-based quantum computation (QC) is an alternative to oracle-based QC. It
has the advantages of being less ``magical'', and being more suited to
implementing space-efficient computation (i.e. computation using the minimum
number of writable qubits). Here we consider a number of small (one and
two-qubit) quantum algorithms illustrating different aspects of ROM-based QC.
They are: (a) a one-qubit algorithm to solve the Deutsch problem; (b) a
one-qubit binary multiplication algorithm; (c) a two-qubit controlled binary
multiplication algorithm; and (d) a two-qubit ROM-based version of the
Deutsch-Jozsa algorithm. For each algorithm we present experimental
verification using NMR ensemble QC. The average fidelities for the
implementation were in the ranges 0.9 - 0.97 for the one-qubit algorithms, and
0.84 - 0.94 for the two-qubit algorithms. We conclude with a discussion of
future prospects for ROM-based quantum computation. We propose a four-qubit
algorithm, using Grover's iterate, for solving a miniature ``real-world''
problem relating to the lengths of paths in a network.Comment: 11 pages, 5 figure
Experimental Realization of A Two Bit Phase Damping Quantum Code
Using nuclear magnetic resonance techniques, we experimentally investigated
the effects of applying a two bit phase error detection code to preserve
quantum information in nuclear spin systems. Input states were stored with and
without coding, and the resulting output states were compared with the
originals and with each other. The theoretically expected result, net reduction
of distortion and conditional error probabilities to second order, was indeed
observed, despite imperfect coding operations which increased the error
probabilities by approximately 5%. Systematic study of the deviations from the
ideal behavior provided quantitative measures of different sources of error,
and good agreement was found with a numerical model. Theoretical questions in
quantum error correction in bulk nuclear spin systems including fidelity
measures, signal strength and syndrome measurements are discussed.Comment: 21 pages, 17 figures, mypsfig2, revtex. Minor changes made to appear
in PR
A Driving Performance Forecasting System Based on Brain Dynamic State Analysis Using 4-D Convolutional Neural Networks.
Vehicle accidents are the primary cause of fatalities worldwide. Most often, experiencing fatigue on the road leads to operator errors and behavioral lapses. Thus, there is a need to predict the cognitive state of drivers, particularly their fatigue level. Electroencephalography (EEG) has been demonstrated to be effective for monitoring changes in the human brain state and behavior. Thirty-seven subjects participated in this driving experiment and performed a perform lane-keeping task in a visual-reality environment. Three domains, namely, frequency, temporal, and 2-D spatial information, of the EEG channel location were comprehensively considered. A 4-D convolutional neural-network (4-D CNN) algorithm was then proposed to associate all information from the EEG signals and the changes in the human state and behavioral performance. A 4-D CNN achieves superior forecasting performance over 2-D CNN, 3-D CNN, and shallow networks. The results showed a 3.82% improvement in the root mean-square error, a 3.45% improvement in the error rate, and a 11.98% improvement in the correlation coefficient with 4-D CNN compared with 3-D CNN. The 4-D CNN algorithm extracts the significant θ and alpha activations in the frontal and posterior cingulate cortices under distinct fatigue levels. This work contributes to enhancing our understanding of deep learning methods in the analysis of EEG signals. We even envision that deep learning might serve as a bridge between translation neuroscience and further real-world applications
Efficient implementation of selective recoupling in heteronuclear spin systems using Hadamard matrices
We present an efficient scheme which couples any designated pair of spins in
heteronuclear spin systems. The scheme is based on the existence of Hadamard
matrices. For a system of spins with pairwise coupling, the scheme
concatenates intervals of system evolution and uses at most pulses
where . Our results demonstrate that, in many systems, selective
recoupling is possible with linear overhead, contrary to common speculation
that exponential effort is always required.Comment: 7 pages, 4 figures, mypsfig2, revtex, submitted April 27, 199
An Alternative Topological Field Theory of Generalized Complex Geometry
We propose a new topological field theory on generalized complex geometry in
two dimension using AKSZ formulation. Zucchini's model is model in the case
that the generalized complex structuredepends on only a symplectic structure.
Our new model is model in the case that the generalized complex structure
depends on only a complex structure.Comment: 29 pages, typos and references correcte
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