17,080 research outputs found
Critical exponents of a three dimensional O(4) spin model
By Monte Carlo simulation we study the critical exponents governing the
transition of the three-dimensional classical O(4) Heisenberg model, which is
considered to be in the same universality class as the finite-temperature QCD
with massless two flavors. We use the single cluster algorithm and the
histogram reweighting technique to obtain observables at the critical
temperature. After estimating an accurate value of the inverse critical
temperature \Kc=0.9360(1), we make non-perturbative estimates for various
critical exponents by finite-size scaling analysis. They are in excellent
agreement with those obtained with the expansion method with
errors reduced to about halves of them.Comment: 25 pages with 8 PS figures, LaTeX, UTHEP-28
From Uncertainty Data to Robust Policies for Temporal Logic Planning
We consider the problem of synthesizing robust disturbance feedback policies
for systems performing complex tasks. We formulate the tasks as linear temporal
logic specifications and encode them into an optimization framework via
mixed-integer constraints. Both the system dynamics and the specifications are
known but affected by uncertainty. The distribution of the uncertainty is
unknown, however realizations can be obtained. We introduce a data-driven
approach where the constraints are fulfilled for a set of realizations and
provide probabilistic generalization guarantees as a function of the number of
considered realizations. We use separate chance constraints for the
satisfaction of the specification and operational constraints. This allows us
to quantify their violation probabilities independently. We compute disturbance
feedback policies as solutions of mixed-integer linear or quadratic
optimization problems. By using feedback we can exploit information of past
realizations and provide feasibility for a wider range of situations compared
to static input sequences. We demonstrate the proposed method on two robust
motion-planning case studies for autonomous driving
Energy-level quantization in YBa2Cu3O7-x phase-slip nanowires
Significant progress has been made in the development of superconducting
quantum circuits, however new quantum devices that have longer decoherence
times at higher temperatures are urgently required for quantum technologies.
Superconducting nanowires with quantum phase slips are promising candidates for
use in novel devices that operate on quantum principles. Here, we demonstrate
ultra-thin YBa2Cu3O7-x nanowires with phase-slip dynamics and study their
switching-current statistics at temperatures below 20 K. We apply theoretical
models that were developed for Josephson junctions and show that our results
provide strong evidence for energy-level quantization in the nanowires. The
crossover temperature to the quantum regime is 12-13 K, while the lifetime in
the excited state exceeds 20 ms at 5.4 K. Both values are at least one order of
magnitude higher than those in conventional Josephson junctions based on
low-temperature superconductors. We also show how the absorption of a single
photon changes the phase-slip and quantum state of a nanowire, which is
important for the development of single-photon detectors with high operating
temperature and superior temporal resolution. Our findings pave the way for a
new class of superconducting nanowire devices for quantum sensing and
computing
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