29,542 research outputs found
Direct sampling of the Susskind-Glogower phase distributions
Coarse-grained phase distributions are introduced that approximate to the
Susskind--Glogower cosine and sine phase distributions. The integral relations
between the phase distributions and the phase-parametrized field-strength
distributions observable in balanced homodyning are derived and the integral
kernels are analyzed. It is shown that the phase distributions can be directly
sampled from the field-strength distributions which offers the possibility of
measuring the Susskind--Glogower cosine and sine phase distributions with
sufficiently well accuracy. Numerical simulations are performed to demonstrate
the applicability of the method.Comment: 10 figures using a4.st
Universal width distributions in non-Markovian Gaussian processes
We study the influence of boundary conditions on self-affine random functions
u(t) in the interval t/L \in [0,1], with independent Gaussian Fourier modes of
variance ~ 1/q^{\alpha}. We consider the probability distribution of the mean
square width of u(t) taken over the whole interval or in a window t/L \in [x,
x+\delta]. Its characteristic function can be expressed in terms of the
spectrum of an infinite matrix. This distribution strongly depends on the
boundary conditions of u(t) for finite \delta, but we show that it is universal
(independent of boundary conditions) in the small-window limit. We compute it
directly for all values of \alpha, using, for \alpha<3, an asymptotic expansion
formula that we derive. For \alpha > 3, the limiting width distribution is
independent of \alpha. It corresponds to an infinite matrix with a single
non-zero eigenvalue. We give the exact expression for the width distribution in
this case. Our analysis facilitates the estimation of the roughness exponent
from experimental data, in cases where the standard extrapolation method cannot
be usedComment: 15 page
Non-equilibrium supercurrent through a quantum dot: current harmonics and proximity effect due to a normal metal lead
We consider a Hamiltonian model for a quantum dot which is placed between two
superconducting leads with a constant bias imposed between these leads. Using
the non-equilibrium Keldysh technique, we focus on the subgap current, where it
is known that multiple Andreev reflections (MAR) are responsible for charge
transfer through the dot. Attention is put on the DC current and on the first
harmonics of the supercurrent. Varying the energy and width of the resonant
level on the dot, we first investigate a cross-over from a quantum dot regime
to a quantum point contact regime when there is zero coupling to the normal
probe. We then study the effect on the supercurrent of the normal probe which
is attached to the dot. This normal probe is understood to lead to dephasing,
or alternatively to induce reverse proximity effect. We describe the full
crossover from zero dephasing to the incoherent case. We also compute the
Josephson current in the presence of the normal lead, and find it in excellent
agreement with the values of the non-equlibrium current extrapolated at zero
voltage
Fast Calculation of the Lomb-Scargle Periodogram Using Graphics Processing Units
I introduce a new code for fast calculation of the Lomb-Scargle periodogram,
that leverages the computing power of graphics processing units (GPUs). After
establishing a background to the newly emergent field of GPU computing, I
discuss the code design and narrate key parts of its source. Benchmarking
calculations indicate no significant differences in accuracy compared to an
equivalent CPU-based code. However, the differences in performance are
pronounced; running on a low-end GPU, the code can match 8 CPU cores, and on a
high-end GPU it is faster by a factor approaching thirty. Applications of the
code include analysis of long photometric time series obtained by ongoing
satellite missions and upcoming ground-based monitoring facilities; and
Monte-Carlo simulation of periodogram statistical properties.Comment: Accepted by ApJ. Accompanying program source (updated since
acceptance) can be downloaded from
http://www.astro.wisc.edu/~townsend/resource/download/code/culsp.tar.g
Dissipation, topology, and quantum phase transition in a one-dimensional Joesphson junction array
We study the phase diagram and quantum critical properties of a resistively
shunted Josephson junction array in one dimension from a strong coupling
analysis. After mapping the dissipative quantum phase model to an effective
sine-Gordon model we study the renormalization group flow and the phase
diagram. We try to bridge the phase diagrams obtained from the weak and the
strong coupling renormalization group calculations to extract a more
comprehensive picture of the complete phase diagram. The relevance of our
theory to experiments in nanowires is discussed.Comment: 13 pages, 3 figures, A few typos are correcte
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