7,875 research outputs found
Efficient quantum computation within a disordered Heisenberg spin-chain
We show that efficient quantum computation is possible using a disordered
Heisenberg spin-chain with `always-on' couplings. Such disorder occurs
naturally in nanofabricated systems. Considering a simple chain setup, we show
that an arbitrary two-qubit gate can be implemented using just three
relaxations of a controlled qubit, which amounts to switching the on-site
energy terms at most twenty-one times.Comment: To appear in Phys. Rev.
Dynamic Matrix Factorization with Priors on Unknown Values
Advanced and effective collaborative filtering methods based on explicit
feedback assume that unknown ratings do not follow the same model as the
observed ones (\emph{not missing at random}). In this work, we build on this
assumption, and introduce a novel dynamic matrix factorization framework that
allows to set an explicit prior on unknown values. When new ratings, users, or
items enter the system, we can update the factorization in time independent of
the size of data (number of users, items and ratings). Hence, we can quickly
recommend items even to very recent users. We test our methods on three large
datasets, including two very sparse ones, in static and dynamic conditions. In
each case, we outrank state-of-the-art matrix factorization methods that do not
use a prior on unknown ratings.Comment: in the Proceedings of 21st ACM SIGKDD Conference on Knowledge
Discovery and Data Mining 201
Effect of Inhomogeneous Heat Flow on the Enhancement of Heat Capacity in Helium-II by Counterflow near Tλ
In 2000 Harter et al. reported the first measurements of the enhancement of the heat capacity ΔCQ[equivalent]C(Q)-C(Q=0) of helium-II transporting a heat flux density Q near Tλ. Surprisingly, their measured ΔCQ was ~7–12 times larger than predicted, depending on which theory was assumed. In this report we present a candidate explanation for this discrepancy: unintended heat flux inhomogeneity. Because C(Q) should diverge at a critical heat flux density Qc, homogeneous heat flow is required for an accurate measurement. We present results from numerical analysis of the heat flow in the Harter et al. cell indicating that substantial inhomogeneity occurred. We determine the effect of the inhomogeneity on ΔCQ and find rough agreement with the observed disparity between prediction and measurement
Testing the Accuracy and Stability of Spectral Methods in Numerical Relativity
The accuracy and stability of the Caltech-Cornell pseudospectral code is
evaluated using the KST representation of the Einstein evolution equations. The
basic "Mexico City Tests" widely adopted by the numerical relativity community
are adapted here for codes based on spectral methods. Exponential convergence
of the spectral code is established, apparently limited only by numerical
roundoff error. A general expression for the growth of errors due to finite
machine precision is derived, and it is shown that this limit is achieved here
for the linear plane-wave test. All of these tests are found to be stable,
except for simulations of high amplitude gauge waves with nontrivial shift.Comment: Final version, as published in Phys. Rev. D; 13 pages, 16 figure
High-fidelity, broadband stimulated-Brillouin-scattering-based slow light using fast noise modulation
We demonstrate a 5-GHz-broadband tunable slow-light device based on
stimulated Brillouin scattering in a standard highly-nonlinear optical fiber
pumped by a noise-current-modulated laser beam. The noise modulation waveform
uses an optimized pseudo-random distribution of the laser drive voltage to
obtain an optimal flat-topped gain profile, which minimizes the pulse
distortion and maximizes pulse delay for a given pump power. Eye-diagram and
signal-to-noise ratio (SNR) analysis show that this new broadband slow-light
technique significantly increases the fidelity of a delayed data sequence,
while maintaining the delay performance. A fractional delay of 0.81 with a SNR
of 5.2 is achieved at the pump power of 350 mW using a 2-km-long highly
nonlinear fiber with the fast noise-modulation method, demonstrating a 50%
increase in eye-opening and a 36% increase in SNR compared to a previous
slow-modulation method
General linear-optical quantum state generation scheme: Applications to maximally path-entangled states
We introduce schemes for linear-optical quantum state generation. A quantum
state generator is a device that prepares a desired quantum state using product
inputs from photon sources, linear-optical networks, and postselection using
photon counters. We show that this device can be concisely described in terms
of polynomial equations and unitary constraints. We illustrate the power of
this language by applying the Grobner-basis technique along with the notion of
vacuum extensions to solve the problem of how to construct a quantum state
generator analytically for any desired state, and use methods of convex
optimization to identify bounds to success probabilities. In particular, we
disprove a conjecture concerning the preparation of the maximally
path-entangled |n,0)+|0,n) (NOON) state by providing a counterexample using
these methods, and we derive a new upper bound on the resources required for
NOON-state generation.Comment: 5 pages, 2 figure
Formation of Kuiper-belt binaries through multiple chaotic scattering encounters with low-mass intruders
The discovery that many trans-neptunian objects exist in pairs, or binaries,
is proving invaluable for shedding light on the formation, evolution and
structure of the outer Solar system. Based on recent systematic searches it has
been estimated that up to 10% of Kuiper-belt objects might be binaries.
However, all examples discovered to-date are unusual, as compared to near-Earth
and main-belt asteroid binaries, for their mass ratios of order unity and their
large, eccentric orbits. In this article we propose a common dynamical origin
for these compositional and orbital properties based on four-body simulations
in the Hill approximation. Our calculations suggest that binaries are produced
through the following chain of events: initially, long-lived quasi-bound
binaries form by two bodies getting entangled in thin layers of dynamical chaos
produced by solar tides within the Hill sphere. Next, energy transfer through
gravitational scattering with a low-mass intruder nudges the binary into a
nearby non-chaotic, stable zone of phase space. Finally, the binary hardens
(loses energy) through a series of relatively gentle gravitational scattering
encounters with further intruders. This produces binary orbits that are well
fitted by Kepler ellipses. Dynamically, the overall process is strongly favored
if the original quasi-bound binary contains comparable masses. We propose a
simplified model of chaotic scattering to explain these results. Our findings
suggest that the observed preference for roughly equal mass ratio binaries is
probably a real effect; that is, it is not primarily due to an observational
bias for widely separated, comparably bright objects. Nevertheless, we predict
that a sizeable population of very unequal mass Kuiper-belt binaries is likely
awaiting discovery.Comment: This is a preprint of an Article accepted for publication in Monthly
Notices of the Royal Astronomical Society, (C) 2005 The Royal Astronomical
Societ
Eye-Safe Solid-State Quasi-CW Raman Laser with Millisecond Pulse Duration
We demonstrate the first quasi-CW (ms-long pulses, pump duty cycle of 10%)
end-diode pumped solid state laser generating eye-safe radiation via
intracavity Raman conversion. The output power at the first Stokes wavelength
(1524 nm) was 250 mW. A theoretical model was applied to analyze the laser
system and provide routes for optimization. The possibility of true CW
operation was discussed.Comment: Preprint accepted for publication in Optics Communications on Feb 6,
201
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