9,234 research outputs found
Cellular Structures for Computation in the Quantum Regime
We present a new cellular data processing scheme, a hybrid of existing
cellular automata (CA) and gate array architectures, which is optimized for
realization at the quantum scale. For conventional computing, the CA-like
external clocking avoids the time-scale problems associated with ground-state
relaxation schemes. For quantum computing, the architecture constitutes a novel
paradigm whereby the algorithm is embedded in spatial, as opposed to temporal,
structure. The architecture can be exploited to produce highly efficient
algorithms: for example, a list of length N can be searched in time of order
cube root N.Comment: 11 pages (LaTeX), 3 figure
A Chandra Study of the Dense Globular Cluster Terzan 5
We report a Chandra ACIS-I observation of the dense globular cluster Terzan
5. The previously known transient low-mass x-ray binary (LMXB) EXO 1745-248 in
the cluster entered a rare high state during our August 2000 observation,
complicating the analysis. Nevertheless nine additional sources clearly
associated with the cluster are also detected, ranging from L_X(0.5-2.5
keV)=5.6*10^{32} down to 8.6*10^{31} ergs/s. Their X-ray colors and
luminosities, and spectral fitting, indicate that five of them are probably
cataclysmic variables, and four are likely quiescent LMXBs containing neutron
stars. We estimate the total number of sources between L_X(0.5-2.5 keV)=10^{32}
and 10^{33} ergs/s as 11.4^{+4.7}_{-1.8} by the use of artificial point source
tests, and note that the numbers of X-ray sources are similar to those detected
in NGC 6440. The improved X-ray position allowed us to identify a plausible
infrared counterpart to EXO 1745-248 on our 1998 Hubble Space Telescope NICMOS
images. This blue star (F110W=18.48, F187W=17.30) lies within 0.2'' of the
boresighted LMXB position. Simultaneous Rossi X-ray Timing Explorer (RXTE)
spectra, combined with the Chandra spectrum, indicate that EXO 1745-248 is an
ultracompact binary system, and show a strong broad 6.55 keV iron line and an 8
keV smeared reflection edge.Comment: 18 pages, 8 figures, accepted to Ap
Exploring Large-scale Structure with Billions of Galaxies
We consider cosmological applications of galaxy number density correlations
to be inferred from future deep and wide multi-band optical surveys. We mostly
focus on very large scales as a probe of possible features in the primordial
power spectrum. We find the proposed survey of the Large Synoptic Survey
Telescope may be competitive with future all-sky CMB experiments over a broad
range of scales. On very large scales the inferred power spectrum is robust to
photometric redshift errors, and, given a sufficient number density of
galaxies, to angular variations in dust extinction and photometric calibration
errors. We also consider other applications, such as constraining dark energy
with the two CMB-calibrated standard rulers in the matter power spectrum, and
controlling the effect of photometric redshift errors to facilitate the
interpretation of cosmic shear data. We find that deep photometric surveys over
wide area can provide constraints that are competitive with spectroscopic
surveys in small volumes.Comment: 11 pages, 7 figures, ApJ accepted, references added, expanded
discussion in Sec. 3.
Linear optics substituting scheme for multi-mode operations
We propose a scheme allowing a conditional implementation of suitably
truncated general single- or multi-mode operators acting on states of traveling
optical signal modes. The scheme solely relies on single-photon and coherent
states and applies beam splitters and zero- and single-photon detections. The
signal flow of the setup resembles that of a multi-mode quantum teleportation
scheme thus allowing the individual signal modes to be spatially separated from
each other. Some examples such as the realization of cross-Kerr nonlinearities,
multi-mode mirrors, and the preparation of multi-photon entangled states are
considered.Comment: 11 pages, 4 eps-figures, using revtex
Nonlinear quantum mechanics implies polynomial-time solution for NP-complete and #P problems
If quantum states exhibit small nonlinearities during time evolution, then
quantum computers can be used to solve NP-complete problems in polynomial time.
We provide algorithms that solve NP-complete and #P oracle problems by
exploiting nonlinear quantum logic gates. It is argued that virtually any
deterministic nonlinear quantum theory will include such gates, and the method
is explicitly demonstrated using the Weinberg model of nonlinear quantum
mechanics.Comment: 10 pages, no figures, submitted to Phys. Rev. Let
Numerical simulation of a Controlled-Controlled-Not (CCN) quantum gate in a chain of three interacting nuclear spins system
We present the study of a quantum Controlled-Controlled-Not gate, implemented
in a chain of three nuclear spins weakly Ising interacting between all of them,
that is, taking into account first and second neighbor spin interactions. This
implementation is done using a single resonant -pulse on the initial state
of the system (digital and superposition). The fidelity parameter is used to
determine the behavior of the CCN quantum gate as a function of the ratio of
the second neighbor interaction coupling constant to the first neighbor
interaction coupling constant (). We found that for we can
have a well defined CCN quantum gate.Comment: 9 pages, 5 fugure
A solvable model of a random spin-1/2 XY chain
The paper presents exact calculations of thermodynamic quantities for the
spin-1/2 isotropic XY chain with random lorentzian intersite interaction and
transverse field that depends linearly on the surrounding intersite
interactions.Comment: 14 pages (Latex), 2 tables, 13 ps-figures included, (accepted for
publication in Phys.Rev.B
Impact of time-ordered measurements of the two states in a niobium superconducting qubit structure
Measurements of thermal activation are made in a superconducting, niobium
Persistent-Current (PC) qubit structure, which has two stable classical states
of equal and opposite circulating current. The magnetization signal is read out
by ramping the bias current of a DC SQUID. This ramping causes time-ordered
measurements of the two states, where measurement of one state occurs before
the other. This time-ordering results in an effective measurement time, which
can be used to probe the thermal activation rate between the two states.
Fitting the magnetization signal as a function of temperature and ramp time
allows one to estimate a quality factor of 10^6 for our devices, a value
favorable for the observation of long quantum coherence times at lower
temperatures.Comment: 14 pages, 4 figure
Mean first-passage times of non-Markovian random walkers in confinement
The first-passage time (FPT), defined as the time a random walker takes to
reach a target point in a confining domain, is a key quantity in the theory of
stochastic processes. Its importance comes from its crucial role to quantify
the efficiency of processes as varied as diffusion-limited reactions, target
search processes or spreading of diseases. Most methods to determine the FPT
properties in confined domains have been limited to Markovian (memoryless)
processes. However, as soon as the random walker interacts with its
environment, memory effects can not be neglected. Examples of non Markovian
dynamics include single-file diffusion in narrow channels or the motion of a
tracer particle either attached to a polymeric chain or diffusing in simple or
complex fluids such as nematics \cite{turiv2013effect}, dense soft colloids or
viscoelastic solution. Here, we introduce an analytical approach to calculate,
in the limit of a large confining volume, the mean FPT of a Gaussian
non-Markovian random walker to a target point. The non-Markovian features of
the dynamics are encompassed by determining the statistical properties of the
trajectory of the random walker in the future of the first-passage event, which
are shown to govern the FPT kinetics.This analysis is applicable to a broad
range of stochastic processes, possibly correlated at long-times. Our
theoretical predictions are confirmed by numerical simulations for several
examples of non-Markovian processes including the emblematic case of the
Fractional Brownian Motion in one or higher dimensions. These results show, on
the basis of Gaussian processes, the importance of memory effects in
first-passage statistics of non-Markovian random walkers in confinement.Comment: Submitted version. Supplementary Information can be found on the
Nature website :
http://www.nature.com/nature/journal/v534/n7607/full/nature18272.htm
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