838 research outputs found
Perturbative analysis of anharmonic chains of oscillators out of equilibrium
We compute the first-order correction to the correlation functions of the
stationary state of a stochastically forced harmonic chain out of equilibrium
when a small on-site anharmonic potential is added. This is achieved by
deriving a suitable formula for the covariance matrix of the invariant state.
We find that the first-order correction of the heat current does not depend on
the size of the system. Second, the temperature profile is linear when the
harmonic part of the on-site potential is zero. The sign of the gradient of the
profile, however, is opposite to the sign of the temperature difference of the
two heat baths.Comment: 26 pages, 2 figures, corrected typo
Real-time Text Queries with Tunable Term Pair Indexes
Term proximity scoring is an established means in information retrieval for improving result quality of full-text queries. Integrating such proximity scores into efficient query processing, however, has not been equally well studied. Existing methods make use of precomputed lists of documents where tuples of terms, usually pairs, occur together, usually incurring a huge index size compared to term-only indexes. This paper introduces a joint framework for trading off index size and result quality, and provides optimization techniques for tuning precomputed indexes towards either maximal result quality or maximal query processing performance, given an upper bound for the index size. The framework allows to selectively materialize lists for pairs based on a query log to further reduce index size. Extensive experiments with two large text collections demonstrate runtime improvements of several orders of magnitude over existing text-based processing techniques with reasonable index sizes
Normal Heat Conductivity in a strongly pinned chain of anharmonic oscillators
We consider a chain of coupled and strongly pinned anharmonic oscillators
subject to a non-equilibrium random forcing. Assuming that the stationary state
is approximately Gaussian, we first derive a stationary Boltzmann equation. By
localizing the involved resonances, we next invert the linearized collision
operator and compute the heat conductivity. In particular, we show that the
Gaussian approximation yields a finite conductivity
, for the anharmonic coupling
strength.Comment: Introduction and conclusion modifie
Surface code architecture for donors and dots in silicon with imprecise and nonuniform qubit couplings
A scaled quantum computer with donor spins in silicon would benefit from a
viable semiconductor framework and a strong inherent decoupling of the qubits
from the noisy environment. Coupling neighbouring spins via the natural
exchange interaction according to current designs requires gate control
structures with extremely small length scales. We present a silicon
architecture where bismuth donors with long coherence times are coupled to
electrons that can shuttle between adjacent quantum dots, thus relaxing the
pitch requirements and allowing space between donors for classical control
devices. An adiabatic SWAP operation within each donor/dot pair solves the
scalability issues intrinsic to exchange-based two-qubit gates, as it does not
rely on sub-nanometer precision in donor placement and is robust against noise
in the control fields. We use this SWAP together with well established global
microwave Rabi pulses and parallel electron shuttling to construct a surface
code that needs minimal, feasible local control.Comment: Published version - more detailed discussions, robustness to
dephasing pointed out additionall
Electrical activation and electron spin coherence of ultra low dose antimony implants in silicon
We implanted ultra low doses (2x10^11 cm-2) of 121Sb ions into isotopically
enriched 28Si and find high degrees of electrical activation and low levels of
dopant diffusion after rapid thermal annealing. Pulsed Electron Spin Resonance
shows that spin echo decay is sensitive to the dopant depths, and the interface
quality. At 5.2 K, a spin decoherence time, T2, of 0.3 ms is found for profiles
peaking 50 nm below a Si/SiO2 interface, increasing to 0.75 ms when the surface
is passivated with hydrogen. These measurements provide benchmark data for the
development of devices in which quantum information is encoded in donor
electron spins
Long Range Anticorrelations and Non-Gaussian Behavior of a Leaky Faucet
We find that intervals between successive drops from a leaky faucet display
scale-invariant, long-range anticorrelations characterized by the same
exponents of heart beat-to-beat intervals of healthy subjects. This behavior is
also confirmed by numerical simulations on lattice and it is faucet-width- and
flow-rate-independent. The histogram for the drop intervals is also well
described by a L\'evy distribution with the same index for both histograms of
healthy and diseased subjects. This additional result corroborates the evidence
for similarities between leaky faucets and healthy hearts underlying dynamics.Comment: Self-extracting uuencoded postscript file. Phys.Rev.E (Rap.Comm.).
Related papers can be found at http://www.if.uff.br/~tjpp/tjppe.htm
Electrical activation and electron spin resonance measurements of implanted bismuth in isotopically enriched silicon-28
We have performed continuous wave and pulsed electron spin resonance
measurements of implanted bismuth donors in isotopically enriched silicon-28.
Donors are electrically activated via thermal annealing with minimal diffusion.
Damage from bismuth ion implantation is repaired during thermal annealing as
evidenced by narrow spin resonance linewidths (B_pp=12uT and long spin
coherence times T_2=0.7ms, at temperature T=8K). The results qualify ion
implanted bismuth as a promising candidate for spin qubit integration in
silicon.Comment: 4 pages, 4 figure
Stark shift and field ionization of arsenic donors in Si-SOI structures
We develop an efficient back gate for silicon-on-insulator (SOI) devices
operating at cryogenic temperatures, and measure the quadratic hyperfine Stark
shift parameter of arsenic donors in isotopically purified Si-SOI layers
using such structures. The back gate is implemented using MeV ion implantation
through the SOI layer forming a metallic electrode in the handle wafer,
enabling large and uniform electric fields up to 2 V/m to be
applied across the SOI layer. Utilizing this structure we measure the Stark
shift parameters of arsenic donors embedded in the Si SOI layer and find
a contact hyperfine Stark parameter of m/V. We also demonstrate electric-field driven dopant ionization in
the SOI device layer, measured by electron spin resonance.Comment: 5 pages, 3 figure
Distributed top-k aggregation queries at large
Top-k query processing is a fundamental building block for efficient ranking in a large number of applications. Efficiency is a central issue, especially for distributed settings, when the data is spread across different nodes in a network. This paper introduces novel optimization methods for top-k aggregation queries in such distributed environments. The optimizations can be applied to all algorithms that fall into the frameworks of the prior TPUT and KLEE methods. The optimizations address three degrees of freedom: 1) hierarchically grouping input lists into top-k operator trees and optimizing the tree structure, 2) computing data-adaptive scan depths for different input sources, and 3) data-adaptive sampling of a small subset of input sources in scenarios with hundreds or thousands of query-relevant network nodes. All optimizations are based on a statistical cost model that utilizes local synopses, e.g., in the form of histograms, efficiently computed convolutions, and estimators based on order statistics. The paper presents comprehensive experiments, with three different real-life datasets and using the ns-2 network simulator for a packet-level simulation of a large Internet-style network
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