9,109 research outputs found
Configurational entropy of network-forming materials
We present a computationally efficient method to calculate the
configurational entropy of network-forming materials. The method requires only
the atomic coordinates and bonds of a single well-relaxed configuration. This
is in contrast to the multiple simulations that are required for other methods
to determine entropy, such as thermodynamic integration. We use our method to
obtain the configurational entropy of well-relaxed networks of amorphous
silicon and vitreous silica. For these materials we find configurational
entropies of 1.02 kb and 0.97 kb per silicon atom, respectively, with kb the
Boltzmann constant.Comment: 4 pages, 4 figure
Structured optical receivers to attain superadditive capacity and the Holevo limit
When classical information is sent over a quantum channel, attaining the
ultimate limit to channel capacity requires the receiver to make joint
measurements over long codeword blocks. For a pure-state channel, we construct
a receiver that can attain the ultimate capacity by applying a single-shot
unitary transformation on the received quantum codeword followed by
simultaneous (but separable) projective measurements on the
single-modulation-symbol state spaces. We study the ultimate limits of
photon-information-efficient communications on a lossy bosonic channel. Based
on our general results for the pure-state quantum channel, we show some of the
first concrete examples of codes and structured joint-detection optical
receivers that can achieve fundamentally higher (superadditive) channel
capacity than conventional receivers that detect each modulation symbol
individually.Comment: 4 pages, 4 figure
pH-Independent, 520 mV Open-Circuit Voltages of Si/Methyl Viologen^(2+/+) Contacts Through Use of Radial n^+p-Si Junction Microwire Array Photoelectrodes
The effects of introducing an n^+-doped emitter layer have been evaluated for both planar Si photoelectrodes and for radial junction Si microwire-array photoelectrodes. In contact with the pH-independent, one-electron, outer-sphere, methyl viologen redox system (denoted MV^(2+/+)), both planar and wire array p-Si photoelectrodes yielded open-circuit voltages, V_(oc), that varied with the pH of the solution. The highest V_(oc) values were obtained at pH = 2.9, with V_(oc) = 0.53 V for planar p-Si electrodes and V_(oc) = 0.42 V for vapor−liquid−solid catalyzed p-Si microwire array samples, under 60 mW cm^(−2) of 808 nm illumination. Increases in the pH of the electrolyte produced a decrease in V_(oc) by approximately −44 mV/pH unit for planar electrodes, with similar trends observed for the Si microwire array electrodes. In contrast, introduction of a highly doped, n^+ emitter layer produced V_(oc) = 0.56 V for planar Si electrodes and V_(oc) = 0.52 V for Si microwire array electrodes, with the photoelectrode properties in each system being essentially independent of pH over six pH units (3 < pH < 9). Hence, formation of an n^+ emitter layer not only produced nearly identical photovoltages for planar and Si microwire array photoelectrodes, but decoupled the band energetics of the semiconductor (and hence the obtainable photovoltage) from the value of the redox potential of the solution. The formation of radial junctions on Si microwire arrays thus provides an approach to obtaining Si-based photoelectrodes with high-photovoltages that can be used for a variety of photoelectrochemical processes, including potentially the hydrogen evolution reaction, under various pH conditions, regardless of the intrinsic barrier height and flat-band properties of the Si/liquid contact
Quantum Capacity Approaching Codes for the Detected-Jump Channel
The quantum channel capacity gives the ultimate limit for the rate at which
quantum data can be reliably transmitted through a noisy quantum channel.
Degradable quantum channels are among the few channels whose quantum capacities
are known. Given the quantum capacity of a degradable channel, it remains
challenging to find a practical coding scheme which approaches capacity. Here
we discuss code designs for the detected-jump channel, a degradable channel
with practical relevance describing the physics of spontaneous decay of atoms
with detected photon emission. We show that this channel can be used to
simulate a binary classical channel with both erasures and bit-flips. The
capacity of the simulated classical channel gives a lower bound on the quantum
capacity of the detected-jump channel. When the jump probability is small, it
almost equals the quantum capacity. Hence using a classical capacity
approaching code for the simulated classical channel yields a quantum code
which approaches the quantum capacity of the detected-jump channel
Travelling-Wave Sub-Doppler Excited Molecule Energy Transfer Spectroscopy
A general formulation of traveling‐wave sub‐Doppler excited molecule energy transferspectroscopy is presented. The line profile analysis is applied to that determined experimentally for the R(22) ν3 HCN transition. Pn the noise equivalent power of the detector is demonstrated to be ⩽10−12 W. Finally, the technique is applied to resolve the KsR(7) ν1 transition head in NH3
Highly optimized tolerance and power laws in dense and sparse resource regimes
Power law cumulative frequency vs. event size distributions
are frequently cited as evidence for complexity and
serve as a starting point for linking theoretical models and mechanisms with
observed data. Systems exhibiting this behavior present fundamental
mathematical challenges in probability and statistics. The broad span of length
and time scales associated with heavy tailed processes often require special
sensitivity to distinctions between discrete and continuous phenomena. A
discrete Highly Optimized Tolerance (HOT) model, referred to as the
Probability, Loss, Resource (PLR) model, gives the exponent as a
function of the dimension of the underlying substrate in the sparse
resource regime. This agrees well with data for wildfires, web file sizes, and
electric power outages. However, another HOT model, based on a continuous
(dense) distribution of resources, predicts . In this paper we
describe and analyze a third model, the cuts model, which exhibits both
behaviors but in different regimes. We use the cuts model to show all three
models agree in the dense resource limit. In the sparse resource regime, the
continuum model breaks down, but in this case, the cuts and PLR models are
described by the same exponent.Comment: 19 pages, 13 figure
On Interferometric Duality in Multibeam Experiments
We critically analyze the problem of formulating duality between fringe
visibility and which-way information, in multibeam interference experiments. We
show that the traditional notion of visibility is incompatible with any
intuitive idea of complementarity, but for the two-beam case. We derive a
number of new inequalities, not present in the two-beam case, one of them
coinciding with a recently proposed multibeam generalization of the inequality
found by Greenberger and YaSin. We show, by an explicit procedure of
optimization in a three-beam case, that suggested generalizations of Englert's
inequality, do not convey, differently from the two-beam case, the idea of
complementarity, according to which an increase of visibility is at the cost of
a loss in path information, and viceversa.Comment: 26 pages, 1 figure, substantial changes in the text, new material has
been added in Section 3. Version to appear in J.Phys.
Direct observation of charge inversion by multivalent ions as a universal electrostatic phenomenon
We have directly observed reversal of the polarity of charged surfaces in
water upon the addition of tri- and quadrivalent ions using atomic force
microscopy. The bulk concentration of multivalent ions at which charge
inversion reversibly occurs depends only very weakly on the chemical
composition, surface structure, size and lipophilicity of the ions, but is
dominated by their valence. These results support the theoretical proposal that
spatial correlations between ions are the driving mechanism behind charge
inversion.Comment: submitted to PRL, 26-04-2004 Changed the presentation of the theory
at the end of the paper. Changed small error in estimate of prefactor ("w" in
first version) of equation
Exceeding classical capacity limit in quantum optical channel
The amount of information transmissible through a communications channel is
determined by the noise characteristics of the channel and by the quantities of
available transmission resources. In classical information theory, the amount
of transmissible information can be increased twice at most when the
transmission resource (e.g. the code length, the bandwidth, the signal power)
is doubled for fixed noise characteristics. In quantum information theory,
however, the amount of information transmitted can increase even more than
twice. We present a proof-of-principle demonstration of this super-additivity
of classical capacity of a quantum channel by using the ternary symmetric
states of a single photon, and by event selection from a weak coherent light
source. We also show how the super-additive coding gain, even in a small code
length, can boost the communication performance of conventional coding
technique.Comment: 4 pages, 3 figure
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