833 research outputs found
Homological Error Correction: Classical and Quantum Codes
We prove several theorems characterizing the existence of homological error
correction codes both classically and quantumly. Not every classical code is
homological, but we find a family of classical homological codes saturating the
Hamming bound. In the quantum case, we show that for non-orientable surfaces it
is impossible to construct homological codes based on qudits of dimension
, while for orientable surfaces with boundaries it is possible to
construct them for arbitrary dimension . We give a method to obtain planar
homological codes based on the construction of quantum codes on compact
surfaces without boundaries. We show how the original Shor's 9-qubit code can
be visualized as a homological quantum code. We study the problem of
constructing quantum codes with optimal encoding rate. In the particular case
of toric codes we construct an optimal family and give an explicit proof of its
optimality. For homological quantum codes on surfaces of arbitrary genus we
also construct a family of codes asymptotically attaining the maximum possible
encoding rate. We provide the tools of homology group theory for graphs
embedded on surfaces in a self-contained manner.Comment: Revtex4 fil
How Sample Completeness Affects Gamma-Ray Burst Classification
Unsupervised pattern recognition algorithms support the existence of three
gamma-ray burst classes; Class I (long, large fluence bursts of intermediate
spectral hardness), Class II (short, small fluence, hard bursts), and Class III
(soft bursts of intermediate durations and fluences). The algorithms
surprisingly assign larger membership to Class III than to either of the other
two classes. A known systematic bias has been previously used to explain the
existence of Class III in terms of Class I; this bias allows the fluences and
durations of some bursts to be underestimated (Hakkila et al., ApJ 538, 165,
2000). We show that this bias primarily affects only the longest bursts and
cannot explain the bulk of the Class III properties. We resolve the question of
Class III existence by demonstrating how samples obtained using standard
trigger mechanisms fail to preserve the duration characteristics of small peak
flux bursts. Sample incompleteness is thus primarily responsible for the
existence of Class III. In order to avoid this incompleteness, we show how a
new dual timescale peak flux can be defined in terms of peak flux and fluence.
The dual timescale peak flux preserves the duration distribution of faint
bursts and correlates better with spectral hardness (and presumably redshift)
than either peak flux or fluence. The techniques presented here are generic and
have applicability to the studies of other transient events. The results also
indicate that pattern recognition algorithms are sensitive to sample
completeness; this can influence the study of large astronomical databases such
as those found in a Virtual Observatory.Comment: 29 pages, 6 figures, 3 tables, Accepted for publication in The
Astrophysical Journa
GUT-Scale Primordial Black Holes: Consequences and Constraints
A population of very light primordial black holes which evaporate before
nucleosynthesis begins is unconstrained unless the decaying black holes leave
stable relics. We show that gravitons Hawking radiated from these black holes
would source a substantial stochastic background of high frequency
gravititational waves ( Hz or more) in the present universe. These
black holes may lead to a transient period of matter dominated expansion. In
this case the primordial universe could be temporarily dominated by large
clusters of "Hawking stars" and the resulting gravitational wave spectrum is
independent of the initial number density of primordial black holes.Comment: 4 pages; grey body factors included in graviton emission
calculations, and a couple of references added, but the conclusions are
unchanged. v3 Minor changes to references and wording; final versio
Dilution effects in HoYSnO: from the Spin Ice to the single-ion magnet
A study of the modifications of the magnetic properties of
HoYSnO upon varying the concentration of diamagnetic
Y ions is presented. Magnetization and specific heat measurements show
that the Spin Ice ground-state is only weakly affected by doping for , even if non-negligible changes in the crystal field at Ho occur.
In this low doping range SR relaxation measurements evidence a
modification in the low-temperature dynamics with respect to the one observed
in the pure Spin Ice. For , or at high temperature, the dynamics
involve fluctuations among Ho crystal field levels which give rise to a
characteristic peak in Sn nuclear spin-lattice relaxation rate. In this
doping limit also the changes in Ho magnetic moment suggest a variation
of the crystal field parameters.Comment: 4 pages, 5 figures, proceedings of HFM2008 Conferenc
Stabilization of single-electron pumps by high magnetic fields
We study the effect of perpendicular magnetic fields on a single-electron
system with a strongly time-dependent electrostatic potential. Continuous
improvements to the current quantization in these electron pumps are revealed
by high-resolution measurements. Simulations show that the sensitivity of
tunnel rates to the barrier potential is enhanced, stabilizing particular
charge states. Nonadiabatic excitations are also suppressed due to a reduced
sensitivity of the Fock-Darwin states to electrostatic potential. The
combination of these effects leads to significantly more accurate current
quantization
The discovery of polarization in the afterglow of GRB 990510 with the ESO Very Large Telescope
Following a BeppoSAX alert (Piro 1999a) and the discovery of the OT at SAAO
(Vreeswijk et al. 1999a), we observed GRB 990510 with the FORS instrument on
ESO's VLT Unit 1 (`Antu'). The burst is unremarkable in gamma rays, but in
optical is the first one to show good evidence for jet-like outflow (Stanek et
al. 1999, Harrison et al. 1999). We report the detection of significant linear
polarization in the afterglow: it is (1.6 +/- 0.2)% 0.86 days after trigger,
and after 1.81 days is consistent with that same value, but much more
uncertain. The polarization angle is constant on a time scale of hours, and may
be constant over one day. We conclude that the polarization is intrinsic to the
source and due to the synchrotron nature of the emission, and discuss the
random and ordered field geometries that may be responsible for it.Comment: submitted to ApJ Lett., 5 pages including 2 figures, uses
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