5,821 research outputs found
Unconditionally secure one-way quantum key distribution using decoy pulses
We report here a complete experimental realization of one-way decoy-pulse
quantum key distribution, demonstrating an unconditionally secure key rate of
5.51 kbps for a 25.3 km fibre length. This is two orders of magnitudes higher
than the value that can be obtained with a non-decoy system. We introduce also
a simple test for detecting the photon number splitting attack and highlight
that it is essential for the security of the technique to fully characterize
the source and detectors used.Comment: 10 pages, 5 figure
Physical Results from Unphysical Simulations
We calculate various properties of pseudoscalar mesons in partially quenched
QCD using chiral perturbation theory through next-to-leading order. Our results
can be used to extrapolate to QCD from partially quenched simulations, as long
as the latter use three light dynamical quarks. In other words, one can use
unphysical simulations to extract physical quantities - in this case the quark
masses, meson decay constants, and the Gasser-Leutwyler parameters L_4-L_8. Our
proposal for determining L_7 makes explicit use of an unphysical (yet
measurable) effect of partially quenched theories, namely the double-pole that
appears in certain two-point correlation functions. Most of our calculations
are done for sea quarks having up to three different masses, except for our
result for L_7, which is derived for degenerate sea quarks.Comment: 26 pages, 12 figures (discussion on discretization errors at end of
sec. IV clarified; minor improvements in presentation; results unchanged
Evidence for quenched chiral logs
Using the pole shifting procedure of the modified quenched approximation
(MQA) to cure the exceptional configuration problem, accurate hadron hadron
spectrum calculations can be obtained at very light quark mass. Here we use the
MQA to extend and improve our previous investigation of chiral logs in the pion
mass. At beta=5.7 for Wilson fermion, we see clear evidence for quenched chiral
logarithms in the pion mass as a function of quark mass. The size of the
observed chiral log exponent delta is in good agreement with the value obtained
from a direct calculation of the eta' hairpin diagram.Comment: 3 pages, 4 figures, Lattice 98 tal
Applications of Partially Quenched Chiral Perturbation Theory
Partially quenched theories are theories in which the valence- and sea-quark
masses are different. In this paper we calculate the nonanalytic one-loop
corrections of some physical quantities: the chiral condensate, weak decay
constants, Goldstone boson masses, B_K and the K+ to pi+ pi0 decay amplitude,
using partially quenched chiral perturbation theory. Our results for weak decay
constants and masses agree with, and generalize, results of previous work by
Sharpe. We compare B_K and the K+ decay amplitude with their real-world values
in some examples. For the latter quantity, two other systematic effects that
plague lattice computations, namely, finite-volume effects and unphysical
values of the quark masses and pion external momenta are also considered. We
find that typical one-loop corrections can be substantial.Comment: 22 pages, TeX, refs. added, minor other changes, version to appear in
Phys. Rev.
Some symmetry classifications of hyperbolic vector evolution equations
Motivated by recent work on integrable flows of curves and 1+1 dimensional
sigma models, several O(N)-invariant classes of hyperbolic equations for an -component vector are considered. In each
class we find all scaling-homogeneous equations admitting a higher symmetry of
least possible scaling weight. Sigma model interpretations of these equations
are presented.Comment: Revision of published version, incorporating errata on geometric
aspects of the sigma model interpretations in the case of homogeneous space
Partially quenched chiral perturbation theory without
This paper completes the argument that lattice simulations of partially
quenched QCD can provide quantitative information about QCD itself, with the
aid of partially quenched chiral perturbation theory. A barrier to doing this
has been the inclusion of , the partially quenched generalization of
the , in previous calculations in the partially quenched effective
theory. This invalidates the low energy perturbative expansion, gives rise to
many new unknown parameters, and makes it impossible to reliably calculate the
relation between the partially quenched theory and low energy QCD. We show that
it is straightforward and natural to formulate partially quenched chiral
perturbation theory without , and that the resulting theory contains
the effective theory for QCD without the . We also show that previous
results, obtained including , can be reinterpreted as applying to the
theory without . We contrast the situation with that in the quenched
effective theory, where we explain why it is necessary to include .
We also compare the derivation of chiral perturbation theory in partially
quenched QCD with the standard derivation in unquenched QCD. We find that the
former cannot be justified as rigorously as the latter, because of the absence
of a physical Hilbert space. Finally, we present an encouraging result:
unphysical double poles in certain correlation functions in partially quenched
chiral perturbation theory can be shown to be a property of the underlying
theory, given only the symmetries and some plausible assumptions.Comment: 45 pages, no figure
Topologically Massive Gauge Theory: A Lorentzian Solution
We obtain a lorentzian solution for the topologically massive non-abelian
gauge theory on AdS space by means of a SU(1, 1) gauge transformation of the
previously found abelian solution. There exists a natural scale of length which
is determined by the inverse topological mass. The topological mass is
proportional to the square of the gauge coupling constant. In the topologically
massive electrodynamics the field strength locally determines the gauge
potential up to a closed 1-form via the (anti-)self-duality equation. We
introduce a transformation of the gauge potential using the dual field strength
which can be identified with an abelian gauge transformation. Then we present
the map from the AdS space to the pseudo-sphere including the topological mass.
This is the lorentzian analog of the Hopf map. This map yields a global
decomposition of the AdS space as a trivial circle bundle over the upper
portion of the pseudo-sphere which is the Hyperboloid model for the Lobachevski
geometry. This leads to a reduction of the abelian field equation onto the
pseudo-sphere using a global section of the solution on the AdS space. Then we
discuss the integration of the field equation using the Archimedes map from the
pseudo-sphere to the cylinder over the ideal Poincare circle. We also present a
brief discussion of the holonomy of the gauge potential and the dual-field
strength on the upper portion of the pseudo-sphere.Comment: 23 pages, 1 postscript figur
Gigahertz quantum key distribution with InGaAs avalanche photodiodes
We report a demonstration of quantum key distribution (QKD) at GHz clock
rates with InGaAs avalanche photodiodes (APDs) operating in a self-differencing
mode. Such a mode of operation allows detection of extremely weak avalanches so
that the detector afterpulse noise is sufficiently suppressed. The system is
characterized by a secure bit rate of 2.37 Mbps at 5.6 km and 27.9 kbps at 65.5
km when the fiber dispersion is not compensated. After compensating the fiber
dispersion, the QKD distance is extended to 101 km, resulting in a secure key
rate of 2.88 kbps. Our results suggest that InGaAs APDs are very well suited to
GHz QKD applications.Comment: 4 pages, 4 figure
Perturbative matching of staggered four-fermion operators with hypercubic fat links
We calculate the one-loop matching coefficients between continuum and lattice
four-fermion operators for lattice operators constructed using staggered
fermions and improved by the use of fattened links. In particular, we consider
hypercubic fat links and SU(3) projected Fat-7 links, and their mean-field
improved versions. We calculate only current-current diagrams, so that our
results apply for operators whose flavor structure does not allow
``eye-diagrams''. We present general formulae, based on two independent
approaches, and give numerical results for the cases in which the operators
have the taste (staggered flavor) of the pseudo-Goldstone pion. We find that
the one-loop corrections are reduced down to the 10-20% level, resolving the
problem of large perturbative corrections for staggered fermion calculations of
matrix elements.Comment: 37 pages, no figure, 20 table
- …