918 research outputs found
Using Tau Polarization to Discriminate between SUSY Models and Determine SUSY Parameters at ILC
In many SUSY models the first SUSY signal in the proposed International
Linear Collider is expected to come from the pair production of ,
followed by its decay into +LSP. We study a simple and robust method of
measuring the polarization of this in its 1-prong hadronic decay
channel,and show how it can be used to discriminate between SUSY models and to
determine SUSY parameters.Comment: 9 pages, 5 figures, minor corrections; version published in Phys.
Lett.
Exponential lower bound on the highest fidelity achievable by quantum error-correcting codes
On a class of memoryless quantum channels which includes the depolarizing
channel, the highest fidelity of quantum error-correcting codes of length n and
rate R is proven to be lower bounded by 1-exp[-nE(R)+o(n)] for some function
E(R). The E(R) is positive below some threshold R', which implies R' is a lower
bound on the quantum capacity.Comment: Ver.4. In vers.1--3, I claimed Theorem 1 for general quantum
channels. Now I claim this only for a slight generalization of depolarizing
channel in this paper because Lemma 2 in vers.1--3 was wrong; the original
general statement is proved in quant-ph/0112103. Ver.5. Text sectionalized.
Appeared in PRA. The PRA article is typographically slightly crude: The LaTeX
symbol star, used as superscripts, was capriciously replaced by the asterisk
in several places after my proof readin
Hopping Conduction in Disordered Carbon Nanotubes
We report electrical transport measurements on individual disordered carbon
nanotubes, grown catalytically in a nanoporous anodic aluminum oxide template.
In both as-grown and annealed types of nanotubes, the low-field conductance
shows as exp[-(T_{0}/T)^{1/2}] dependence on temperature T, suggesting that
hopping conduction is the dominant transport mechanism, albeit with different
disorder-related coefficients T_{0}. The field dependence of low-temperature
conductance behaves an exp[-(xi_{0}/xi)^{1/2}] with high electric field xi at
sufficiently low T. Finally, both annealed and unannealed nanotubes exhibit
weak positive magnetoresistance at low T = 1.7 K. Comparison with theory
indicates that our data are best explained by Coulomb-gap variable range
hopping conduction and permits the extraction of disorder-dependent
localization length and dielectric constant.Comment: 10 pages, 5 figure
Gate errors in solid state quantum computer architectures
We theoretically consider possible errors in solid state quantum computation
due to the interplay of the complex solid state environment and gate
imperfections. In particular, we study two examples of gate operations in the
opposite ends of the gate speed spectrum, an adiabatic gate operation in
electron-spin-based quantum dot quantum computation and a sudden gate operation
in Cooper pair box superconducting quantum computation. We evaluate
quantitatively the non-adiabatic operation of a two-qubit gate in a
two-electron double quantum dot. We also analyze the non-sudden pulse gate in a
Cooper-pair-box-based quantum computer model. In both cases our numerical
results show strong influences of the higher excited states of the system on
the gate operation, clearly demonstrating the importance of a detailed
understanding of the relevant Hilbert space structure on the quantum computer
operations.Comment: 6 pages, 2 figure
Continuity theorems for the queueing system
In this paper continuity theorems are established for the number of losses
during a busy period of the queue. We consider an queueing
system where the service time probability distribution, slightly different in a
certain sense from the exponential distribution, is approximated by that
exponential distribution. Continuity theorems are obtained in the form of one
or two-sided stochastic inequalities. The paper shows how the bounds of these
inequalities are changed if further assumptions, associated with specific
properties of the service time distribution (precisely described in the paper),
are made. Specifically, some parametric families of service time distributions
are discussed, and the paper establishes uniform estimates (given for all
possible values of the parameter) and local estimates (where the parameter is
fixed and takes only the given value). The analysis of the paper is based on
the level crossing approach and some characterization properties of the
exponential distribution.Comment: Final revision; will be published as i
Multiplicativity of completely bounded p-norms implies a new additivity result
We prove additivity of the minimal conditional entropy associated with a
quantum channel Phi, represented by a completely positive (CP),
trace-preserving map, when the infimum of S(gamma_{12}) - S(gamma_1) is
restricted to states of the form gamma_{12} = (I \ot Phi)(| psi >< psi |). We
show that this follows from multiplicativity of the completely bounded norm of
Phi considered as a map from L_1 -> L_p for L_p spaces defined by the Schatten
p-norm on matrices; we also give an independent proof based on entropy
inequalities. Several related multiplicativity results are discussed and
proved. In particular, we show that both the usual L_1 -> L_p norm of a CP map
and the corresponding completely bounded norm are achieved for positive
semi-definite matrices. Physical interpretations are considered, and a new
proof of strong subadditivity is presented.Comment: Final version for Commun. Math. Physics. Section 5.2 of previous
version deleted in view of the results in quant-ph/0601071 Other changes
mino
Superconductor coupled to two Luttinger liquids as an entangler for electron spins
We consider an s-wave superconductor (SC) which is tunnel-coupled to two
spatially separated Luttinger liquid (LL) leads. We demonstrate that such a
setup acts as an entangler, i.e. it creates spin-singlets of two electrons
which are spatially separated, thereby providing a source of electronic
Einstein-Podolsky-Rosen pairs. We show that in the presence of a bias voltage,
which is smaller than the energy gap in the SC, a stationary current of
spin-entangled electrons can flow from the SC to the LL leads due to Andreev
tunneling events. We discuss two competing transport channels for Cooper pairs
to tunnel from the SC into the LL leads. On the one hand, the coherent
tunneling of two electrons into the same LL lead is shown to be suppressed by
strong LL correlations compared to single-electron tunneling into a LL. On the
other hand, the tunneling of two spin-entangled electrons into different leads
is suppressed by the initial spatial separation of the two electrons coming
from the same Cooper pair. We show that the latter suppression depends
crucially on the effective dimensionality of the SC. We identify a regime of
experimental interest in which the separation of two spin-entangled electrons
is favored. We determine the decay of the singlet state of two electrons
injected into different leads caused by the LL correlations. Although the
electron is not a proper quasiparticle of the LL, the spin information can
still be transported via the spin density fluctuations produced by the injected
spin-entangled electrons.Comment: 15 pages, 2 figure
Associated charged Higgs and W boson production in the MSSM at the CERN Large Hadron Collider
We investigate the viability of observing charged Higgs bosons (H^+/-)
produced in association with W bosons at the CERN Large Hadron Collider, using
the leptonic decay H^+ -> tau^+ nu_tau and hadronic W-decay, within different
scenarios of the Minimal Supersymmetric Standard Model (MSSM) with both real
and complex parameters. Performing a parton level study we show how the
irreducible Standard Model background from W+2 jets can be controlled by
applying appropriate cuts and find that the size of a possible signal depends
on the cuts needed to suppress QCD backgrounds and misidentifications. In the
standard maximal mixing scenario of the MSSM we find a viable signal for large
tan(beta) and intermediate H^+/- masses (~m_t) when using optimistic cuts
whereas for more pessimistic ones we only find a viable signal for very large
tan(beta) (>~50). We have also investigated a special class of MSSM scenarios
with large mass-splittings among the heavy Higgs bosons where the cross-section
can be resonantly enhanced by factors up to one hundred, with a strong
dependence on the CP-violating phases. Even so we find that the signal after
cuts remains small except for small masses (~< m_t) with optimistic cuts.
Finally, in all the scenarios we have investigated we have only found small
CP-asymmetries.Comment: 28 pages, 12 figures, version to appear in Euro. Phys. J.
Implication of Super-Kamiokande Data on R-parity Violation
R-parity violating bilinear (soft) terms in the supersymmetric standard model
would be the leading source for nonzero neutrino masses and mixing. We point
out that the mixing between neutralinos (charginos) and neutrinos (charged
leptons) driven by the bilinear terms take factorized forms, which may enable
us to probe the neutrino mixing parameters in a collider. It is then shown that
the Super-Kamiokande data on atmospheric neutrinos require all the baryon
number violating couplings to be substantially suppressed: .Comment: 11 pages, LaTeX, to appear in Phys. Rev.
Flavor changing Z-decays from scalar interactions at a Giga-Z Linear Collider
The flavor changing decay Z -> d_I \bar{d}_J is investigated with special
emphasis on the b \bar{s} final state. Various models for flavor violation are
considered: two Higgs doublet models (2HDM's), supersymmetry (SUSY) with flavor
violation in the up and down-type squark mass matrices and SUSY with flavor
violation mediated by R-parity-violating interaction. We find that, within the
SUSY scenarios for flavor violation, the branching ratio for the decay Z -> b
\bar{s} can reach 10^{-6} for large \tan\beta values, while the typical size
for this branching ratio in the 2HDM's considered is about two orders of
magnitudes smaller at best. Thus, flavor changing SUSY signatures in radiative
Z decays such as Z -> b \bar{s} may be accessible to future ``Z factories''
such as a Giga-Z version of the TESLA design.Comment: 27 pages, 15 figures, REVTeX4. A new section added and a few minor
corrections were made in the tex
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