12,442 research outputs found
A very brief introduction to quantum computing and quantum information theory for mathematicians
This is a very brief introduction to quantum computing and quantum
information theory, primarily aimed at geometers. Beyond basic definitions and
examples, I emphasize aspects of interest to geometers, especially connections
with asymptotic representation theory. Proofs of most statements can be found
in standard references
Unconditional security at a low cost
By simulating four quantum key distribution (QKD) experiments and analyzing
one decoy-state QKD experiment, we compare two data post-processing schemes
based on security against individual attack by L\"{u}tkenhaus, and
unconditional security analysis by Gottesman-Lo-L\"{u}tkenhaus-Preskill. Our
results show that these two schemes yield close performances. Since the Holy
Grail of QKD is its unconditional security, we conclude that one is better off
considering unconditional security, rather than restricting to individual
attacks.Comment: Accepted by International Conference on Quantum Foundation and
Technology: Frontier and Future 2006 (ICQFT'06
QKD in Standard Optical Telecommunications Networks
To perform Quantum Key Distribution, the mastering of the extremely weak
signals carried by the quantum channel is required. Transporting these signals
without disturbance is customarily done by isolating the quantum channel from
any noise sources using a dedicated physical channel. However, to really profit
from this technology, a full integration with conventional network technologies
would be highly desirable. Trying to use single photon signals with others that
carry an average power many orders of magnitude bigger while sharing as much
infrastructure with a conventional network as possible brings obvious problems.
The purpose of the present paper is to report our efforts in researching the
limits of the integration of QKD in modern optical networks scenarios. We have
built a full metropolitan area network testbed comprising a backbone and an
access network. The emphasis is put in using as much as possible the same
industrial grade technology that is actually used in already installed
networks, in order to understand the throughput, limits and cost of deploying
QKD in a real network
Allowable Low-Energy E_6 Subgroups from Leptogenesis
There are only two viable low-energy subgroups: or , which would not erase any preexisting lepton asymmetry of
the Universe that may have been created by the decay of heavy singlet
(right-handed) neutrinos or any other mechanism. They are also the two most
favored subgroups from a recent analysis of present neutral-current data.
We study details of the leptogenesis, as well as some salient experimental
signatures of the two models.Comment: 12 page
Heavy Triplet Leptons and New Gauge Boson
A heavy triplet of leptons per family is
proposed as the possible anchor of a small seesaw neutrino mass. A new U(1)
gauge symmetry is then also possible, and the associated gauge boson may be
discovered at or below the TeV scale. We discuss the phenomenology of this
proposal, with and without possible constraints from the NuTeV and atomic
parity violation experiments, which appear to show small discrepancies from the
predictions of the standard model.Comment: 20 pages including 4 figure
Postponement of dark-count effects in practical quantum key-distribution by two-way post-processing
The influence of imperfections on achievable secret-key generation rates of
quantum key distribution protocols is investigated. As examples of relevant
imperfections, we consider tagging of Alice's qubits and dark counts at Bob's
detectors, while we focus on a powerful eavesdropping strategy which takes full
advantage of tagged signals. It is demonstrated that error correction and
privacy amplification based on a combination of a two-way classical
communication protocol and asymmetric Calderbank-Shor-Steane codes may
significantly postpone the disastrous influence of dark counts. As a result,
the distances are increased considerably over which a secret key can be
distributed in optical fibres reliably. Results are presented for the
four-state, the six-state, and the decoy-state protocols.Comment: Fully revised version (12 pages and 8 figures). Improved figures and
discussion added. To appear in Eur. Phys. J.
Recombination Induced Softening and Reheating of the Cosmic Plasma
The atomic recombination process leads to a softening of the matter equation
of state as reflected by a reduced generalized adiabatic index, with
accompanying heat release. We study the effects of this recombination softening
and reheating of the cosmic plasma on the ionization history, visibility
function, Cold Dark Matter (CDM) transfer function, and the Cosmic Microwave
Background (CMB) spectra. The resulting modifications of the CMB spectrm is
1/10 of WMAP's current error and is comparable to PLANCK's error. Therefore,
this effect should be considered when data with higher accuracy are analysed.Comment: 11 pages, 6 figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Society; as advised by referee, omit high-baryon mode
Alternative schemes for measurement-device-independent quantum key distribution
Practical schemes for measurement-device-independent quantum key distribution
using phase and path or time encoding are presented. In addition to immunity to
existing loopholes in detection systems, our setup employs simple encoding and
decoding modules without relying on polarization maintenance or optical
switches. Moreover, by employing a modified sifting technique to handle the
dead-time limitations in single-photon detectors, our scheme can be run with
only two single-photon detectors. With a phase-postselection technique, a
decoy-state variant of our scheme is also proposed, whose key generation rate
scales linearly with the channel transmittance.Comment: 30 pages, 5 figure
Bounds of concurrence and their relation with fidelity and frontier states
The bounds of concurrence in [F. Mintert and A. Buchleitner, Phys. Rev. Lett.
98 (2007) 140505] and [C. Zhang \textit{et. al.}, Phys. Rev. A 78 (2008)
042308] are proved by using two properties of the fidelity. In two-qubit
systems, for a given value of concurrence, the states achieving the maximal
upper bound, the minimal lower bound or the maximal difference upper-lower
bound are determined analytically
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