2,209 research outputs found
Photon-Number-Splitting versus Cloning Attacks in Practical Implementations of the Bennett-Brassard 1984 protocol for Quantum Cryptography
In practical quantum cryptography, the source sometimes produces multi-photon
pulses, thus enabling the eavesdropper Eve to perform the powerful
photon-number-splitting (PNS) attack. Recently, it was shown by Curty and
Lutkenhaus [Phys. Rev. A 69, 042321 (2004)] that the PNS attack is not always
the optimal attack when two photons are present: if errors are present in the
correlations Alice-Bob and if Eve cannot modify Bob's detection efficiency, Eve
gains a larger amount of information using another attack based on a 2->3
cloning machine. In this work, we extend this analysis to all distances
Alice-Bob. We identify a new incoherent 2->3 cloning attack which performs
better than those described before. Using it, we confirm that, in the presence
of errors, Eve's better strategy uses 2->3 cloning attacks instead of the PNS.
However, this improvement is very small for the implementations of the
Bennett-Brassard 1984 (BB84) protocol. Thus, the existence of these new attacks
is conceptually interesting but basically does not change the value of the
security parameters of BB84. The main results are valid both for Poissonian and
sub-Poissonian sources.Comment: 11 pages, 5 figures; "intuitive" formula (31) adde
Encoding a qubit in an oscillator
Quantum error-correcting codes are constructed that embed a
finite-dimensional code space in the infinite-dimensional Hilbert space of a
system described by continuous quantum variables. These codes exploit the
noncommutative geometry of phase space to protect against errors that shift the
values of the canonical variables q and p. In the setting of quantum optics,
fault-tolerant universal quantum computation can be executed on the protected
code subspace using linear optical operations, squeezing, homodyne detection,
and photon counting; however, nonlinear mode coupling is required for the
preparation of the encoded states. Finite-dimensional versions of these codes
can be constructed that protect encoded quantum information against shifts in
the amplitude or phase of a d-state system. Continuous-variable codes can be
invoked to establish lower bounds on the quantum capacity of Gaussian quantum
channels.Comment: 22 pages, 8 figures, REVTeX, title change (qudit -> qubit) requested
by Phys. Rev. A, minor correction
Quantum error-correcting codes associated with graphs
We present a construction scheme for quantum error correcting codes. The
basic ingredients are a graph and a finite abelian group, from which the code
can explicitly be obtained. We prove necessary and sufficient conditions for
the graph such that the resulting code corrects a certain number of errors.
This allows a simple verification of the 1-error correcting property of
fivefold codes in any dimension. As new examples we construct a large class of
codes saturating the singleton bound, as well as a tenfold code detecting 3
errors.Comment: 8 pages revtex, 5 figure
Topological Protection and Quantum Noiseless Subsystems
Encoding and manipulation of quantum information by means of topological
degrees of freedom provides a promising way to achieve natural fault-tolerance
that is built-in at the physical level. We show that this topological approach
to quantum information processing is a particular instance of the notion of
computation in a noiseless quantum subsystem. The latter then provide the most
general conceptual framework for stabilizing quantum information and for
preserving quantum coherence in topological and geometric systems.Comment: 4 Pages LaTeX. Published versio
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
Simple Quantum Error Correcting Codes
Methods of finding good quantum error correcting codes are discussed, and
many example codes are presented. The recipe C_2^{\perp} \subseteq C_1, where
C_1 and C_2 are classical codes, is used to obtain codes for up to 16
information qubits with correction of small numbers of errors. The results are
tabulated. More efficient codes are obtained by allowing C_1 to have reduced
distance, and introducing sign changes among the code words in a systematic
manner. This systematic approach leads to single-error correcting codes for 3,
4 and 5 information qubits with block lengths of 8, 10 and 11 qubits
respectively.Comment: Submitted to Phys. Rev. A. in May 1996. 21 pages, no figures. Further
information at http://eve.physics.ox.ac.uk/ASGhome.htm
Classicality in discrete Wigner functions
Gibbons et al. [Phys. Rev. A 70, 062101(2004)] have recently defined a class
of discrete Wigner functions W to represent quantum states in a Hilbert space
with finite dimension. We show that the only pure states having non-negative W
for all such functions are stabilizer states, as conjectured by one of us
[Phys. Rev. A 71, 042302 (2005)]. We also show that the unitaries preserving
non-negativity of W for all definitions of W form a subgroup of the Clifford
group. This means pure states with non-negative W and their associated unitary
dynamics are classical in the sense of admitting an efficient classical
simulation scheme using the stabilizer formalism.Comment: 10 pages, 1 figur
Preparing encoded states in an oscillator
Recently a scheme has been proposed for constructing quantum error-correcting
codes that embed a finite-dimensional code space in the infinite-dimensional
Hilbert space of a system described by continuous quantum variables. One of the
difficult steps in this scheme is the preparation of the encoded states. We
show how these states can be generated by coupling a continuous quantum
variable to a single qubit. An ion trap quantum computer provides a natural
setting for a continuous system coupled to a qubit. We discuss how encoded
states may be generated in an ion trap.Comment: 5 pages, 4 figures, RevTe
Teleportation via thermally entangled state of a two-qubit Heisenberg XX chain
We find that quantum teleportation, using the thermally entangled state of
two-qubit Heisenberg XX chain as a resource, with fidelity better than any
classical communication protocol is possible. However, a thermal state with a
greater amount of thermal entanglement does not necessarily yield better
fidelity. It depends on the amount of mixing between the separable state and
maximally entangled state in the spectra of the two-qubit Heisenberg XX model.Comment: 5 pages, 1 tabl
Mirax: A Brazilian X-Ray Astronomy Satellite Mission
We describe the ``Monitor e Imageador de Raios-X'' (MIRAX), an X-ray
astronomy satellite mission proposed by the high energy astrophysics group at
the National Institute for Space Research (INPE) in Brazil to the Brazilian
Space Agency. MIRAX is an international collaboration that includes, besides
INPE, the University of California San Diego, the University of Tuebingen in
Germany, the Massachusetts Institute of Technology and the Space Research
Organization Netherlands. The payload of MIRAX will consist in two identical
hard X-ray cameras (10 -200 keV) and one soft X-ray camera (2-28 keV), both
with angular resolution of ~ 5-6 arcmin. The basic objective of MIRAX is to
carry out continuous broadband imaging spectroscopy observations of a large
source sample (~ 9 months/yr) in the central Galactic plane region. This will
allow the detection, localization, possible identification, and
spectral/temporal study of the entire history of transient phenomena to be
carried out in one single mission. MIRAX will have sensitivities of ~ 5
mCrab/day in the 2-10 keV band (~2 times better than the All Sky Monitor on
Rossi X-ray Timing Explorer) and 2.6 mCrab/day in the 10-100 keV band (~40
times better than the Earth Occultation technique of the Burst and Transient
Source Experiment on the Compton Gamma-Ray Observatory). The MIRAX spacecraft
will weigh about 200 kg and is expected to be launched in a low-altitude (~ 600
km) circular equatorial orbit around 2007/2008.Comment: 6 pages, 1 table, 3 figures, presented at 2002 COSPAR meeting in
Houston. Submitted to Adv. Space Re
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