Vol.7
No.7
September 1, 2007
Research Articles:
Estimation of the heating rate of
ions due to laser fluctuations when implementing quantum algorithms (pp573583)
S.
Fujiwara and S. Hasegawa
We analyze numerically the heating of trapped ions due to
laser intensity and phase fluctuations when implementing Grover's
algorithm and the Quantum Fourier Transform. For a simpler analysis we
assume that the stochastic processes are white noise processes and
average over each noise as in [Phys. Rev. A. \textbf{57}, 3748, (1998)].
We investigate the fidelity and the heating rate for these algorithms
using parameters estimated from experiments, and we can see the order of
magnitude difference in the heating rate depending on the quantum
algorithms.
Genuine tripartite entanglement
semimonotone for 2x2xndimensional systems (pp584593)
C.S.
Yu, H.S. Song, and Y.H. Wang
In this paper, we present a new approach to study genuine
tripartite entanglement existing in $(2\times 2\times n)$dimensional
quantum pure states. By utilizing the approach, we introduce a
particular quantity to measure genuine tripartite entanglement. The
quantity is shown to be an entanglement monotone in 2dimensional
subsystems (semimonotone) and reaches zero for separable states and
$(2\times 2\times 2)$dimensional $W$ states, hence is a good criterion
to characterize genuine tripartite entanglement. Furthermore, the
formulation for pure states can be conveniently extended to the case of
mixed states by utilizing the kronecker product approximation technique.
As applications, we give the analytic approximation for weakly mixed
states, and study the genuine tripartite entanglement of two given
weakly mixed states.
Highfidelity singlequbit gates
using nonadiabatic rapid passage (pp594608)
R.
Li, M. Hoover, and F. Gaitan
Numerical simulation results are presented which suggest
that a class of nonadiabatic rapid passage sweeps first realized
experimentally in 1991 should be capable of implementing a set of
quantum gates that is universal for onequbit unitary operations and
whose elements operate with error probabilities $P_{e}<10^{4}$. The
sweeps are noncomposite and generate controllable quantum interference
effects which allow the onequbit gates produced to operate
nonadiabatically while maintaining high accuracy. The simulations
suggest that the onequbit gates produced by these sweeps show promise
as possible elements of a faulttolerant scheme for quantum computing.
Probabilistic purification of noisy
coherent states (pp609623)
P.
Marek and R. Filip
A basic feasible probabilistic purification of unknown
noisy coherent states, outgoing from different state preparations with
unknown mean number of thermal photons, is proposed. The scheme is based
only on a linearoptical network with an avalanche photodiode or
heterodyne (homodyne) detection used to postselect a successful
processing. The suggested probabilistic method can produce an output
state with a lower noise than both quantum deterministic and classical
probabilistic purification method. The purification applied in the state
preparation can increase classical capacity of communication and
security of quantum key distribution.
Separability criteria based on the
Bloch representation of
density matrices
(pp624638)
J.
de Vicente
We study the separability of bipartite quantum systems in
arbitrary dimensions using the Bloch representation of their density
matrix. This approach enables us to find an alternative characterization
of the separability problem, from which we derive a necessary condition
and sufficient conditions for separability. For a certain class of
states the necessary condition and a sufficient condition turn out to be
equivalent, therefore yielding a necessary and sufficient condition. The
proofs of the sufficient conditions are constructive, thus providing
decompositions in pure product states for the states that satisfy them.
We provide examples that show the ability of these conditions to detect
entanglement. In particular, the necessary condition is proved to be
strong enough to detect bound entangled states.
Zeroerror attacks and detection
statistics in the coherent oneway protocol for quantum cryptography (pp639664)
C.
Branciard, N. Gisin, N. Lutkenhaus, and V. Scarani
This is a study of the security of the Coherent OneWay
(COW) protocol for quantum cryptography, proposed recently as a simple
and fast experimental scheme. In the zeroerror regime, the eavesdropper
Eve can only take advantage of the losses in the transmission. We
consider new attacks, based on unambiguous state discrimination, which
perform better than the basic beamsplitting attack, but which can be
detected by a careful analysis of the detection statistics. These
results stress the importance of testing several statistical parameters
in order to achieve higher rates of secret bits.
Sequential attacks against
differentialphaseshift quantum key distribution with weak coherent
states (pp665688)
M.
Curty, L.L. Zhang, H.K. Lo, and N. Lutkenhaus
We investigate limitations imposed by sequential attacks
on the performance of differentialphaseshift quantum key distribution
protocols that use pulsed coherent light. In particular, we analyze two
sequential attacks based on unambiguous state discrimination and minimum
error discrimination, respectively, of the signal states emitted by the
source. Sequential attacks represent a special type of interceptresend
attacks and, therefore, they provide ultimate upper bounds on the
maximal distance achievable by quantum key distribution schemes.
Book Review:
On “” (pp689690)
G.J.
Milburn
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