Vol.11
No.5&6 May 1, 2011
Research Articles:
MultiBloch
vector representation of the qutrit
(pp03610373)
Pawel
Kurzynski
An ability to describe quantum states directly by average values of
measurement outcomes is provided by the Bloch vector. For an
informationally complete set of measurements one can construct unique
Bloch vector for any quantum state. However, not every Bloch vector
corresponds to a quantum state. It seems that only for twodimensional
quantum systems it is easy to distinguish proper Bloch vectors from
improper ones, i.e. the ones corresponding to quantum states from the
other ones. I propose an alternative approach to the problem in which
more than one vector is used. In particular, I show that a state of the
qutrit can be described by the three qubitlike Bloch vectors.
Finitekey
analysis for quantum key distribution with decoy states
(pp03740389)
TingTing Song,
Jie Zhang, SuJuan Qin, Fei Gao, and QiaoYan Wen
We analyze the security of finiteresource quantum key distribution with
decoy states, and present the security bound for the practical
implementations by introducing the deviations of the probability of
sending a $k$photon pulse and the error rate of the quantum state. The
bound is simulated under reasonable values of the observed parameters.
Compared with the previous works, the security bound is more stringent.
Evolution of
entanglement
(pp03900419)
M.
Merkli, G.P. Berman, F. Borgonovi, and K. Gebresellasie
We analyze the dynamics of entanglement between two qubits which
interact through collective and local environments. Our approach is
based on a resonance theory which assumes a small interaction between
qubits and environments and which gives rigorous perturbation theory
results, valid for all times. We obtain expressions for (i)
characteristic timescales for decoherence, relaxation, disentanglement,
and for (ii) the evolution of observables, valid uniformly in time $t\geq
0$. We introduce a classification of decoherence times based on
clustering of the reduced density matrix elements, persisting on all
timescales. We examine characteristic dynamical properties such as
creation, death and revival of entanglement. We discuss possible
applications of our results for superconducting quantum computation and
quantum measurement technologies.
Coherence
preservation in 3level atom
(pp04200433)
Fei Yang and
Shuang Cong
Coherence preservation of a multilevel system subject to Markovian
decoherence is studied. A Lambdatype threelevel atom is selected as
the system model. Coherence preservation between a ground state and the
excited state of such a system is defined as the control object. A
control field is designed by means of constraining the constant
coherence condition. For the singularities of the control field, we
qualitatively analyze the breakdown time, i.e. the time of control
diverging. We obtain the region in which the state stays to maintain
coherence forever in the case that the threelevel system is reduced to
a twolevel one. For other cases, we investigate how different
parameters affect the breakdown time qualitatively. Numerical
experiments are implemented on a threelevel quantum system and the
experimental results are analyzed.
Security of a
kind of quantum secret sharing with single photons
(pp04340443)
TianYin Wang and
QiaoYan Wen
The security of a kind of quantum secret sharing with single photons was
analyzed recently, and it was shown that almost all the present schemes
in this kind were not secure in the sense that an unauthorized set of
participants can gain access to the dealer's secret without introducing
any error. In this paper, we give a general model for this kind of
quantum secret sharing. Then we analyze the conditions that make it
immune to all the present attacks. Finally, we give a feasible way to
design secure quantum secret sharing schemes in the model.
HQC with the AC
setup associated with topological defects
(pp04440455)
Knut Bakke and
Cláudio Furtado
In this work, we propose a new formulation allowing to realize the
holonomic quantum computation with neutral particles with a permanent
magnetic dipole moments interacting with an external electric field in
the presence of a topological defect. We show that both the interaction
of the electric field with the magnetic dipole moment and the presence
of topological defect generate independent contributions to the
geometric quantum phases which can be used to describe any arbitrary
rotation on the magnetic dipole moment without using the adiabatic
approximation.
Quantum memory
for light with a quantum dot system coupled to a nanomechanical
resonator
(pp04560465)
JinJin Li and
KaDi Zhu
The specific features including high factor and long vibration lifetime
of nanomechanical resonator (NR) in nanooptomechanical systems have
stimulated research to realize some optical devices. In this work, we
demonstrate theoretically that it is possible to achieve quantum memory
for light on demand via a quantum dot system coupled to a nanomechanical
resonator. This quantum memory for light is based on mechanically
induced exciton polaritons, which makes the darkstate polariton
reaccelerated and converted back into a photon pulse. Our presented
device could open the door to alloptical routers for light memory
devices and quantum information processing.
Depolarizing behavior of quantum
channels in higher dimensions
(pp04660484)
Easwar Magesan
The paper analyzes the behavior of quantum channels, particularly in
large dimensions, by proving various properties of the quantum gate
fidelity. Many of these properties are of independent interest in the
theory of distance measures on quantum operations. A nonuniqueness
result for the gate fidelity is proven, a consequence of which is the
existence of nondepolarizing channels that produce a constant gate
fidelity on pure states. Asymptotically, the gate fidelity associated
with any quantum channel is shown to converge to that of a depolarizing
channel. Methods for estimating the minimum of the gate fidelity are
also presented.
Localization of
quantum walks via the CGMV method
(pp04850495)
Norio Konno and
Etsuo Segawa
We study discretetime quantum walks on a half line by means of spectral
analysis. Cantero et al. showed that the CMV matrix, which gives a
recurrence relation for the orthogonal Laurent polynomials on the unit
circle, expresses the dynamics of the quantum walk. Using the CGMV
method introduced by them, the name is taken from their initials, we
obtain the spectral measure for the quantum walk. As a corollary, we
give another proof for localization of the quantum walk on homogeneous
trees shown by Chisaki et al.
Assisted
entanglement distillation
(pp04960520)
Nicolas Dutil and
Patrick Hayden
Motivated by the problem of designing quantum repeaters,
we study entanglement distillation between two parties, Alice and Bob,
starting from a mixed state and with the help of ``repeater'' stations.
To treat the case of a single repeater, we extend the notion of
entanglement of assistance to arbitrary mixed tripartite states and
exhibit a protocol, based on a random coding strategy, for extracting
pure entanglement. The rates achievable by this protocol formally
resemble those achievable if the repeater station could merge its state
to one of Alice and Bob even when such merging is impossible. This rate
is provably better than the hashing bound for sufficiently pure
tripartite states. We also compare our assisted distillation protocol to
a hierarchical strategy consisting of entanglement distillation followed
by entanglement swapping. We demonstrate by the use of a simple example
that our random measurement strategy outperforms hierarchical
distillation strategies when the individual helper stations' states fail
to individually factorize into portions associated specifically with
Alice and Bob. Finally, we use these results to find achievable rates
for the more general scenario, where many spatially separated repeaters
help two recipients distill entanglement.
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