Vol.5 No.4/No.5
July 1,
2005
Editorial:
Quantum
Information and Quantum Control
(pp273274)
P. Brumer,
D. Lidar, HK Lo and A Steinberg
Research Articles:
Minimum entangled state dimension
required for pseudotelepathy
(pp275284)
G. Brassard, A.A. Methot and A. Tapp
Pseudotelepathy provides an intuitive way of
looking at Bell's inequalities, in which it is
often obvious that feats achievable by use of
quantum entanglement would be classically impossible.
A~twoplayer pseudotelepathy game proceeds as follows:
Alice and Bob are individually asked a question
and they must provide an answer. They
are \emph{not} allowed any form of
communication once the questions are asked, but they may
have agreed on a common strategy prior to the execution of the
game. We~say that they \emph{win} the game if
the questions and answers fulfil a specific
relation. A~game exhibits \emph{pseudotelepathy}
if there is a quantum strategy that makes Alice and
Bob win the game for all possible questions, provided
they share prior entanglement, whereas it would be impossible to
win this game systematically in a classical
setting. In~this paper, we show that any
twoplayer pseudotelepathy game requires the
quantum players to share an entangled quantum
system of dimension at least~\mbox{$3 \times 3$}.
This is optimal for twoplayer games, but the most efficient
pseudotelepathy game possible, in terms of total dimension,
involves \emph{three} players who share a
quantum system of dimension~\mbox{$2 \times 2
\times 2$}.
Universal
dynamical control of decay and decoherence for weak and strong
systembath coupling (pp285317)
G. Gordon, G. Kurizki, A.G. Kofman and
S. Pellegrin
A unified theory is given of dynamically modified
decay and decoherence in driven twolevel and
multilevel quantum systems that are weakly
coupled to arbitrary finitetemperature
reservoirs and undergo random phase fluctuations.
Criteria for the optimization of decoherence suppression and the
limitations of this approach are obtained.
For a driven qubit that is strongly coupled to the
continuum edge of
reservoir's spectrum, we demonstrate that only an appropriately
ordered sequence of abrupt changes of the resonance frequency,
near the continuum edge, can effectively
protect the qubit state from decoherence.
Coherent optical
control of electronic excitations in functionalized semiconductor
nanostructures (pp318334)
L.G.C. Rego, S.G. Abuabara and V.S.
Batista
The feasibility of creating and manipulating coherent
quantum states on surfaces of functionalized semiconductor
nanostructures is computationally investigated. Quantum dynamics
simulations of electronhole transfer between catechol molecules
adsorbed on TiO_2 anatase nanostructures under cryogenic and vacuum
conditions indicate that laser induced coherent excitations can be
prepared and manipulated to exhibit controllable spatial Rabi
oscillations. The presented computational methods and results are
particularly relevant to explore the basic model components of
quantuminformation electrooptic devices based on inexpensive and
readily available semiconductor materials.
Onsite
localization of excitations (pp335349)
M.I. Dykman, L.F. Santos, M. Shapiro and
F.M. Izrailev
We demonstrate that, in a quantum computer with perpetually
coupled qubits, all excitations can be confined to their sites (qubits)
even without refocusing. The onsite localization is obtained by
constructing a sequence of qubit energies that efficiently suppresses
resonant hopping. The time during which a manyexcitation state remains
strongly localized in an infinite chain can exceed the reciprocal
hopping frequency by $\agt 10^5$ already for a moderate bandwidth of
qubit energies. The proposed energy sequence is also convenient for
performing quantum operations on the qubits.
Stabilizing qubit
coherence via trackingcontrol (pp350363)
D.A. Lidar and S. Schneider
We consider the problem of
stabilizing the coherence of a single qubit
subject to Markovian
decoherence, via the application of a control
Hamiltonian, without any
additional resources. In this case neither quantum
error
correction/avoidance, nor dynamical decoupling applies. We show that
using trackingcontrol,
i.e., the conditioning of the control field on the
state of the qubit, it is
possible to maintain coherence for finite time
durations, until the
control field diverges.
Phase control for
entanglement preparation in twoqubit systems (pp364379)
V.S. Malinovsky and I.R. Sola
The theory of Quantum Control is starting to lay
bridges with the field of Quantum Information
and Quantum Computation. Using key ideas of
laser control of the dynamics by means of phase manipulation and
adiabatic passage, we review laser schemes that allow
entanglement preparation in a twoqubit
system. The schemes are based on sequences
that use four timedelayed pulses, with or without concerted decay,
in or off resonance with the intermediate levels of the qubit
space. We show how to control the fidelity and
phase of the entanglement, as well as the
sensitivity of the preparation to the different pulse
parameters. In general the schemes provide an improvement in
robustness and in the finesse of the control
to phase, with respect to previously proposed
schemes based on sequences of $\pi$ pulses.
Bidirectional
coherent classical communication (pp380395)
A.W. Harrow and D.W. Leung
A unitary interaction coupling two parties enables
quantum or classical
communication in both the forward and backward directions.
Each communication capacity can be thought of as a tradeoff
between the achievable rates of specific types
of forward and backward communication.
Our first result shows that for any bipartite unitary gate,
bidirectional coherent classical communication is no more
difficult than bidirectional classical
communication  they have the same achievable rate
regions. Previously this result was known only for the
unidirectional capacities (i.e., the
boundaries of the tradeoff). We then relate
the tradeoff for twoway coherent communication to the
tradeoff for twoway quantum communication and the tradeoff for
coherent communication in one direction and quantum communication
in the other.
Information vs.
disturbance in dimension D (pp396412)
P.O. Boykin and V.P. Roychowdhury
We show that for Eve to get information in
one basis about a state, she must cause errors in \emph{all} bases that
are mutually unbiased to that basis. Our result holds in any dimension.
We also show that this result holds for \emph{all functions of messages}
that are encrypted with a key.
Getting something
out of nothing (pp413418)
HK Lo
We study quantum key distribution
with standard weak coherent states and
show, rather counterintuitively, that the detection events
originated from vacua can contribute to secure key generation
rate, over and above the best previous result.
Our proof is based on a communication complexity/quantum memory
argument. The key observation is that Eve does
not have to store anything, if Alice sends out
a vacuum state.
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