Vol.4 No.2 March 30,
2004
Research and
Review Articles:
Experimental realization of entangled qutrits for quantum communication
(pp093101)
R. Thew, A. Acin, H. Zbinden and N.
Gisin
We have experimentally realized a technique to generate, control and
measure entangled qutrits, 3dimensional quantum systems. This scheme
uses spontaneous parametric down converted photons and unbalanced 3arm
fiber optic interferometers in a scheme analogous to the Franson
interferometric arrangement for qubits. The results reveal a source
capable of generating maximally entangled states with a net state
fidelity, F = 0.985 $\pm$ 0.018. Further the control over the system
reveals a high, net, 2photon interference fringe visibility, V = 0.919
$\pm$ 0.026. This has all been done at telecom wavelengths thus
facilitating the advancement towards long distance higher dimensional
quantum communication.
Quantum dynamics of
the oscillating cantileverdriven adiabatic reversals in magnetic
resonance force microscopy (pp102113)
G.P. Berman, F. Borgonovi and V.I.
Tsifrinovich
We simulated the quantum dynamics for magnetic resonance force
microscopy (MRFM) in the oscillating cantileverdriven adiabatic
reversals (OSCAR) technique. We estimated the frequency shift of the
cantilever vibrations and demonstrated that this shift causes the
formation of a Schr\"odinger cat state which has some similarities and
differences from the conventional MRFM technique which uses cyclic
adiabatic reversals of spins. The interaction of the cantilever with the
environment is shown to quickly destroy the coherence between the two
possible cantilever trajectories. We have shown that using partial
adiabatic reversals, one can produce a significant increase in the OSCAR
signal.
Reconciling cloning
fidelities (pp114121)
I.Ali Khan and J.C. Howell
In most theoretical literature on Quantum Cloning fidelity is defined in
terms of density matrices by assuming clones which are produced in
distinguishable spatial modes. Recent optical implementations of cloning
\cite{Simon99, LamasLinares}, however, do not produce clones in
distinct spatial modes, therefore another simpler expression for
fidelity was proposed. However, no clear theoretical justification was
provided for the equivalence of the two expressions. It is important to
be able to compare experimental results with theoretical predictions,
therefore the theoretical justification is given in this paper, along
with the circumstances under which the two expressions for fidelity are
equivalent. They are shown to be equivalent for all symmetric N$\rightarrow$M
QC, with the symmetry requirement being lifted for ancillafree cloners.
The fidelity is verified explicitly for the 1$\rightarrow$2 UQC based on
stimulated emission proposed in \cite{Simon99}, where the spatial
indistinguishability of the output clones is also discussed.
Relation between
discrete and continuous teleportation using linear elements
(pp122133)
D. Witthaut and M. Fleischhauer
We discuss the relation between discrete and continuous linear
teleportation. For this a specific generalization of existing protocols
to qudits with a discrete and finite spectrum but with an arbitrary
number of states or alternatively to continuous variables is introduced.
Correspondingly a generalization of linear operations and detection is
made on an abstract level. It is shown that linear teleportation is only
possible in a probabilistic sense. An expression for the success
probability of this teleportation protocol is derived which is shown to
depend only on the relevant size of the input and ancilla Hilbert
spaces. From this the known results $P=1/2$ and $P=1$ for the discrete
and continuous cases can be recovered. We also discuss the probabilistic
teleportation scheme of Knill, Laflame and Milburn and argue that it
does not make optimum use of ancilla resources.
Adaptive Quantum
Computation, Constant Depth Quantum Circuits and ArthurMerlin Games
(pp134145)
B.M. Terhal and D.P. DiVincenzo
We present evidence that there exist quantum computations that can be
carried out in constant depth, using 2qubit gates, that cannot be
simulated classically with high accuracy. We prove that if one can
simulate these circuits classically efficiently then ${\rm BQP} \subseteq
{\rm AM}$.
Distributed
construction of quantum fingerprints (pp146151)
A. Ambainis and YY Shi
Quantum fingerprints are useful quantum encodings introduced by Buhrman,
Cleve, Watrous and de Wolf (Physical Review Letters, Volume 87, Number
16, Article 167902, 2001) to obtain an efficient quantum communication
protocol. We design a protocol for constructing the fingerprint in a
distributed scenario. As an application, this protocol gives rise to a
communication protocol more efficient than the best known classical
protocol for a communication problem.
On the structure of
nonstabilizer Cliford codes (pp152160)
A.A. Klappenecker and M. Rotteler
Clifford codes are a class of quantum error control codes that form a
natural generalization of stabilizer codes. These codes were introduced
in 1996 by Knill, but only a single Clifford code was known, which was
not already a stabilizer code. We derive a necessary and sufficient
condition that allows one to decide when a Clifford code is a stabilizer
code, and compile a table of all true Clifford codes for error groups of
small order.
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