Vol.4 No.2 March 30, 2004
Research and Review Articles:
Experimental realization of entangled qutrits for quantum communication (pp093-101)
        R. Thew, A. Acin, H. Zbinden and N. Gisin
We have experimentally realized a technique to generate, control and measure entangled qutrits, 3-dimensional quantum systems. This scheme uses spontaneous parametric down converted photons and unbalanced 3-arm 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, 2-photon 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 cantilever-driven adiabatic reversals in magnetic resonance force microscopy (pp102-113)
        G.P. Berman, F. Borgonovi and V.I. Tsifrinovich
We simulated the quantum dynamics for magnetic resonance force microscopy (MRFM) in the oscillating cantilever-driven 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 (pp114-121)
        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, Lamas-Linares}, 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 ancilla-free 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 (pp122-133)
        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 Arthur-Merlin Games (pp134-145)
        B.M. Terhal and D.P. DiVincenzo
We present evidence that there exist quantum computations that can be carried out in constant depth, using 2-qubit 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 (pp146-151)
        A. Ambainis and Y-Y 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 (pp152-160)
        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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