Vol.3 No. Special  October 1, 2003
Editorial Note (pp469-470)
          J.H. Shapiro and H.-K. Lo
Research and Review Articles:
Generalized Bell inequalities with parametric down-conversion (pp471-479)
         A. Lamas-Linares, W.T.M. Irvine, J.C. Howell and D. Bouwmeester
We analize the suitability of states generated by stimulated parametric down-conversion for the testing of high dimensional Bell-type inequalities. Two families of bipartite inequalities are considered. For both it is found that the states are good candidates for the tests and some experimental results for one spin-1 inequality are given. The influence of noise and the possibility of supurious results are discussed.

Photon engineering for quantum information processing (pp480-502)
        A.B. U'Ren, K. Banaszek and I.A. Walmsley
We study distinguishing information in the context of quantum interference involving more than one parametric downconversion (PDC) source and in the context of generating polarization-entangled photon pairs based on PDC. We arrive at specific design criteria for two-photon sources so that when used as part of complex optical systems, such as photon-based quantum information processing schemes, distinguishing information between the photons is eliminated guaranteeing high visibility interference. We propose practical techniques which lead to suitably engineered two-photon states that can be realistically implemented with available technology. Finally, we study an implementation of the nonlinear-sign shift (NS) logic gate with PDC sources and show the effect of distinguishing information on the performance of the gate.

Precise creation, characterization, and manipulation of single optical qubits (pp503-517)
        N. Peters, J. Altepeter, E. Jeffrey, D. Branning and P. Kwiat
We present the theoretical basis for and experimental verification of arbitrary single-qubit state generation, using the polarization of photons generated via spontaneous parametric downconversion. Our precision measurement and state reconstruction system has the capability to distinguish over 3 million states, all of which can be reproducibly generated using our state creation apparatus. In order to complete the triumvirate of single qubit control, there must be a way to not only manipulate single qubits after creation and before measurement, but a way to characterize the manipulations themselves. We present a general representation of arbitrary processes, and experimental techniques for generating a variety of single qubit manipulations, including unitary, decohering, and (partially) polarizing operations.

Atomic spins as a storage medium for quantum fluctuations of light  (pp518-534)
        B. Julsgaard, C. Schori, J. L. Sorensen, and E.S. Polzik
We review recent results showing the possibility to use off-resonant light/matter interaction for the purpose of quantum memory. A quantum state of atomic spins can be read out by light in a process which is a quantum analogue of the classical Faraday effect. Conversely, the dynamic Stark effect opens up the opportunity for recording the polarization state of light onto the atomic spin memory. We demonstrate that a sample of cesium atoms under appropriate conditions has the sensitivity to record properties of just a few photons, thus being a feasible candidate for quantum memory for light.

Virtual entanglement and reconciliation protocols for  quantum cryptography with continuous variables  (pp535-552)
        F. Grosshans, N.J. Cerf, J.  Wenger, R. Tualle-Brouri and Ph. Grangier
We discuss quantum key distribution protocols using quantum continuous variables. We show that such protocols can be made secure against individual gaussian attacks regardless the transmission of the optical line between Alice and Bob. %while other ones require that the line transmission is larger than 50%. This is achieved by reversing the reconciliation procedure subsequent to the quantum transmission, that is, using Bob's instead of Alice's data to build the key. Although squeezing or entanglement may be helpful to improve the resistance to noise, they are not required for the protocols to remain secure with high losses. Therefore, these protocols can be implemented very simply by transmitting coherent states and performing homodyne detection. Here, we show that entanglement nevertheless plays a crucial role in the security analysis of coherent state protocols. Every cryptographic protocol based on displaced gaussian states turns out to be equivalent to an entanglement-based protocol, even though no entanglement is actually present. This equivalence even holds in the absence of squeezing, for coherent state protocols. This ``virtual'' entanglement is important to assess the security of these protocols as it provides an upper bound on the mutual information between Alice and Bob if they had used entanglement. The resulting security criteria are compared to the separability criterion for bipartite gaussian variables. It appears that the security thresholds are well within the entanglement region. This supports the idea that coherent state quantum cryptography may be unconditionally secure.

Experimental Progress in Linear Optics Quantum Computing  (pp553-562)
        J.D. Franson J.D. Franson, M.M. Donegan, M.J. Fitch, B.C. Jacobs and T.B. Pittman
Probabilistic quantum logic operations can be performed using linear optical elements and post-selection based on the results of measurements on ancilla photons. We review the results of a number of recent experiments in this area, including the demonstration of several quantum logic gates, the use of feed-forward control, a new source of single photons, and a quantum memory device for single photons. A high-fidelity approach in which the logic gates always produce an output will also be discussed.
 

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