Vol.3 No. Special
J.H. Shapiro and H.-K. Lo
Generalized Bell inequalities with
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
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
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
Atomic spins as a storage medium for
quantum fluctuations of light
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
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
J.D. Franson J.D. Franson, M.M. Donegan, M.J. Fitch, B.C. Jacobs and T.B.
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
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