Vol.2 No.6 Oct 15, 2002 (print:
November 15,
2002)
Researches:
Creating highdimensional timebin entanglement using modelocked
lasers
(pp425433)
H. de Riedmatten, I. Marcikic,
H.
Zbinden and N. Gisin
We present a new scheme to generate high dimensional
entanglement between two photonic systems. The idea is based on
parametric down conversion with a sequence of pump pulses generated by a
modelocked laser. We prove experimentally the feasibility of this
scheme by performing a Fransontype Bell test using a 2way
interferometer with pathlength difference equal to the distance between
2 pump pulses. With this experiment, we can demonstrate entanglement for
a twophoton state of at least dimension D=11. Finally, we propose a
feasible experiment to show a FabryPerot like effect for a high
dimensional twophoton state.
A practical trojan horse for
Bellinequalitybased quantum cryptography
(pp434442)
J. Larsson
Quantum
Cryptography, or more accurately, Quantum Key Distribution (QKD) is
based on using an unconditionally secure ``quantum channel'' to share a
secret key
among two users. A manufacturer of QKD devices could, intentionally or
not, use a (semi)classical channel instead of the quantum channel,
which would remove the supposedly unconditional security. One example is
the BB84 protocol, where the quantum channel can be implemented in
polarization of single photons. Here, use of several photons instead of
one to encode each bit of the key provides a similar but insecure
system. For protocols based on violation of a Bell inequality (e.g., the
Ekert protocol) the situation is somewhat different. While the
possibility is mentioned by some authors, it is generally thought that
an implementation of a (semi)classical channel will differ
significantly from that of a quantum channel. Here, a counterexample
will be given using an identical physical setup as is used in
photonpolarization Ekert QKD. Since the physical implementation is
identical, a manufacturer may include this modification as a Trojan
Horse in manufactured systems, to be activated at will by an
eavesdropper. Thus, the old truth of cryptography still holds: you have
to trust the manufacturer of your cryptographic device. Even when you do
violate the Bell inequality.
Computational model underlying the oneway quantum computer
(pp443486)
R.
Raussendorf and H. Briegel
In this paper we present the computational model
underlying the oneway quantum computer which we introduced recently
[Phys. Rev. Lett. {\bf{86}}, 5188 (2001)]. The oneway quantum computer
has the property that any quantum logic network can be simulated on it.
Conversely, not all ways of quantum information processing that are
possible with the oneway quantum computer can be understood properly in
network model terms. We show that the logical depth is, for certain
algorithms, lower than has so far been known for networks. For example,
every quantum circuit in the Clifford group can be performed on the
oneway quantum computer in a single step. Lorentz Invariance
of Entanglement (pp487512)
P.M.
Alsing and G. Milburn
We study the
transformation of maximally entangled states under the action of Lorentz
transformations in a fully relativistic setting. By explicit calculation
of the Wigner rotation, we describe the relativistic analog of the Bell
states as viewed from two inertial frames moving with constant velocity
with respect to each other. Though the finite dimensional matrices
describing the Lorentz transformations are nonunitary, each single
particle state of the entangled pair undergoes an effective, momentum
dependent, local unitary rotation, thereby preserving the entanglement
fidelity of the bipartite state. The details of how these unitary
transformations are manifested are explicitly worked out for the Bell
states comprised of massive spin $1/2$ particles and massless photon
polarizations. The relevance of this work to noninertial frames is
briefly discussed.
The density matrix for mixed state qubits and hyperbolic geometry
(pp513514)
A.A. Ungar
Density matrices for
mixed state qubits, parametrized by the Bloch vector in the open unit
ball of the Euclidean 3space, are well known in quantum information and
computation theory. By presenting new identities for the qubit density
matrix we indicate its intimate relationship with M\"obius addition and
scalar multiplication. The latter, in turn, form the algebraic setting
for the Poincar\'e ball model of hyperbolic geometry so that, as a
result, the qubit density matrix is linked to hyperbolic geometry.
Book Review:
On "A
First Course in Information Theory"
by
Raymond W. Yeung
(pp515516)
A. Winter
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