Vol.2 No.2 Feb. 15, 2002 (print: March 15,
2002)
Researches:
Superposition, entanglement and quantum computation
(pp97116)
T.M. Forcer, A.J.G. Hey, D.A. Ross and P.G.R. Smith
The paper examines the roles played by superposition and
entanglement in quantum computing. The analysis is illustrated by
discussion of a "classical" electronic implementation of Grover's
quantum search algorithm. It is shown explicitly that the absence of
multiparticle entanglement leads to exponentially rising resources for
implementing such quantum algorithms.
Simulating
arbitrary pairinteractions by a given Hamiltonian:
graphtheoretical
bounds on the timecomplexity
(pp117132)
P. Wocjan, D. Janzing and T. Beth We consider a quantum computer consisting of n
spins with an arbitrary but fixed pairinteraction Hamiltonian and
describe how to simulate other pairinteractions by interspersing the
natural time evolution with fast local transformations. Calculating the
minimal time overhead of such a simulation leads to a convex
optimization problem. Lower and upper bounds on the minimal time
overhead are derived in terms of chromatic indices of interaction graphs
and spectral majorization criteria. These results classify Hamiltonians
with respect to their computational power. For a specific
Hamiltonian, namely \sigma_z\otimes\sigma_zinteractions between all
spins, the optimization is mathematically equivalent to a separability
problem of nqubit density matrices. We compare the complexity defined
by such a quantum computer with the usual gate complexity.
Universal
simulation of Hamiltonians using a finite set of control operations
(pp133150)
P. Wocjan, M. Rotteler, D. Janzing and T. Beth
Any quantum system with a nontrivial Hamiltonian is
able to simulate any other Hamiltonian evolution provided that a
sufficiently large group of unitary control operations is available. We
show that there exist finite groups with this property and present a
sufficient condition in terms of group characters. We give examples of
such groups in dimension 2 and 3. Furthermore, we show that it is
possible to simulate an arbitrary bipartite interaction by a given one
using such groups acting locally on the subsystems.
The quantum state
of a laser field
(pp151165)
S.J. van Enk and C.A. Fuchs Optical implementations of quantum communication
protocols typically involve laser fields. However, the standard
description of the quantum state of a laser field is surprisingly
insufficient to understand the quantum nature of such implementations.
In this paper, we give a quantum informationtheoretic description of a
propagating continuouswave laser field and reinterpret various
quantumoptical experiments in light of this. A timely example is found
in a recent controversy about the quantum teleportation of continuous
variables. We show that contrary to the claims of T. Rudolph and B.C.
Sanders [Phys. Rev. Lett. {\bf 87}, 077903 (2001)], a
conventional laser can be used for quantum teleportation with continuous
variables and for generating continuousvariable quantum entanglement.
Furthermore, we show that optical coherent states do play a privileged
role in the description of propagating laser fields even though they
cannot be ascribed such a role for the intracavity field.
Perspective:
NMR quantum
information processing and entanglement
(pp166176)
R. Laflamme, D. Cory, C. Negrevergne and L. Viola
In this essay we discuss the issue of quantum information and recent
nuclear magnetic resonance (NMR) experiments. We explain why these
experiments should be regarded as quantum information processing (QIP)
despite the fact that, in present liquid state NMR experiments, no
entanglement is found. We comment on how these experiments contribute to
the future of QIP and include a brief discussion on the origin of the
power of quantum computers.
Book Review:
On “Introduction
to Quantum Computation and Information”
edited by HoiKwong
Lo, Sandu Popescu, and Tim Spiller
(pp177168)
D. Gottesman
QIC Webcorner:
Update
(pp179180)
P. Kok
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