Vol.6 No.6
September 1, 2006
ABSTRACTS
Research Articles:
Nearperfect simultaneous measurement
of a qubit register (pp465482)
M. Acton, K.A. Brickman, P.C. Haljan,
P.J. Lee, L. Deslauriers, and C. Monroe
Simultaneous measurement of multiple qubits stored in
hyperfine levels of trapped ^{111}Cd^+ ions is realized with an
intensified chargecoupled device (CCD) imager. A general theory of
fluorescence detection for hyperfine qubits is presented and applied to
experimental data. The use of an imager for photon detection allows for
multiple qubit state measurement with detection fidelities of greater
than $98\%$. Improvements in readout speed and fidelity are discussed in
the context of scalable quantum computation architectures.
A new algorithm for fixed point
quantum search (pp483494)
T. Tulsi, L.K. Grover, and A. Patel
The standard quantum search lacks a feature, enjoyed by
many classical algorithms, of having a fixed point, i.e. monotonic
convergence towards the solution. Recently a fixed point quantum search
algorithm has been discovered, referred to as the Phase\pi/3
search algorithm, which gets around this limitation. While searching a
database for a target state, this algorithm reduces the error
probability from \epsilon to \epsilon^{2q+1} using q
oracle queries, which has since been proved to be asymptotically
optimal. A different algorithm is presented here, which has the same
worstcase behavior as the Phase\pi/3 search algorithm but much
better averagecase behavior. Furthermore the new algorithm gives
\epsilon^{2q+1} convergence for all integral q, whereas the
Phase\pi/3 search algorithm requires q to be
(3^{n}1)/2 with n a positive integer. In the new algorithm,
the operations are controlled by two ancilla qubits, and fixed point
behavior is achieved by irreversible measurement operations applied to
these ancillas. It is an example of how measurement can allow us to
bypass some restrictions imposed by unitarity on quantum computing.
Universal quantum
computation with shutter logic
(pp495515)
J.C. GarciaEscartin and P.
ChamorroPosada
We show that universal quantum logic can be achieved
using only linear optics and a quantum shutter device. With these
elements, we design a quantum memory for any number of qubits and a CNOT
gate which are the basis of a universal quantum computer. An
interactionfree model for a quantum shutter is given.
Quality of a quantum error correcting
scheme and memory error threshold estimation
(pp516526)
P.J. Salas
The error correcting
capabilities of the CalderbankShorSteane [[7,1,3]] quantum code,
together with a faulttolerant syndrome extraction by means of several
ancilla states, have been numerically studied. A simple probability
expression to characterize the code ability for correcting an encoded
qubit has been considered. This probability, as a correction quality
criterion, permits the error correction capabilities among different
recovery schemes to be compared. The memory error threshold is
calculated by means of the best method of those considered.
Entanglement probability distribution
of bipartite randomised stabilizer states
(pp527538)
O.C.O. Dahlsten and M.B. Plenio
We study the entanglement properties of random pure
stabilizer states in spin1/2 particles. We obtain a compact and exact
expression for the probability distribution of the entanglement values
across any bipartite cut. This allows for exact derivations of the
average entanglement and the degree of concentration of measure around
this average. We also give simple bounds on these quantities. We find
that for large systems the average entanglement is near maximal and the
measure is concentrated around it.
The distillability problem revisited
(pp539560)
L. Clarisse
An important open problem in quantum information
theory is the question of the existence of NPT bound entanglement. In
the past years, little progress has been made, mainly because of the
lack of mathematical tools to address the problem. (i) In an
attempt to overcome this, we show how the distillability problem can be
reformulated as a special instance of the separability problem, for
which a large number of tools and techniques are available. (ii)
Building up to this we also show how the problem can be formulated as a
Schmidt number problem. (iii) A numerical method for detecting
distillability is presented and strong evidence is given that all 1copy
undistillable Werner states are also 4copy undistillable. (iv) The same
method is used to estimate the volume of distillable states, and the
results suggest that bound entanglement is primarily a phenomenon found
in low dimensional quantum systems. (v) Finally, a set of one parameter
states is presented which we conjecture to exhibit all forms of
distillability.
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