4,467 research outputs found
Whole-gear efficiency of a benthic survey trawl for flatfish
Whole-gear efficiency (the proportion of fish passing between the otter doors of a bottom trawl that are subsequently captured) was estimated from data collected during experiments to measure the herding efficiency of bridles and doors, the capture efficiency of the net, and the length of the bridles sufficiently close to the seafloor to elicit a herding response. The experiments were focused on four species of flatfish: arrowtooth flounder (Atheresthes stomias), flathead sole (Hippoglossoides elassodon), rex sole (Glyptocephalus zachirus), and Dover sole (Microstomus pacificus). Whole-gear efficiency varied with fish length and reached maximum values between 40% and 50% for arrowtooth flounder, flathead sole, and rex sole. For Dover sole, however, whole-gear efficiency declined from a maximum of 33% over the length range sampled. Such efficiency estimates can be used to determine catchability, which, in turn, can be used to improve the accuracy of stock assessment models when the time series of a survey is short
The efficiencies of generating cluster states with weak non-linearities
We propose a scalable approach to building cluster states of matter qubits
using coherent states of light. Recent work on the subject relies on the use of
single photonic qubits in the measurement process. These schemes can be made
robust to detector loss, spontaneous emission and cavity mismatching but as a
consequence the overhead costs grow rapidly, in particular when considering
single photon loss. In contrast, our approach uses continuous variables and
highly efficient homodyne measurements. We present a two-qubit scheme, with a
simple bucket measurement system yielding an entangling operation with success
probability 1/2. Then we extend this to a three-qubit interaction, increasing
this probability to 3/4. We discuss the important issues of the overhead cost
and the time scaling. This leads to a "no-measurement" approach to building
cluster states, making use of geometric phases in phase space.Comment: 21 pages, to appear in special issue of New J. Phys. on
"Measurement-Based Quantum Information Processing
Efficient optical quantum information processing
Quantum information offers the promise of being able to perform certain
communication and computation tasks that cannot be done with conventional
information technology (IT). Optical Quantum Information Processing (QIP) holds
particular appeal, since it offers the prospect of communicating and computing
with the same type of qubit. Linear optical techniques have been shown to be
scalable, but the corresponding quantum computing circuits need many auxiliary
resources. Here we present an alternative approach to optical QIP, based on the
use of weak cross-Kerr nonlinearities and homodyne measurements. We show how
this approach provides the fundamental building blocks for highly efficient
non-absorbing single photon number resolving detectors, two qubit parity
detectors, Bell state measurements and finally near deterministic control-not
(CNOT) gates. These are essential QIP devicesComment: Accepted to the Journal of optics B special issue on optical quantum
computation; References update
Hybrid quantum repeater based on dispersive CQED interactions between matter qubits and bright coherent light
We describe a system for long-distance distribution of quantum entanglement,
in which coherent light with large average photon number interacts dispersively
with single, far-detuned atoms or semiconductor impurities in optical cavities.
Entanglement is heralded by homodyne detection using a second bright light
pulse for phase reference. The use of bright pulses leads to a high success
probability for the generation of entanglement, at the cost of a lower initial
fidelity. This fidelity may be boosted by entanglement purification techniques,
implemented with the same physical resources. The need for more purification
steps is well compensated for by the increased probability of success when
compared to heralded entanglement schemes using single photons or weak coherent
pulses with realistic detectors. The principle cause of the lower initial
fidelity is fiber loss; however, spontaneous decay and cavity losses during the
dispersive atom/cavity interactions can also impair performance. We show that
these effects may be minimized for emitter-cavity systems in the weak-coupling
regime as long as the resonant Purcell factor is larger than one, the cavity is
over-coupled, and the optical pulses are sufficiently long. We support this
claim with numerical, semiclassical calculations using parameters for three
realistic systems: optically bright donor-bound impurities such as 19-F:ZnSe
with a moderate-Q microcavity, the optically dim 31-P:Si system with a high-Q
microcavity, and trapped ions in large but very high-Q cavities.Comment: Please consult the published version, where assorted typos are
corrected. It is freely available at http://stacks.iop.org/1367-2630/8/18
Single photon quantum non-demolition in the presence of inhomogeneous broadening
Electromagnetically induced transparency (EIT) has been often proposed for
generating nonlinear optical effects at the single photon level; in particular,
as a means to effect a quantum non-demolition measurement of a single photon
field. Previous treatments have usually considered homogeneously broadened
samples, but realisations in any medium will have to contend with inhomogeneous
broadening. Here we reappraise an earlier scheme [Munro \textit{et al.} Phys.
Rev. A \textbf{71}, 033819 (2005)] with respect to inhomogeneities and show an
alternative mode of operation that is preferred in an inhomogeneous
environment. We further show the implications of these results on a potential
implementation in diamond containing nitrogen-vacancy colour centres. Our
modelling shows that single mode waveguide structures of length in single-crystal diamond containing a dilute ensemble of NV
of only 200 centres are sufficient for quantum non-demolition measurements
using EIT-based weak nonlinear interactions.Comment: 21 pages, 9 figures (some in colour) at low resolution for arXiv
purpose
Weak nonlinearities: A new route to optical quantum computation
Quantum information processing (QIP) offers the promise of being able to do
things that we cannot do with conventional technology. Here we present a new
route for distributed optical QIP, based on generalized quantum non-demolition
measurements, providing a unified approach for quantum communication and
computing. Interactions between photons are generated using weak
non-linearities and intense laser fields--the use of such fields provides for
robust distribution of quantum information. Our approach requires only a
practical set of resources, and it uses these very efficiently. Thus it
promises to be extremely useful for the first quantum technologies, based on
scarce resources. Furthermore, in the longer term this approach provides both
options and scalability for efficient many-qubit QIP.Comment: 7 Pages, 4 Figure
Normal, Abby Normal, Prefix Normal
A prefix normal word is a binary word with the property that no substring has
more 1s than the prefix of the same length. This class of words is important in
the context of binary jumbled pattern matching. In this paper we present
results about the number of prefix normal words of length , showing
that for some and
. We introduce efficient
algorithms for testing the prefix normal property and a "mechanical algorithm"
for computing prefix normal forms. We also include games which can be played
with prefix normal words. In these games Alice wishes to stay normal but Bob
wants to drive her "abnormal" -- we discuss which parameter settings allow
Alice to succeed.Comment: Accepted at FUN '1
A Time-Space Tradeoff for Triangulations of Points in the Plane
In this paper, we consider time-space trade-offs for reporting a triangulation of points in the plane. The goal is to minimize the amount of working space while keeping the total running time small. We present the first multi-pass algorithm on the problem that returns the edges of a triangulation with their adjacency information. This even improves the previously best known random-access algorithm
Quantum Computation by Communication
We present a new approach to scalable quantum computing--a ``qubus
computer''--which realises qubit measurement and quantum gates through
interacting qubits with a quantum communication bus mode. The qubits could be
``static'' matter qubits or ``flying'' optical qubits, but the scheme we focus
on here is particularly suited to matter qubits. There is no requirement for
direct interaction between the qubits. Universal two-qubit quantum gates may be
effected by schemes which involve measurement of the bus mode, or by schemes
where the bus disentangles automatically and no measurement is needed. In
effect, the approach integrates together qubit degrees of freedom for
computation with quantum continuous variables for communication and
interaction.Comment: final published versio
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