5,638 research outputs found
Interpretation of coronal synoptic observations
Three-dimensional reconstruction techniques used to determine coronal density distributions from synoptic data are complicated and time consuming to employ. Current techniques also assume time invariant structures and thus mix both temporal and spatial variations present in the coronal data. The observed distribution of polarized brightness, pB, and brightness, B, of coronal features observed either at eclipses or with coronagraphs depends upon both the three-dimensional distribution of electron density within the structure and the location of the feature with respect to the plane-of-the-sky. By theoretically studying the signature of various coronal structures as they would appear during a limb transit, it is possible to recognize these patterns in real synoptic data as well as estimate temporal evolutionary effects
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
Threshold criteria for incipient sediment motion on an inclined bedform in the presence of oscillating-grid turbulence
Here, we report laboratory experiments to investigate the threshold criteria for incipient sediment motion in the presence of oscillating-grid turbulence, with the bed slope inclined at angles between the horizontal and the repose limit for the sediment. A set of nine mono-disperse sediment types was used with size ranges normally associated with either the hydraulically-smooth or transitional regimes. Measurements of the (turbulent) fluid velocity field, in the region between the grid and bedform's surface, were obtained using two-dimensional particle imaging velocimetry. Statistical analysis of the velocity data showed that the turbulence had a anisotropic structure, due to the net transfer of energy from the normal to the tangential velocity components in the near-bed region, and that the fluctuations were dominant compared to the secondary mean flow. The sediment threshold criteria for horizontal bedforms were compared with, and found to be in good qualitative agreement with the standard Shields curve. For non-horizontal bedforms, the bed mobility was found to increase with increasing bed slope, and the threshold criteria were compared with previously-reported theoretical models, based on simple force-balance arguments
Ensemble averaged entanglement of two-particle states in Fock space
Recent results, extending the Schmidt decomposition theorem to wavefunctions
of identical particles, are reviewed. They are used to give a definition of
reduced density operators in the case of two identical particles. Next, a
method is discussed to calculate time averaged entanglement. It is applied to a
pair of identical electrons in an otherwise empty band of the Hubbard model,
and to a pair of bosons in the the Bose-Hubbard model with infinite range
hopping. The effect of degeneracy of the spectrum of the Hamiltonian on the
average entanglement is emphasised.Comment: 19 pages Latex, changed title, references added in the conclusion
Light-dependent magnetoreception: orientation behaviour of migratory birds under dim red light
Magnetic compass orientation in migratory birds has been shown to be based on radical pair processes and to require light from the short wavelength part of the spectrum up to 565 nm Green. Under dim red light of 645 nm wavelength and 1 mW m(-2) intensit
Applications of Coherent Population Transfer to Quantum Information Processing
We develop a theoretical framework for the exploration of quantum mechanical
coherent population transfer phenomena, with the ultimate goal of constructing
faithful models of devices for classical and quantum information processing
applications. We begin by outlining a general formalism for weak-field quantum
optics in the Schr\"{o}dinger picture, and we include a general
phenomenological representation of Lindblad decoherence mechanisms. We use this
formalism to describe the interaction of a single stationary multilevel atom
with one or more propagating classical or quantum laser fields, and we describe
in detail several manifestations and applications of electromagnetically
induced transparency. In addition to providing a clear description of the
nonlinear optical characteristics of electromagnetically transparent systems
that lead to ``ultraslow light,'' we verify that -- in principle -- a
multi-particle atomic or molecular system could be used as either a low power
optical switch or a quantum phase shifter. However, we demonstrate that the
presence of significant dephasing effects destroys the induced transparency,
and that increasing the number of particles weakly interacting with the probe
field only reduces the nonlinearity further. Finally, a detailed calculation of
the relative quantum phase induced by a system of atoms on a superposition of
spatially distinct Fock states predicts that a significant quasi-Kerr
nonlinearity and a low entropy cannot be simultaneously achieved in the
presence of arbitrary spontaneous emission rates. Within our model, we identify
the constraints that need to be met for this system to act as a one-qubit and a
two-qubit conditional phase gate.Comment: 25 pages, 14 figure
Limits in the characteristic function description of non-Lindblad-type open quantum systems
In this paper I investigate the usability of the characteristic functions for
the description of the dynamics of open quantum systems focussing on
non-Lindblad-type master equations. I consider, as an example, a non-Markovian
generalized master equation containing a memory kernel which may lead to
nonphysical time evolutions characterized by negative values of the density
matrix diagonal elements [S.M. Barnett and S. Stenholm, Phys. Rev. A {\bf 64},
033808 (2001)]. The main result of the paper is to demonstrate that there exist
situations in which the symmetrically ordered characteristic function is
perfectly well defined while the corresponding density matrix loses positivity.
Therefore nonphysical situations may not show up in the characteristic
function. As a consequence, the characteristic function cannot be considered an
{\it alternative complete} description of the non-Lindblad dynamics.Comment: Revised version. 4 pages, 1 figur
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
Unifying static and dynamic approaches to evolution through the Compliant Systems Architecture
©2004 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.Support for evolution can be classified as static or dynamic. Static evolvability is principally concerned with structuring systems as separated abstractions. Dynamic evolvability is concerned with the means by which change is effected. Dynamic evolution provides the requisite flexibility for application evolution, however, the dynamic approach is not scalable in the absence of static measures to achieve separation of abstractions. This separation comes at a price in that issues of concern become trapped within static abstraction boundaries, thereby inhibiting dynamic evolution. The need for a unified approach has long been recognised but existing systems that attempt to address this need do so in an ad-hoc manner. The principal reason for this is that these approaches fail to resolve the incongruence in the underlying models. Our contention is that this disparity is incidental rather than fundamental to the problem. To this end we propose an alternative model based on the Compliant Systems Architecture (CSA), a structuring methodology for constructing software systems. The overriding benefit of this work is increased flexibility. Specifically our contribution is an instantiation of the CSA that supports unified static and dynamic evolution techniques. Our model is explored through a worked example in which we evolve an application’s concurrency model.Falkner, K.; Detmold, H.; Howard, D.; Munro, D.S.; Morrison, R.; Norcross, S
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
- …