1,283 research outputs found
Global consensus Monte Carlo
To conduct Bayesian inference with large data sets, it is often convenient or
necessary to distribute the data across multiple machines. We consider a
likelihood function expressed as a product of terms, each associated with a
subset of the data. Inspired by global variable consensus optimisation, we
introduce an instrumental hierarchical model associating auxiliary statistical
parameters with each term, which are conditionally independent given the
top-level parameters. One of these top-level parameters controls the
unconditional strength of association between the auxiliary parameters. This
model leads to a distributed MCMC algorithm on an extended state space yielding
approximations of posterior expectations. A trade-off between computational
tractability and fidelity to the original model can be controlled by changing
the association strength in the instrumental model. We further propose the use
of a SMC sampler with a sequence of association strengths, allowing both the
automatic determination of appropriate strengths and for a bias correction
technique to be applied. In contrast to similar distributed Monte Carlo
algorithms, this approach requires few distributional assumptions. The
performance of the algorithms is illustrated with a number of simulated
examples
Entanglement of Pure Two-Mode Gaussian States
The entanglement of general pure Gaussian two-mode states is examined in
terms of the coefficients of the quadrature components of the wavefunction. The
entanglement criterion and the entanglement of formation are directly evaluated
as a function of these coefficients, without the need for deriving local
unitary transformations. These reproduce the results of other methods for the
special case of symmetric pure states which employ a relation between squeezed
states and Einstein-Podolsky-Rosen correlations. The modification of the
quadrature coefficients and the corresponding entanglement due to application
of various optical elements is also derived.Comment: 12 page
Global consensus Monte Carlo
To conduct Bayesian inference with large data sets, it is often convenient or necessary to distribute the data across multiple machines. We consider a likelihood function expressed as a product of terms, each associated with a subset of the data. Inspired by global variable consensus optimisation, we introduce an instrumental hierarchical model associating auxiliary statistical parameters with each term, which are conditionally independent given the top-level parameters. One of these top-level parameters controls the unconditional strength of association between the auxiliary parameters. This model leads to a distributed MCMC algorithm on an extended state space yielding approximations of posterior expectations. A trade-off between computational tractability and fidelity to the original model can be controlled by changing the association strength in the instrumental model. We further propose the use of a SMC sampler with a sequence of association strengths, allowing both the automatic determination of appropriate strengths and for a bias correction technique to be applied. In contrast to similar distributed Monte Carlo algorithms, this approach requires few distributional assumptions. The performance of the algorithms is illustrated with a number of simulated examples
Entanglement of a Double Dot with a Quantum Point Contact
Entanglement between particle and detector is known to be inherent in the
measurement process. Gurvitz recently analyzed the coupling of an electron in a
double dot (DD) to a quantum point contact (QPC) detector. In this paper we
examine the dynamics of entanglement that result between the DD and QPC. The
rate of entanglement is optimized as a function of coupling when the electron
is initially in one of the dots. It decreases asymptotically towards zero with
increased coupling. The opposite behavior is observed when the DD is initially
in a superposition: the rate of entanglement increases unboundedly as the
coupling is increased. The possibility that there are conditions for which
measurement occurs versus entanglement is considered
Prospective memory functioning among ecstasy/polydrug users: evidence from the Cambridge Prospective Memory Test (CAMPROMPT)
Rationale:
Prospective memory (PM) deficits in recreational drug users have been documented in recent years. However, the assessment of PM has largely been restricted to self-reported measures that fail to capture the distinction between event-based and time-based PM. The aim of the present study is to address this limitation.
Objectives:
Extending our previous research, we augmented the range laboratory measures of PM by employing the CAMPROMPT test battery to investigate the impact of illicit drug use on prospective remembering in a sample of cannabis only, ecstasy/polydrug and non-users of illicit drugs, separating event and time-based PM performance. We also administered measures of executive function and retrospective memory in order to establish whether ecstasy/polydrug deficits in PM were mediated by group differences in these processes.
Results:
Ecstasy/polydrug users performed significantly worse on both event and time-based prospective memory tasks in comparison to both cannabis only and non-user groups. Furthermore, it was found that across the whole sample, better retrospective memory and executive functioning was associated with superior PM performance. Nevertheless, this association did not mediate the drug-related effects that were observed. Consistent with our previous study, recreational use of cocaine was linked to PM deficits.
Conclusions:
PM deficits have again been found among ecstasy/polydrug users, which appear to be unrelated to group differences in executive function and retrospective memory. However, the possibility that these are attributable to cocaine use cannot be excluded
Structure of the Phase in Pure Two-Mode Gaussian States
The two-mode relative phase associated with Gaussian states plays an
important role in quantum information processes in optical, atomic and
electronic systems. In this work, the origin and structure of the two-mode
relative phase in pure Gaussian states is studied in terms of its dependences
on the quadratures of the modes. This is done by constructing local canonical
transformations to an associated two-mode squeezed state. The results are
illustrated by studying the time dependence of the phase under a nonlocal
unitary model evolution containing correlations between the modes. In a more
general context, this approach may allow the two-mode phase to be studied in
situations sensitive to different physical parameters within experimental
configurations relevant to quantum information processing tasks
Gate voltage dependent Rashba spin splitting in hole transverse magnetic focussing
Magnetic focussing of charge carriers in two-dimensional systems provides a
solid state version of a mass spectrometer. In the presence of a spin-orbit
interaction, the first focussing peak splits into two spin dependent peaks,
allowing focussing to be used to measure spin polarisation and the strength of
the spin-orbit interaction. In hole systems, the k^3 dependence of the Rashba
spin-orbit term allows the spatial separation of spins to be changed in-situ
using a voltage applied to an overall top gate. Here we demonstrate that this
can be used to control the splitting of the magnetic focussing peaks.
Additionally, we compare the focussing peak splitting to that predicted by
Shubnikov-de Haas oscillations and k.p bandstructure calculations. We find that
the focussing peak splitting is consistently larger than expected, suggesting
further work is needed on understanding spin dependent magnetic focussing
Surface shape resonances in lamellar metallic gratings
The specular reflectivity of lamellar gratings of gold with grooves 0.5
microns wide separated by a distance of 3.5 microns was measured on the 2000
cm - 7000 cm spectral range for p-polarized light. For the first
time, experimental evidence of the excitation of electromagnetic surface shape
resonances for optical frequencies is given. In these resonances the electric
field is highly localized inside the grooves and is almost zero in all other
regions. For grooves of depth equal to 0.6 microns, we have analyzed one of
these modes whose wavelength (3.3 microns) is much greater than the lateral
dimension of the grooves.Comment: 4 pages (LaTex), 5 postscript figures, to be published in Physical
Review Letter
Spin polarisation and spin dependent scattering of holes in transverse magnetic focussing
In 2D systems with a spin-orbit interaction, magnetic focussing can be used
to create a spatial separation of particles with different spin. Here we
measure hole magnetic focussing for two different magnitudes of the Rashba
spin-orbit interaction. We find that when the Rashba spin-orbit magnitude is
large there is significant attenuation of one of the focussing peaks, which is
conventionally associated with a change in the spin polarisation. We instead
show that in hole systems with a spin-orbit interaction, this peak
suppression is due to a change in the scattering of one spin state, not a
change in spin polarisation. We also show that the change in scattering length
extracted from magnetic focussing is consistent with results obtained from
measurements of Shubnikov-de Haas oscillations. This result suggests that
scattering must be considered when relating focussing peak amplitude to spin
polarisation in hole system
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