10,657 research outputs found
Dynamics of atomic spin-orbit-state wave packets produced by short-pulse laser photodetachment
We analyse the experiment by Hultgren et al. [Phys. Rev. A {\bf 87}, 031404
(2013)] on orbital alignment and quantum beats in coherently excited atomic
fine-structure manifolds produced by short-pulse laser photodetachment of
C, Si and Ge negative ions, and derive a formula that describes the
beats. Analysis of the experimental data enables us to extract the non-coherent
background contribution for each species, and indicates the need for a full
density matrix treatment of the problem
Comment on "Direct photodetachment of F by mid-infrared few-cycle femtosecond laser pulses"
Multiphoton detachment of F by strong few-cycle laser pulses was studied
by Shearer and Monteith using a Keldysh-type approach [Phys. Rev. A 88, 033415
(2013)]. We believe that this work contained errors in the calculation of the
detachment amplitude and photoelectron spectra. We describe the necessary
corrections to the theory and show that the results, in particular, the
interference features of the photoelectron spectra, appear noticeably
different.Comment: 9 pages, 4 figure
Stability of Filters for the Navier-Stokes Equation
Data assimilation methodologies are designed to incorporate noisy
observations of a physical system into an underlying model in order to infer
the properties of the state of the system. Filters refer to a class of data
assimilation algorithms designed to update the estimation of the state in a
on-line fashion, as data is acquired sequentially. For linear problems subject
to Gaussian noise filtering can be performed exactly using the Kalman filter.
For nonlinear systems it can be approximated in a systematic way by particle
filters. However in high dimensions these particle filtering methods can break
down. Hence, for the large nonlinear systems arising in applications such as
weather forecasting, various ad hoc filters are used, mostly based on making
Gaussian approximations. The purpose of this work is to study the properties of
these ad hoc filters, working in the context of the 2D incompressible
Navier-Stokes equation. By working in this infinite dimensional setting we
provide an analysis which is useful for understanding high dimensional
filtering, and is robust to mesh-refinement. We describe theoretical results
showing that, in the small observational noise limit, the filters can be tuned
to accurately track the signal itself (filter stability), provided the system
is observed in a sufficiently large low dimensional space; roughly speaking
this space should be large enough to contain the unstable modes of the
linearized dynamics. Numerical results are given which illustrate the theory.
In a simplified scenario we also derive, and study numerically, a stochastic
PDE which determines filter stability in the limit of frequent observations,
subject to large observational noise. The positive results herein concerning
filter stability complement recent numerical studies which demonstrate that the
ad hoc filters perform poorly in reproducing statistical variation about the
true signal
Optimizing photon indistinguishability in the emission from incoherently-excited semiconductor quantum dots
Most optical quantum devices require deterministic single-photon emitters.
Schemes so far demonstrated in the solid state imply an energy relaxation which
tends to spoil the coherent nature of the time evolution, and with it the
photon indistinguishability. We focus our theoretical investigation on
semiconductor quantum dots embedded in microcavities. Simple and general
relations are identified between the photon indistinguishability and the
collection efficiency. The identification of the key parameters and of their
interplay provides clear indications for the device optimization
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