1,216 research outputs found
Unpolarized, incoherent repumping light for prevention of dark states in a trapped and laser-cooled single ion
Many ion species commonly used for laser-cooled ion trapping studies have a
low-lying metastable 2D3/2 state that can become populated due to spontaneous
emission from the 2P1/2 excited state. This requires a repumper laser to
maintain the ion in the Doppler cooling cycle. Typically the 2D3/2 state, or
some of its hyperfine components if the ion has nuclear spin, has a higher
multiplicity than the upper state of the repumping transition. This can lead to
dark states, which have to be destabilized by an external magnetic field or by
modulating the polarization of the repumper laser. We propose using
unpolarized, incoherent amplified spontaneous emission (ASE) to drive the
repumping transition. An ASE source offers several advantages compared to a
laser. It prevents the buildup of dark states without external polarization
modulation even in zero magnetic field, it can drive multiple hyperfine
transitions simultaneously, and it requires no frequency stabilization. These
features make it very compact and robust, which is essential for the
development of practical, transportable optical ion clocks. We construct a
theoretical model for the ASE radiation, including the possibility of the
source being partially polarized. Using 88Sr+ as an example, the performance of
the ASE source compared to a single-mode laser is analyzed by numerically
solving the eight-level density matrix equations for the involved energy
levels. Finally a reduced three-level system is derived, yielding a simple
formula for the excited state population and scattering rate, which can be used
to optimize the experimental parameters. The required ASE power spectral
density can be obtained with current technology
Dark-state suppression and optimization of laser cooling and fluorescence in a trapped alkaline-earth-metal single ion
We study the formation and destabilization of dark states in a single trapped
88Sr+ ion caused by the cooling and repumping laser fields required for Doppler
cooling and fluorescence detection of the ion. By numerically solving the
time-dependent density matrix equations for the eight-level system consisting
of the sublevels of the 5s 2S1/2, 5p 2P1/2, and 4d 2D3/2 states, we analyze the
different types of dark states and how to prevent them in order to maximize the
scattering rate, which is crucial for both the cooling and the detection of the
ion. The influence of the laser linewidths and ion motion on the scattering
rate and the dark resonances is studied. The calculations are then compared
with experimental results obtained with an endcap ion trap system located at
the National Research Council of Canada and found to be in good agreement. The
results are applicable also to other alkaline earth ions and isotopes without
hyperfine structure
Goal-oriented sensitivity analysis for lattice kinetic Monte Carlo simulations
In this paper we propose a new class of coupling methods for the sensitivity
analysis of high dimensional stochastic systems and in particular for lattice
Kinetic Monte Carlo. Sensitivity analysis for stochastic systems is typically
based on approximating continuous derivatives with respect to model parameters
by the mean value of samples from a finite difference scheme. Instead of using
independent samples the proposed algorithm reduces the variance of the
estimator by developing a strongly correlated-"coupled"- stochastic process for
both the perturbed and unperturbed stochastic processes, defined in a common
state space. The novelty of our construction is that the new coupled process
depends on the targeted observables, e.g. coverage, Hamiltonian, spatial
correlations, surface roughness, etc., hence we refer to the proposed method as
em goal-oriented sensitivity analysis. In particular, the rates of the coupled
Continuous Time Markov Chain are obtained as solutions to a goal-oriented
optimization problem, depending on the observable of interest, by considering
the minimization functional of the corresponding variance. We show that this
functional can be used as a diagnostic tool for the design and evaluation of
different classes of couplings. Furthermore the resulting KMC sensitivity
algorithm has an easy implementation that is based on the Bortz-Kalos-Lebowitz
algorithm's philosophy, where here events are divided in classes depending on
level sets of the observable of interest. Finally, we demonstrate in several
examples including adsorption, desorption and diffusion Kinetic Monte Carlo
that for the same confidence interval and observable, the proposed
goal-oriented algorithm can be two orders of magnitude faster than existing
coupling algorithms for spatial KMC such as the Common Random Number approach
Broadband lasers for photo-ionization and repumping of trapped ions
A frequency-stable, broadband laser is presented for experiments on trapped
ions. Since the design is based on widely available semiconductor optical
amplifier technology, similar lasers can be realized for virtually any
wavelength in the near-infrared, and the coherence properties and output power
allow for efficient second harmonic generation. No closed-loop frequency
stabilization for addressing Doppler- or naturally-broadened, dipole-allowed
transitions is needed, and the light source can be turned on and off during a
measurement cycle with sub-microsecond response time. As a case study, a
921.7-nm laser with an output power of 20mW and a linewidth of 10GHz is
realized, which is then frequency doubled to 460.9nm for excitation of
strontium as the first step in photo-ionization. The excitation efficiency is
compared to that achievable with a narrow-linewidth distributed Bragg reflector
laser as well as to theory.Comment: JOSA B accepted manuscript (9 pages, 5 figures
Broadband, unpolarized repumping and clearout light sources for Sr single-ion clocks
Future transportable optical clocks require compact and reliable light
sources. Here, broadband, unpolarized repumper and state clearout sources for
Sr+ single-ion optical clocks are reported. These turn-key devices require no
frequency stabilization nor external modulators. They are fiber based,
inexpensive, and compact. Key characteristics for clock operation are
presented, including optical spectra, induced light shifts and required
extinction ratios. Tests with an operating single-ion standard show a clearout
efficiency of 100%. Compared to a laser-based repumper, the achievable
fluorescence rates for ion detection are a few tens of per cent lower. The
resulting ion kinetic temperature is 1--1.5 mK, near the Doppler limit of the
ion system. Similar repumper light sources could be made for Ca+ (866 nm) and
Ba+ (650 nm) using semiconductor gain media.Comment: 4 pages, 6 figure
Trapping atoms on a transparent permanent-magnet atom chip
We describe experiments on trapping of atoms in microscopic magneto-optical
traps on an optically transparent permanent-magnet atom chip. The chip is made
of magnetically hard ferrite-garnet material deposited on a dielectric
substrate. The confining magnetic fields are produced by miniature magnetized
patterns recorded in the film by magneto-optical techniques. We trap Rb atoms
on these structures by applying three crossed pairs of counter-propagating
laser beams in the conventional magneto-optical trapping (MOT) geometry. We
demonstrate the flexibility of the concept in creation and in-situ modification
of the trapping geometries through several experiments.Comment: Modifications: A) Reference I. Barb et al., Eur. Phys. JD, 35, 75
(2005) added. B)Sentence rewritten: We routinely capture more than 10^6 atoms
in a micro-MOT on a magnetized pattern. C) The magnetic field strengths are
now given in Teslas. D) The second sentence in the fourth paragraph has been
rewritten in order to more clearly describe the geometry and purpose of the
compensation coils.E) In the seventh paragraph we have rewritten the sentence
about the creation of the external magnetic field for the magnetic-domain
patterning. F) In the ninth paragraph, we clarify the way to shift the trap
center. G) Caption of Fig. 4 changed. H) We have modified paragraph 12 to
improve the description on the guiding of the trap center along a toroidal
pattern. I) The last two sentences of the manuscript have been rewritte
Relational reasoning via probabilistic coupling
Probabilistic coupling is a powerful tool for analyzing pairs of
probabilistic processes. Roughly, coupling two processes requires finding an
appropriate witness process that models both processes in the same probability
space. Couplings are powerful tools proving properties about the relation
between two processes, include reasoning about convergence of distributions and
stochastic dominance---a probabilistic version of a monotonicity property.
While the mathematical definition of coupling looks rather complex and
cumbersome to manipulate, we show that the relational program logic pRHL---the
logic underlying the EasyCrypt cryptographic proof assistant---already
internalizes a generalization of probabilistic coupling. With this insight,
constructing couplings is no harder than constructing logical proofs. We
demonstrate how to express and verify classic examples of couplings in pRHL,
and we mechanically verify several couplings in EasyCrypt
Surface ruptures on cross-faults in the 24 November 1987 Superstition Hills, California, earthquake sequence
Left-lateral slip occurred on individual surface breaks along northeast-trending faults associated with the 24 November 1987 earthquake sequence in the Superstition Hills, Imperial Valley, California. This sequence included the M_s = 6.2 event on a left-lateral, northeast-trending “cross-fault” between the Superstition Hills fault (SHF) and Brawley seismic zone, which was spatially associated with the left-lateral surface breaks. Six distinct subparallel cross-faults broke at the surface, with rupture lengths ranging from about Formula to 10 km and maximum displacements ranging from 30 to 130 mm. About half a day after the M_s = 6.2 event, an M_s = 6.6 earthquake nucleated near the intersection of the cross-faults with the SHF, and rupture propagated southeast along the SHF. Whereas right-lateral slip on the SHF occurred dominantly on a single trace in a narrow zone, the cross-fault surface slip was distributed over several stands across a 10-km-wide zone. Also, whereas afterslip accounted for a large proportion of total slip on the SHF, there is no evidence for afterslip on the cross-faults. We present documentation of these surface ruptures. A simple mechanical model of faulting illustrates how the foreshock sequence may have triggered the main rupture. Displacement on other cross-faults could trigger an event on the southern San Andreas fault by a similar mechanism in the future
- …