2,316 research outputs found
Bose-Einstein condensation of metastable helium: some experimental aspects
We describe our recent realization of BEC using metastable helium. All
detection is done with a micruchannel plate which detects the metastables or
ions coming from the trapped atom cloud. This discussion emphasizes some of the
diagnostic experiments which were necessary to quantitatively analyse our
results.Comment: 5 pages, 3 figure
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Estimating drizzle drop size and precipitation rate using two-colour lidar measurements
A method to estimate the size and liquid water content of drizzle drops using lidar measurements at two wavelengths is described. The method exploits the differential absorption of infrared light by liquid water at 905 nm and 1.5 μm, which leads to a different backscatter cross section for water drops larger than ≈50 μm. The ratio of backscatter measured from drizzle samples below cloud base at these two wavelengths (the colour ratio) provides a measure of the median volume drop diameter D0. This is a strong effect: for D0=200 μm, a colour ratio of ≈6 dB is predicted. Once D0 is known, the measured backscatter at 905 nm can be used to calculate the liquid water content (LWC) and other moments of the drizzle drop distribution.
The method is applied to observations of drizzle falling from stratocumulus and stratus clouds. High resolution (32 s, 36 m) profiles of D0, LWC and precipitation rate R are derived. The main sources of error in the technique are the need to assume a value for the dispersion parameter μ in the drop size spectrum (leading to at most a 35% error in R) and the influence of aerosol returns on the retrieval (≈10% error in R for the cases considered here). Radar reflectivities are also computed from the lidar data, and compared to independent measurements from a colocated cloud radar, offering independent validation of the derived drop size distributions
Hanbury Brown and Twiss correlations in atoms scattered from colliding condensates
Low energy elastic scattering between clouds of Bose condensed atoms leads to
the well known s-wave halo with atoms emerging in all directions from the
collision zone. In this paper we discuss the emergence of Hanbury Brown and
Twiss coincidences between atoms scattered in nearly parallel directions. We
develop a simple model that explains the observations in terms of an
interference involving two pairs of atoms each associated with the elementary s
wave scattering process.Comment: Minor corrections. reference update
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The formation of ice in a long-lived supercooled layer cloud
This article focuses on the characteristics of persistent thin single-layer mixed-phase clouds. We seek to answer two important questions: (i) how does ice continually nucleate and precipitate from these clouds, without the available ice nuclei becoming depleted? (ii) how do the supercooled liquid droplets persist in spite of the net flux of water vapour to the growing ice crystals? These questions are answered quantitatively using in situ and radar observations of a long-lived mixed-phase cloud layer over the Chilbolton Observatory.
Doppler radar measurements show that the top 500 m of cloud (the top 250 m of which is mixed-phase, with ice virga beneath) is turbulent and well-mixed, and the liquid water content is adiabatic. This well-mixed layer is bounded above and below by stable layers. This inhibits entrainment of fresh ice nuclei into the cloud layer, yet our in situ and radar observations show that a steady flux of ≈100 m−2s−1 ice crystals fell from the cloud over the course of ∼1 day. Comparing this flux to the concentration of conventional ice nuclei expected to be present within the well-mixed layer, we find that these nuclei would be depleted within less than 1 h. We therefore argue that nucleation in these persistent supercooled clouds is strongly time-dependent in nature, with droplets freezing slowly over many hours, significantly longer than the few seconds residence time of an ice nucleus counter.
Once nucleated, the ice crystals are observed to grow primarily by vapour deposition, because of the low liquid water path (21 g m−2) yet vapour-rich environment. Evidence for this comes from high differential reflectivity in the radar observations, and in situ imaging of the crystals. The flux of vapour from liquid to ice is quantified from in situ measurements, and we show that this modest flux (3.3 g m−2h−1) can be readily offset by slow radiative cooling of the layer to space
Theory and observations of ice particle evolution in cirrus using Doppler radar: evidence for aggregation
Vertically pointing Doppler radar has been used to study the evolution of ice
particles as they sediment through a cirrus cloud. The measured Doppler fall
speeds, together with radar-derived estimates for the altitude of cloud top,
are used to estimate a characteristic fall time tc for the `average' ice
particle. The change in radar reflectivity Z is studied as a function of tc,
and is found to increase exponentially with fall time. We use the idea of
dynamically scaling particle size distributions to show that this behaviour
implies exponential growth of the average particle size, and argue that this
exponential growth is a signature of ice crystal aggregation.Comment: accepted to Geophysical Research Letter
Hanbury Brown Twiss effect for ultracold quantum gases
We have studied 2-body correlations of atoms in an expanding cloud above and
below the Bose-Einstein condensation threshold. The observed correlation
function for a thermal cloud shows a bunching behavior, while the correlation
is flat for a coherent sample. These quantum correlations are the atomic
analogue of the Hanbury Brown Twiss effect. We observe the effect in three
dimensions and study its dependence on cloud size.Comment: Figure 1 availabl
Fast production of Bose-Einstein condensates of metastable Helium
We report on the Bose-Einstein condensation of metastable Helium-4 atoms
using a hybrid approach, consisting of a magnetic quadrupole and a crossed
optical dipole trap. In our setup we cross the phase transition with 2x10^6
atoms, and we obtain pure condensates of 5x10^5 atoms in the optical trap. This
novel approach to cooling Helium-4 provides enhanced cycle stability, large
optical access to the atoms and results in production of a condensate every 6
seconds - a factor 3 faster than the state-of-the-art. This speed-up will
dramatically reduce the data acquisition time needed for the measurement of
many particle correlations, made possible by the ability of metastable Helium
to be detected individually
Thermal counting statistics in an atomic two-mode squeezed vacuum state
We measure the population distribution in one of the atomic twin beams
generated by four-wave mixing in an optical lattice.
Although the produced two-mode squeezed vacuum state is pure, each individual
mode is described as a statistical mixture.
We confirm the prediction that the particle number follows an exponential
distribution when only one spatio-temporal mode is selected.
We also show that this distribution accounts well for the contrast of an
atomic Hong--Ou--Mandel experiment.
These experiments constitute an important validation of our twin beam source
in view of a future test of a Bell inequalities.Comment: SciPost submissio
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