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arXiv:1006.0917 · Physical Review A

Measurement of the atom number distribution in an optical tweezer using single-photon counting

Andreas Fuhrmanek, Yvan R. P. Sortais, Philippe Grangier, Antoine Browaeys

Published Aug 30, 2010• 24 citations
arXivHTMLPDF

Abstract

We demonstrate in this paper a method to reconstruct the atom number distribution of a cloud containing a few tens of cold atoms. The atoms are first loaded from a magneto-optical trap into a microscopic optical dipole trap and then released in a resonant light probe where they undergo a Brownian motion and scatter photons. We count the number of photon events detected on an image intensifier. Using the response of our detection system to a single atom as a calibration, we extract the atom number distribution when the trap is loaded with more than one atom. The atom number distribution is found to be compatible with a Poisson distribution.

DOI
10.1103/physreva.82.023623

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We demonstrate in this paper a method to reconstruct the atom number distribution of a cloud containing a few tens of cold atoms. The atoms are first loaded from a magneto-optical trap into a microscopic optical dipole trap and then released in a resonant light probe where they undergo a Brownian motion and scatter photons. We count the number of photon events detected on an image intensifier. Using the response of our detection system to a single atom as a calibration, we extract the atom number distribution when the trap is loaded with more than one atom. The atom number distribution is found to be compatible with a Poisson distribution.
We perform fluorescence imaging of a single 87 Rb atom after its release from an optical dipole trap. The time-of-flight expansion of the atomic spatial density distribution is observed by accumulating many single atom images. The position of the atom is revealed with a spatial resolution close to 1 μm by a single-photon event, induced by a short resonant probe. The expansion yields a measure of the temperature of a single atom, which is in very good agreement with the value obtained by an independent measurement based on a release-and-recapture method. The analysis presented in this paper provides a way of calibrating an imaging system useful for experimental studies involving a few atoms confined in a dipole trap.
We describe a simple experimental technique which allows us to store a small and deterministic number of neutral atoms in an optical dipole trap. The desired atom number is prepared in a magneto-optical trap overlapped with a single focused Nd:YAG laser beam. Dipole trap loading efficiency of 100% and storage times of about one minute have been achieved. We have also prepared atoms in a certain hyperfine state and demonstrated the feasibility of a state-selective detection via resonance fluorescence at the level of a few neutral atoms. A spin relaxation time of the polarized sample of 4.2+/-0.7 s has been measured. Possible applications are briefly discussed.
This thesis covers experimental work conducted within two distinct fields of cold-atom physics. The first part describes an experiment capable of spatially-selective dispersive measurements of a cold-atomic system in part or in whole, by virtue of a digital micromirror device (DMD). Atom clouds are trapped in optical tweezers made and controlled by an acousto-optical deflector. Two types of high-precision magnetometers are realised. One is vectorial and reaches a single-shot precision of dB = (100, 200) µG for the field components parallel and transversal to the probe light, exceeding a previous realisation by two orders of magnitude. The other is a scalar magnetometer based on Larmor precession and reaches dB = 30 µG, putting it on equal footing with other state-of-the-art cold-atom magnetometers. The second part is devoted to the characterisation and the first results of a new quantum gas microscope experiment. It features a 0.69 NA microscope objective for high-resolution fluorescence imaging of individual atoms trapped in deep optical lattices. The imaging system is shown to perform near the diffraction limit. By means of DMD-generated off-resonant tight optical tweezers, projected through the high-resolution optics, we can load only a few planes of a co-propagating 1D lattice. In a single realisation of the experiment we acquire multiple fluorescence images, where the objective is translated between images, bringing different planes of the optical lattices in focus. In this way we can tomographically reconstruct the atom distribution in 3D.
We investigate experimentally the number statistics of a mesoscopic ensemble of cold atoms in a microscopic dipole trap loaded from a magneto-optical trap and find that the atom-number fluctuations are reduced with respect to a Poisson distribution due to light-assisted two-body collisions. For numbers of atoms $N\ensuremath{\gtrsim}2$, we measure a reduction factor (Fano factor) of $0.72\ifmmode\pm\else\textpm\fi{}0.07$, which differs from 1 by more than four standard deviations. We analyze this fact by a general stochastic model describing the competition between the loading of the trap from a reservoir of cold atoms and multiatom losses, leading to a master equation. Applied to our experimental regime, this model indicates an asymptotic value of 3/43/43/4 for the Fano factor at large NNN and in the steady state. We thus show that we have reached the ultimate level of reduction in number fluctuations in our system.
The number of cold atoms trapped in magneto-optical trap(MOT) can be derived from probe absorption signal. After the MOT is switched off, the cold cloud freely expands in gravitational field with an initial velocity distribution. So the measurement process can be described with the interaction model between the individual stationary two-level atom and single mode resonate laser. Absorption method can largely improved the measurement precision. In this paper we introduce the physical principle of measurement and demonstrate the whole experimental procedure in detail. The number of atoms trapped in our MOT, (8±0.3)×106 is measured in experiment. And the number of steady-state atoms and the lifetime of atom trapped in MOT was used to deduce the capture rate of MOT.
Based on the real sense of the time-of-flight , we demonstrate an alternative method of measuring the temperature of cold atoms in magneto-optical traps (MOTs) using a high-finesse optical microcavity, which acts as a pointlike single-atom counter. A cloud of atoms trapped in magneto-optical traps is positioned about 5 mm above the cavity, and the atoms fall freely down through the cavity. The temperature of the cold atoms in the MOT is determined by counting the exact arrival times of the single atoms. A theoretical model based on a ballistic expansion of a cloud of trapped atoms falling in the earth’s gravitational field is used to fit the probability distribution of atom arrivals, and the fittings agree very well with the experimental results. This method could be used for systems with little room, where an extra probe beam is hard to involve, or with fewer atoms initially.