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Physical Review Letters

Direct Measurement of The Spatial Correlation Function of Ultracold Atoms

Bruno Saubaméa, Tom W. Hijmans, S. A. Kulin, Ernst Maria Rasel, E. Peik, Michèle Leduc, Claude N. Cohen-Tannoudji

Published Oct 27, 1997• 52 citations
Publisher

Abstract

We demonstrate a new method to directly measure, from the overlap of two atomic wave packets, the spatial correlation function of metastable helium atoms cooled by velocity selective coherent population trapping. This approach allows us to determine precisely the effective temperature of the atoms and to make quantitative comparison with theory. Temperatures as low as $1/800$ of the recoil temperature are found. Moreover, we extend this method to realize a temporal interferometer with de Broglie matter waves in the subrecoil regime.

DOI
10.1103/physrevlett.79.3146

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We demonstrate a new method to directly measure, from the overlap of two atomic wave packets, the spatial correlation function of metastable helium atoms cooled by velocity selective coherent population trapping. This approach allows us to determine precisely the effective temperature of the atoms and to make quantitative comparison with theory. Temperatures as low as 1/8001/8001/800 of the recoil temperature are found. Moreover, we extend this method to realize a temporal interferometer with de Broglie matter waves in the subrecoil regime.
We present the first detailed investigation of the line shape of the momentum distribution of atoms cooled far below the recoil limit. This distribution is deduced from a direct measurement of the atomic spatial correlation function of metastable helium atoms cooled by velocity-selective coherent population trapping. The measured line shape is then compared to the prediction of an analytical model of this cooling based on L\'evy statistics. A very good agreement is found between experiment and theory and fundamental features such as self-similarity and nonergodicity are identified.
Trapped gases of ultracold metastable helium atoms are a unique experimental sys- tem. The trapped cloud’s low temperatures, the ability to selectively trap and ma- nipulate individual atomic states and the isolation provided in a vacuum chamber are all conducive to the measurement of precision atomic parameters, such as atomic transition rates. Due to it’s atomic structure helium - the simplest multi-electron atom - is a favourite test-bed for precision QED theoretical calculations compared to experimentally determined parameters. This thesis reports on a number of tran- sition rates measured for the first time with high accuracy, including the first excited metastable state lifetime, found to be 7870(510)s, the longest of any neutral atomic species yet measured. Transition rates from the 23P level to the ground state were also measured for the first time and found to be in excellent agreement with theory. Further cooling of helium produces a Bose-Einstein condensate (BEC), where all atoms are in an identical quantum state, the analogue of an optical laser. One par- ticularly interesting property a BEC posesses is long-range coherence to all orders, meaning all component particles are described by a single wavefunction. While first order coherence is measured in an interference experiment like Young’s double slit, higher order coherence is the ability for correlations to exist between groups of two, three or more particles, quantified by the measurement of correlation functions. Prior to the work in this thesis correlation functions for atoms had only been measured for second order. Here we develop a new technique which we use to show that a BEC is coherent to second and third order to 0.1% accuracy, compared to a thermal cloud which we show is incoherent. At small inter-particle spacings, there is an enhanced probability of measuring double and triple events in the thermal cloud (termed bunching). A drawback of all previous atomic correlation measurements is that the small de-Broglie wavelengths compared to the detector resolution leads to a significant reduction in the measured bunching amplitudes compared to theory. By altering our trapping configuration and temperature we are able to measure a two-particle bunching amplitude which is 88% of the theoretical maximum. This system also allowed the measurement of correlations up to 5th order in thermal gases. The near ideal bunching allowed us to demonstrate perturbations to the correlation functions due to the 1D nature of the system. These advances should lead to correlation measurements being used as a probe to characterise more exotic system properties such as low dimensionality, engineered quantum states and paticle pairing. Finally, a waveguide was created using a dipole laser beam and used to guide atoms in analogy to an optical laser in a fibre. Guided ‘atom lasers’ are likely to be important components for practical devices such as precision sensors. We demonstrate that a guided atom laser has a gaussian profile matching the lowest transverse mode of the waveguide and is first and second order coherent. In contrast guided thermal atoms show matter-wave speckle and are incoherent.
We use laser induced adiabatic passage to coherently manipulate the atomic wave packets resulting from subrecoil laser cooling in one, two, and three dimensions by velocity selective coherent population trapping. Using metastable helium atoms, we demonstrate adiabatic transfer into a single wave packet or into two coherent wave packets, while retaining the subrecoil momentum dispersion of the initial wave packets. The efficiency of the transfer is nearly perfect in one and two dimensions, and of the order of 75% in three dimensions. To a large extent, the direction of the momentum and the internal state of the atoms described by each of the final wave packets can be chosen at will.
We report the first observation of subrecoil laser cooling in two dimensions by velocity selective coherent population trapping. We have compressed the momentum distribution of a cloud of trapped metastable helium atoms along two orthogonal axes to $\ensuremath{\delta}p=\frac{\ensuremath{\hbar}k}{4}$, where $\ensuremath{\hbar}k$ is the photon momentum. The corresponding temperature, 16 times below the single-photon recoil temperature, is 250 nK. Atoms are pumped into a nonabsorbing state exhibiting four well-resolved momentum peaks with subrecoil widths. The momentum space density in the peaks is significantly greater than that in the uncooled distribution.
We report the first observation of subrecoil laser cooling in three dimensions, where we use the method of velocity-selective coherent population trapping. We have cooled a cloud of trapped metastable helium atoms to 180 nK, 22 times below the single-photon recoil temperature, with an enhancement in the momentum-space density of 16. We find that the efficiency with which the velocity-selective trap is populated can be augmented by Sisyphus-type frictional forces, while the cooling process is accompanied by the diffusion of atoms to states of very high velocity.
We present a new laser-cooling scheme based on velocity-selective optical pumping of atoms into a nonabsorbing coherent superposition of states. This method has allowed us to achieve transverse cooling of metastable 4He^{4}\mathrm{He}4He atoms to a temperature of 2 \ensuremath{\mu}K, lower than both the usual Doppler cooling limit (23 \ensuremath{\mu}K) and the one-photon recoil energy (4 \ensuremath{\mu}K). The corresponding de Broglie wavelength (1.4 \ensuremath{\mu}m) is larger than the atomic-transition optical wavelength.