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Laser Spectroscopy

ULTRACOLD METASTABLE HELIUM-4 AND HELIUM-3 GASES

W. Vassen, T. Jeltes, J. M. MCNAMARA, A. S. Tychkov, W. Hogervorst, K. A. H. van Leeuwen, Valentina Krachmalnicoff, M. Schellekens, A. Perrin, H. Chang, Denis Boiron, Alain Aspect, Christoph Ira Westbrook

Published Apr 1, 2008• 1 citation
Publisher

Abstract

We discuss our work to obtain a condensate containing more than 107 atoms and the first degenerate Fermi gas in a metastable state. Sympathetic cooling with Helium-4 is used to cool 106 Helium-3 atoms to a temperature T/TF < 0.5. The ultracold bosonic and fermionic gases have been used to observe the Hanbury Brown and Twiss effect for both isotopes, showing bunching for the bosons and antibunching for the fermions. A proposal for high resolution spectroscopy at 1.557 μm, connecting both metastable states directly, is discussed at the end.

DOI
10.1142/9789812813206_0021

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We discuss our work to obtain a condensate containing more than 107 atoms and the first degenerate Fermi gas in a metastable state. Sympathetic cooling with Helium-4 is used to cool 106 Helium-3 atoms to a temperature T/TF < 0.5. The ultracold bosonic and fermionic gases have been used to observe the Hanbury Brown and Twiss effect for both isotopes, showing bunching for the bosons and antibunching for the fermions. A proposal for high resolution spectroscopy at 1.557 μm, connecting both metastable states directly, is discussed at the end.
We report the observation of simultaneous quantum degeneracy in a dilute gaseous Bose-Fermi mixture of metastable atoms. Sympathetic cooling of helium-3 (fermion) by helium-4 (boson), both in the lowest triplet state, allows us to produce ensembles containing more than 10(6) atoms of each isotope at temperatures below 1 microK, and achieve a fermionic degeneracy parameter of T/TF = 0.45. Because of their high internal energy, the detection of individual metastable atoms with subnanosecond time resolution is possible, permitting the study of bosonic and fermionic quantum gases with unprecedented precision. This may lead to metastable helium becoming the mainstay of quantum atom optics.
We give an overview of the experiments at the Laser Centre of the Vrije Universiteit in Amsterdam on ultracold gases of metastable helium-4 and helium-3 as well as mixtures of both isotopes. We describe our experimental setup and discuss our experiments on Bose-Einstein condensation of 4He* (more than 10 million atoms in a BEC) and sympathetic cooling of 3He* towards Fermi degeneracy (more than 1 million atoms below the Fermi temperature as well as realization of a degenerate boson-fermion mixture). We also present our results on Penning Ionization in ~1 mK clouds containing a single isotope (homonuclear losses) or a mixture of both isotopes (heteronuclear losses).
We report on the achievement of quantum degeneracy in both components of a Bose-Fermi mixture of metastable helium atoms, 4He∗^{4}\mathrm{He}^{*}4He∗ and 3He∗^{3}\mathrm{He}^{*}3He∗. Degeneracy is achieved via Doppler cooling and forced evaporation for 4He∗^{4}\mathrm{He}^{*}4He∗, and sympathetically cooling 3He∗^{3}\mathrm{He}^{*}3He∗ with 4He∗^{4}\mathrm{He}^{*}4He∗. We discuss our modified implementation, along with the high versatility of our system. This technique is able to produce a degenerate Fermi gas with a minimum reduced temperature of T/TF=0.14(1)T/{T}_{F}=0.14(1)T/TF​=0.14(1), consisting of $2.5\ifmmode\times\else\texttimes\fi{}{10}^{4}$ 3He∗^{3}\mathrm{He}^{*}3He∗ atoms. Due to the high internal energy of both isotopes single atom detection is possible, opening the possibility of a large number of experiments into Bose-Fermi mixtures.
Summary form only given. The importance of helium atoms in the metastable state 2/sup 3/S/sub 1/ (He*, lifetime 8000 s) for studies of ultracold atomic clouds in the quantum degeneracy regime has been pointed out. For /sup 4/He* Bose-Einstein condensation (BEC) is expected, whereas the /sup 3/He isotope allows the study of a fermionic system, if the technique of sympathetic cooling is feasible. We plan to reach the BEC transition using forced evaporative cooling in our magnetostatic trap (MST) of cloverleaf geometry.
This thesis presents two experiments on ultracold gases of metastable Helium that constitute the extension, to matter waves, of experiments that have been fundamental for the development of quantum optics. Both experiments have been realized by using a single atom detector, that allows one to reconstruct the position of the atoms in three dimensions. In the first experiment we directly compared the two-body correlation function for a cold cloud of identical fermions and for a cold cloud of identical bosons at the same temperature issued from the same experimental apparatus. Since bosons and fermions obey different quantum statistics, the two-body correlation functions are different: identical bosons tend to be detected bunched in pairs, while identical fermions tend to antibunch because of the Pauli exclusion principle. In the second experiment we studied the correlation between pairs of bosonic atoms generated in the collision of two Bose-Einstein condensates. The measurement of the correlation function allows us to demonstrate that atoms flying back to back in the centre of mass frame are correlated in pairs. Furthermore the observation of a Hanbury Brown Twiss correlation between atoms with collinear velocity demonstrates that our system cannot be interpreted in terms of classical mechanics, but that the bosonic statistics plays a crucial role. A new version of the experiment will allow us to measure the angular dependence of the population of the scattering modes and the number difference squeezing between two correlated modes.
The study of low density, ultracold atomic Fermi gases is a promising avenue to understand fermion superfluidity from first principles. One technique currently used to bring Fermi gases in the degenerate regime is sympathetic cooling through a reservoir made of an ultracold Bose gas. We discuss a proposal for trapping and cooling of two-species Fermi-Bose mixtures into optical dipole traps made from combinations of laser beams having two different wavelengths. In these bichromatic traps it is possible, by a proper choice of the relative laser powers, to selectively trap the two species in such a way that fermions experience a stronger confinement than bosons. As a consequence, a deep Fermi degeneracy can be reached having at the same time a softer degenerate regime for the Bose gas. This leads to an increase in the sympathetic cooling efficiency and allows for higher precision thermometry of the Fermi - Bose mixture.