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arXiv:cond-mat/0606638 · Physical Review Letters

Degenerate Bose-Fermi Mixture of Metastable Atoms

J. M. McNamara, T. Jeltes, A. S. Tychkov, W. Hogervorst, W. Vassen

Published Aug 24, 2006• 99 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevlett.97.080404

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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 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.
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 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).
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.
We simultaneously confine fermionic metastable 3He^{3}\mathrm{H}\mathrm{e}3He atoms and bosonic metastable 4He^{4}\mathrm{H}\mathrm{e}4He atoms in a magneto-optical trap. The trapped clouds, containing up to $1.5\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms of each isotope, are characterized by measuring ions and metastable helium atoms escaping from the trap. Optical pumping of 3He^{3}\mathrm{H}\mathrm{e}3He atoms to a nontrapped hyperfine state is investigated and it is shown that large atom numbers can be confined without additional repumping lasers. Unique possibilities for quantum degeneracy experiments with mixtures of spin-polarized metastable 3He^{3}\mathrm{H}\mathrm{e}3He and 4He^{4}\mathrm{H}\mathrm{e}4He atoms are indicated.
This thesis presents studies of quantum degenerate atomic gases of fermionic 6Li and bosonic 7Li. Degeneracy is reached by evaporative cooling of 7Li in a strongly confining magnetic trap. Since at low temperatures direct evaporative cooling is not possible for a polarized fermionic gas, 6Li is sympathetically cooled by thermal contact with 7Li. In a first series of experiments both isotopes are trapped in their low-field seeking higher hyperfine states. A Fermi degeneracy of is achieved for 105 fermions. For more than ~300 atoms, the 7Li condensate collapses, due to the attractive interatomic interaction in this state. This limits the degeneracy reached for both species. To overcome this limit, in a second series of experiments 7Li and 6Li atoms are transferred to their low field seeking lower hyperfine states, where the boson-boson interaction is repulsive but weak. The inter-isotope collisions are used to thermalize the mixture. A 7Li Bose-Einstein condensate (BEC) of 104 atoms immersed in a Fermi sea is produced. The BEC is quasi-one-dimensional and the thermal fraction can be negligible. The measured degeneracies are . The temperature is measured using the bosonic thermal fraction, which vanishes at the lowest temperatures, limiting our measurement sensitivity. In a third series of experiments, the bosons are transferred into an optical trap and their internal state is changed to , the lowest energy state. A Feshbach resonance is detected and used to produce a BEC with tunable atomic interactions. When the effective interaction between atoms is tuned to be small and attractive, we observe the formation of a matter-wave bright soliton. Propagation of the soliton without spreading over a macroscopic distance of 1.1~mm is observed.