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Atomic gas mixtures at absolute zero
Over the last decade, the field of cold degenerate gases has been one of the most active areas in physics. From the production of Bose-Einstein Condensates (BEC) in 1995, to the recent demonstration of superfluidity in a strongly interacting mixture of Degenerate Fermi Gases (DFG), the research has progressed to increasingly sophisticated and complex systems. This interest is driven by the desire to understand strongly interacting and strongly correlated systems, with applications in solid-state physics, nuclear physics, astrophysics, quantum computing, and nanotechnologies.
Image of a cold Rubidium cloud taken with a CCD camera.
Precise control of inter-particle interactions and correlations is crucial for such investigations. With cold atomic and molecular gases, such control is possible by manipulation of external magnetic (Feshbach resonances) and laser fields (optical lattices). Feshbach resonances have enabled the investigation of homonuclear molecular BEC, and the study of the BEC-BCS crossover in mixtures of DFG. Control of optical lattices has permitted the observation of the superfluid to Mott-insulator phase transition. For the past decade tools from atomic and molecular physics, have been used to produce and manipulate cold quantum degenerate atomic gases. Today, the combination of these methods with quantum degenerate mixtures paves the way towards new physics of cold degenerate molecular gases.
This project investigates the interaction in a mixed sample of rubidium and potassium atoms. It will contribute to the understanding of a large family of atomic and molecular systems, with exciting prospects for future experiments. It paves a clear path towards ultra-cold chemistry and, one can envision the use of cold molecules for molecular optics, molecular interferometry and a molecular quantum computer.
The experiment
To realize these goals we cool two atomic species to temperatures close to absolute zero: Rubidium-87 (Boson) and K-40 (Fermion). Close to absolute zero, at a few hundred Nanokelvin, the Rubidium atoms form a Bose-Einstein-condensate (BEC) and the Potassium atoms become a quantum-degenerate Fermi gas (DFG). The size of the condensate is about a tenth millimeter and we image the atoms with a CCD camera. Figure 1 shows how a Bose-Einstein-Condensate emerges from a thermal cloud for three different temperatures. (indicated here by the final radiofrequency of the evaporation ramp).
Fig. 1: Emergence of a BEC from a thermal cloud. The frequencies indicate the final RF frequency of the evaporation ramp.
We produce Bose-Einstein condensates with 7*105 Rb87 atoms and a degenerate Fermi-sea containing about 5*105 K40 atoms. The temperature of the mixture is less than 200 nK.
Fig. 2: Images of the atomic clouds. A Rb Bose-Einstein-condensate is shown on the left and a K Fermi gas on the right.
A comparision of a fit with gaussian distribution and a with a Thomas-Fermi distribution of the cold K40 cloud on the right indicates the quantum degeneracy:
Fig. 3: Fit of a K DFG indicating a final temperature of T/TF = 0.26 (black curve). A Gaussian fit (red curve) clearly performs less well.
Experimental goals
The main goals of our experiment are
- to povide a next generation apparatus to produce ultracold gas mixtures of the Rb87 (Boson) and the K40 (Fermion) and K41 (Boson).
- to develop methods for the production of ultracold heteronuclear molecules
- to investigate molecules at ultralow temperatures