Atomic Physics Seminars
Dr. Smair Bali , Duke University
[Host: Olivier Pfister]
A quantum degenerate sample of cold dilute fermions is expected to yield new exciting physics since Pauli's exclusion principle forbids congregation of fermions in the same quantum state. A fascinating possibility is the formation of Cooper pairs, analogous to the BCS phase transition responsible for superconductivity and for superfluidity in liquid He-3. Lithium-6, a stable and naturally abundant fermionic isotope, is an excellent candidate because it has large and attractive interatomic interactions, a necessary requirement for the superfluid transition. However, the lowest lying spin states of lithium-6 are not magnetically trappable. This precludes the use of a magnetic trap, the only kind of trap in which BEC has been achieved for bosonic atoms. The possibility of achieving quantum degeneracy, whether boson or fermion, in an optical trap has been a long sought goal and is of intense current interest. This is because, unlike their magnetic counterparts, optical traps can trap all spin states and offer the possibility of arbitrary control of interatomic interactions via external magnetic fields. However, owing to the presence of unexplained heating rates, optical traps have failed to be stable. We have identified some of the important heating mechanisms. By minimizing their effect we have constructed an ultrastable optical trap consisting of a focused far-detuned CO2 laser beam, in which we confine lithium-6 atoms with a life-time of 300 secs. This is nearly a two order of magnitude improvement in stability over all previous optical traps, rivalling that of magnetic traps. We simultaneously confine the two lowest lying spin states of lithium-6 thus enabling efficient evaporative cooling toward Fermi degeneracy and, possibly, the superfluid transition. Measurements of anomalously large elastic collision cross-sections and observation of evaporative cooling of lithium-6 will be presented.
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