Double-degenerate Bose-Fermi mixture of Strontium

Double-degenerate Bose-Fermi mixture of Strontium

Fermionic atoms with two valence electrons, as ⁸⁷Sr, ¹⁷¹Yb, and ¹⁷³Yb, have a ground state that possesses only the nuclear magnetic moment. ⁸⁷Sr has a nuclear spin of 9/2, which is the largest of these. This nuclear spin is at the heart of proposals for quantum computation and simulation. It allows the protected storage of quantum information, which can then be manipulated using the rich electronic structure of those two-electron elements. It enables also quantum simulations that are highly interesting but impossible to perform using alkali atoms, as the simulation of SU(10) symmetric lattice spin models. For all these applications, quantum degenerate samples prepared in a well defined initial state are required.

We have cooled a spin-polarized sample of ⁸⁷Sr atoms into quantum degeneracy. Since identical fermions do not collide, thermalization is obtained through interisotope collisions with bosonic ⁸⁴Sr. We developed a new and very simple scheme for the detection of spin-polarization of the fermions. Fermionic degeneracy manifests itself in subtle changes in the momentum distribution of atoms. Pauli blocking limits the occupation of low momentum states to one fermion per state, leading to a flatter and wider momentum distribution than expected classically. This change is evident in the azimuthally averaged time-of-flight distribution show in figure 1.

Figure 1: Azimuthally averaged density distributions of a degenerate Fermi gas at T/TF = 0.28 (left) and a thermal fermionic cloud at T/TF = 0.95 (right) after 10 ms of free expansion. Fermi-Dirac distributions (solid red line) fit both measurements well. For low T/TF, the Fermi-Dirac fit deviates from both a Gaussian fit (dotted blue line), and a Gaussian fit to the wing of the distribution (dashed grey line). For T/TF = 0.95, the difference between the three fits is barely visible.

We achieve a degeneracy of T/TF = 0.30(5) with 2 x 10^{4 87}Sr atoms at the end of the evaporation. The degenerate Fermi gas is accompanied by an almost pure ⁸⁴Sr BEC of 10⁵ atoms. Figure 2 illustrates the difference in the density distribution of a BEC and a Fermi gas in thermal equilibrium after free expansion.

Figure 2: Time-of-flight absorption images of a nearly pure ⁸⁴Sr BEC and a ⁸⁷Sr Fermi gas at T/TF of 0.45 after 15 ms expansion. The BEC expansion is mean field driven and leads to an inverted aspect ratio. The momentum distribution of the fermionic sample is much broader than the one of the bosonic sample because of the Pauli exclusion principle forcing the fermions to higher momentum states.

The Team

Meng Khoon Tey, Simon Stellmer, Rudolf Grimm and Florian Schreck

References

• Double-degenerate Bose-Fermi mixture of strontium: Phys. Rev. A 82, 011608(R) (2010), arXiv:1006.1131