Degenerate Fermi gases (DFGs) are typically achieved under similar conditions as BEC. The main difference is Fermi gases can only be cooled to low temperature if two different “kinds of atoms” (different elements, different isotopes or different internal states) are simultaneously trapped. The typical shape of a degenerate Fermi gas is significantly different from the BEC shape and so are the physical properties. Unlike Bosons identical Fermions cannot occupy a quantum state with more than one particle. So there is no macroscopic occupation of the lowest energy state with fermions. While BECs are not observed in nature, degenerate Fermi gases of non-atomic particles are present in neutron stars or in metals. Thanks to atom cooling results neutral atoms can now be studied as model systems for these systems or degenerate Fermi gases in general.
Bose-Einstein condensates or degenerate Fermi gases were realized with the following atomic elements sorted by atomic number with year of first production: (feel free to inform us about missing species and win a TOPTICA cup)
H (1998), He* (2001), Li (1995), Na (1995), K (2001), Ca (2009), Cr (2004), Rb (1995), Sr (2009), Cs (2002), Dy (2011), Er (2012) and Yb (2003).
Tuning of interactions gives not only a new twist to the physics of degenerate quantum gases but pushes the door open wide for many fundamental studies. Most of the BEC (or degenerate Fermi gases) species show only weak interaction between the atoms (“weakly interacting BEC”). This interaction is the standard “molecular” interaction which at low temperatures can be modeled as a hard sphere interaction. Two atoms behave as if they were “billiard” balls feeling the presence of each other only if they approach very closely (typ. 10-100 nm). This short range interaction, called contact interaction, is isotropic and the dominant interaction in BECs. Close to a “Feshbach resonance”, where a molecular bound state is magnetically shifted to the energy of the two free atoms, things can change dramatically. With tiny changes in the magnetic field, one can control the strength and the sign of this interaction, e.g. have strong repulsive, zero or even attractive interaction. Using this Feshbach resonances, one could observe interaction induced collapse of BECs, dipolar BECs and transition between molecular BECs to “High Tc super conducting” degenerate Fermi gases.
Molecular BECs so far have been achieved starting from atomic BECs or DFGs gases. Direct trapping of molecules and cooling them down to sub-µK temperatures has not been achieved so far. Having an optically trapped atomic BEC or DFG, one can coherently transform pairs of unbound atoms into a vibrationally highly excited molecules by sweeping the magnetic field over a Feshbach resonance. Applying laser pulses, one can then de-excite the molecules until they are in the lowest energy state. Especially interesting are diatomic molecules with a large electric dipole moment confined in optical lattices. They allow one to study or simulate unique quantum phases like super solid or checker board.
Degenerate quantum gases in optical lattices can be used to investigate solid state physics. While the atoms play the role of electrons, the optical lattice takes the part of the periodic potentials in solid states stemming from the ions. The advantages of the degenerate quantum gases are that one can tune the interaction between the particles and that one can generate very clean potentials of many different kinds and strengths. This way, one can study theoretically predicted phenomena (e.g. transition between superfluid and Mott-Insulator, Anderson localization, High Tc super conductivity) or even “quantum simulate” solid state problems which do not have an analytical solution and cannot be treated numerically.
