Our group created the world's first ultracold Fermi gas of ⁴⁰K atoms in 1999; this accomplishment was selected as one of the top 10 scientific breakthroughs of that year by Science magazine. To create the quantum gas, we extended the magnetic trapping and evaporative cooling techniques used to make Bose-Einstein condensates (BECs). In so doing, we had to solve the problem of cooling a spin-polarized gas of fermions that wouldn't collide at low temperatures. (Without collisions, evaporative cooling cannot progress.) We defeated this problem by trapping two spin states of ⁴⁰K in the f=9/2 hyperfine ground state. Even though fermionic atoms in the same spin state cannot collide at ultralow temperatures, atoms in different spin states will.

Methods

Our initial experimental procedure is shown on the right. First we amass atoms from a room-temperature vapor (1) and cool them with a magneto-optical trap (MOT). Atoms in the mf=9/2 and mf=7/2 Zeeman levels in the f=9/2 ground state are then loaded into a magnetic trap that provides a harmonic-trapping potential. In the harmonic trap (2) we cool the atoms to below 100 nK by ejecting the highest energy atoms from the trap via a microwave transition between hyperfine ground states. As the highest atoms are forced to leave, the remaining atoms rethermalize to a lower temperature. For experiments using the Feshbach resonance, the gas is transferred from the magnetic trap to a far-detuned optical trap where the final evaporation takes place.

Ultracold Fermions

At ultralow temperature, the Pauli Exclusion Principle comes into play. Essentially, it states that identical fermions cannot occupy the same quantum state. In our experiment, this forces the atoms to stack up in the energy levels of the harmonic trap, much like the electrons in an atom stack up in orbitals. Near absolute zero, the atoms stack up to the Fermi Energy, E_F, as shown at left. E_F also defines the Fermi temperature. Below this temperature, the laws of quantum mechanics start to dominate the properties of the gas.

Information about the Fermi gas is extracted from shadow images (3) obtained after the magnetic trap is turned off and the Fermi gas is allowed to expand. We measure energy, temperature, number, and momentum distribution from these images.

Quantum Behavior

We observe the quantum behavior of the gas via measurements of the thermodynamics and collisional dynamics. Thermodynamically, the quantum gas has excess energy compared to a classical gas because the Pauli Exclusion Principle prohibits the atoms from all falling into the lowest energy state in the trap. It also affects collisions in the quantum regime. Collisions that would result in an atom moving into a low-energy state in the trap are prevented since those states are already occupied.

Molecular Condensation

We create a molecular Bose-Einstein condensate in an ultracold degenerate Fermi gas of atoms via a magnetic sweep across a Feshbach resonance. If the initial atom gas is at a sufficiently low temperature compared to the Fermi temperature, T_F, we observe a molecular condensate. The slow sweep of the applied magnetic field converts most of the fermionic atoms into bosonic molecules, which condense merely by traversing the BCS-BEC crossover regime. The molecules are not actively cooled.

Condensation with Correlated Atom Pairs

To observe condensation of fermionic atom pairs in the BCS-BEC crossover regime, we first evaporatively cool a trapped gas of fermionic ⁴⁰K atoms to quantum degeneracy. Then we use a magnetic-field Feshbach resonance to control the atom-atom interactions. We detect condensation of fermionic atom pairs near and on both sides of the Feshbach resonance, in the region of the BCS-BEC crossover. Here, many-body effects give rise to a condensation of fermion pairs.