Indistinguishable fermions don’t typically interact much. Because their wavefunctions are antisymmetric, they tend to stay away from one another. That behavior manifests with particular strength in quantum gases cooled to ultralow temperatures. Bosons, on the other hand, can occupy the same energy state (see the left side in the figure), which leads to physical proximity and much more interaction. What new physics would emerge if fermions could act more like bosons?
Alkaline-earth fermionic atoms can do just that. Because their nuclear spin states are decoupled from their electronic states, atoms with different nuclear spin states have identical energy levels and wavefunctions in an optical trap. But the atoms aren’t identical, so fermionic atoms, each with a different spin, can cluster together in the same energy level and interact (right side of the figure). Researchers are interested in interacting fermions as a model to explore a range of condensed-matter systems—for example, cuprates and other transition-metal oxides that display high-temperature superconductivity.
Now the University of Colorado Boulder’s Jun Ye and his colleagues have created a gas with a record-high number of fermions in each energy level. Their ultracold strontium-87 gas with 10 distinct nuclear spin states shows clear signs of interatom interactions in its thermodynamics.
To prepare their gas, the researchers used two stages of laser cooling down to 2 μK and then introduced a single dimple trap for the atoms to pool into. To finish the cooling process, they left the gas to evaporate for as short as 0.6 s, down from about 10 s for two-spin gases. The more fermions that are colliding in each energy level, the faster they cool.
The researchers prepared Sr atoms with all 10 possible nuclear spin states—that is, 10 atoms per energy level—and 5 × 104 atoms per spin state by the end of evaporation. Although the fermion interactions are weak, they measurably change the system’s density fluctuations, compressibility, and time-of-flight dynamics, in agreement with theoretical models.
Now that researchers have an efficient preparation method and a basic understanding of the properties of interacting Fermi gases, says Ye, the gases will be “premium fuel for a quantum simulator.” Different combinations of kinetic energy, interaction energy, and nuclear spins can be used to systematically explore the phase diagram of, for example, the Fermi–Hubbard model (see the article by Gabriel Kotliar and Dieter Vollhardt, Physics Today, March 2004, page 53), whose phase diagram is still unknown despite its common use to describe strongly correlated materials. (L. Sonderhouse et al., Nat. Phys., 2020, doi:10.1038/s41567-020-0986-6; thumbnail photo credit: Christian Sanner.)
