Anomalous Behavior of Dark States in Quantum Gases of 6Li

We create atom-molecule dark states in a degenerate Fermi gas of 6Li in both weakly and strongly interacting regimes using two-photon Raman scattering to couple fermion pairs to bound molecular states in the ground singlet and triplet potential. Near the unitarity point in the BEC-BCS crossover regime, the atom number revival height associated with the dark state abruptly and unexpectedly decreases and remains low for magnetic fields below the Feshbach resonance center at 832.2~G. With a weakly interacting Fermi gas at 0~G we perform precision dark-state spectroscopy of the least-bound vibrational levels of the lowest singlet and triplet potentials. From these spectra, we obtain binding energies of the $v''=9$, $N''=0$ level of the $a(1^3\Sigma_{u}^+)$ potential and the $v''=38$, $N''=0$ level of the $X(1^1\Sigma_{g}^+)$ potential with absolute uncertainty as low as $20$ kHz. For the triplet potential we resolve the molecular hyperfine structure.