Anomalous Behavior of Dark States in Quantum Gases of 6Li

TL;DR

This paper investigates the behavior of dark states in a quantum gas of 6Li, revealing unexpected anomalies near the unitarity point and providing precise spectroscopic data on molecular energy levels.

Contribution

It demonstrates anomalous dark state behavior in a degenerate Fermi gas and performs high-precision spectroscopy of molecular vibrational levels in 6Li.

Findings

Dark state revival height decreases near unitarity

Precise binding energies of specific vibrational levels obtained

Resolved hyperfine structure of triplet potential

Abstract

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.