Optical Control of Magnetic Feshbach Resonances by Closed-Channel Electromagnetically Induced Transparency.

We control magnetic Feshbach resonances in an optically trapped mixture of the two lowest hyperfine states of a ^{6}Li Fermi gas, using two optical fields to create a dark state in the closed molecular channel. In the experiments, the narrow Feshbach resonance is tuned by up to 3 G. For the broad resonance, the spontaneous lifetime is increased to 0.4 s at the dark-state resonance, compared to 0.5 ms for single-field tuning. We present a new model of light-induced loss spectra, employing continuum-dressed basis states, which agrees in shape and magnitude with loss measurements for both broad and narrow resonances. Using this model, we predict the trade-off between tunability and loss for the broad resonance in ^{6}Li, showing that our two-field method substantially reduces the two-body loss rate compared to single-field methods for the same tuning range.