Many-body gap-protection of a strontium optical transition against Doppler dephasing via all-to-all exchange interactions mediated by a ring cavity*

We realize a new method to extend the coherence time of a collective optical dipole moment in strontium by engineering a collective recoil mechanism that suppresses dephasing due to Doppler decoherence. The mechanism is akin to but distinct from, Mossbauer spectroscopy. Such suppression was first observed recently in the context of a Bragg matterwave interferometer by applying dressing lasers to generate cavity-mediated momentum-exchange interactions [1].  Here, we realize a large all-to-all spin-exchange interaction by loading 0.9 million ⁸⁸Sr into a high finesse ring cavity that is detuned from resonance with the optical 7.5 kHz linewidth ¹S₀ to ³P₁ transition.  We observe that the coherence of a spin-wave, which is excited by driving the transition, is extended well beyond the Doppler dephasing time scale due to the spin exchange interactions. This opens a potential alternative to Lamb-Dicke confinement for suppressing Doppler dephasing in cold gases with possible applications in quantum sensing and quantum memories, as well as demonstrating an interesting platform for exploring quantum many-body physics in spin-orbit coupled systems.