Angular momentum transfer via virtual atomic excitations

The tunability of atomic parameters has made atomic systems an appealing experimental platform. We utilize the tunability of atomic energy levels to transfer angular momentum to photons. By spatiotemporally modulating a cloud of ⁸⁷Rb at the waist of a twisted cavity with a far-detuned Stark-shifting beam, photons that pass through the cloud are converted to different angular momentum modes of the cavity. We observe this conversion and achieve the maximum theoretical conversion efficiency, demonstrating the coherent modification of photonic degrees of freedom and prospects for engineering couplings between arbitrary modes.