Programmable cavity QED with a fiber-integrated atomic array

This research demonstrates a fiber-integrated cavity-QED platform that combines a twelve-site one-dimensional array of laser-cooled 87Rb atoms with an 85 µm Fabry–Pérot microcavity formed by fiber mirrors. The cavity has a measured single-atom cooperativity of 173, placing the system deep in the strong-coupling regime while retaining microscope access for site-resolved fluorescence imaging. By shifting each optical tweezer through the cavity’s standing wave, the experiment continuously programs the atom–photon coupling: transmission oscillates with the λ/2 spatial period, and positional stability reaches 1.1% × λ. For a five-site array, the measured single-atom couplings average 103(4) MHz with variations below 4%, and spectra for up to five coupled atoms show the expected Tavis–Cummings enhancement, with collective coupling scaling as √N. The array can also be read out nondestructively through collective polariton resonances: zero versus one coupled atom is distinguished with 99.0(4)% fidelity in 100 µs, while fluorescence imaging independently achieves 99.91(3)% fidelity. Unlike fixed intracavity lattices, this architecture offers continuous, site-specific coupling control, enabling engineered collective modes, cavity-mediated interactions, mid-circuit measurement, and future fiber-based quantum-network nodes.