The spectral addressing of many individual rare‐earth dopants in optical resonators offers great potential for realizing distributed quantum information processors. To this end, it is required to understand and control the spectral properties of the emitters in micron‐scale devices. Here, erbium emitters are investigated in a Fabry–Perot resonator that contains a 10 µm thin membrane of crystalline yttrium orthosilicate that is co‐doped with europium. The co‐doping allows for tailoring the inhomogeneous distribution of the emitter frequency. With this approach, more than 360 spectrally resolved emitters are observed with Purcell factors exceeding 35, each of which constitutes an individually addressable qubit within the micron‐scale resonator. In addition to this spectral multiplexing, the optical coherence is preserved up to 0.62(3) ms under dynamical decoupling. Without decoupling, the coherence still reaches the lifetime limit for the emitters with the strongest Purcell enhancement that leads up to a 110‐fold lifetime reduction, down to 0.104(9) ms. Future work may combine this with long‐lived nuclear spin memories, which makes the investigated co‐doped membranes a promising platform for quantum repeaters and distributed quantum computers.     «

The spectral addressing of many individual rare‐earth dopants in optical resonators offers great potential for realizing distributed quantum information processors. To this end, it is required to understand and control the spectral properties of the emitters in micron‐scale devices. Here, erbium emitters are investigated in a Fabry–Perot resonator that contains a 10 µm thin membrane of crystalline yttrium orthosilicate that is co‐doped with europium. The co‐doping allows for tailoring the inhomo...     »