The interaction of single quantum emitters with an optical cavity enables the realization of efficient spin-photon interfaces, an essential resource for quantum networks. The dynamical control of the spontaneous emission rate of quantum emitters in cavities has important implications in quantum technologies, e.g. for shaping the emitted photons waveform, for generating quantum entanglement, or for driving coherently the optical transition while preventing photon emission. Here we demonstrate the dynamical control of the Purcell enhanced emission of a small ensemble of erbium ions doped into nanoparticles. By embedding the doped nanoparticles into a fully tunable high finesse fiber based optical microcavity, we show that we can tune the cavity on- and out of-resonance by controlling its length with sub-nanometer precision, on a time scale more than two orders of magnitude faster than the natural lifetime of the erbium ions. This allows us to shape in real time the Purcell enhanced emission of the ions and to achieve full control over the emitted photons waveforms. This capability opens prospects for the realization of efficient nanoscale quantum interfaces between solid-state spins and single telecom photons with controllable waveform, and for the realization of quantum gates between rare-earth ion qubits coupled to an optical cavity.
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