Purcell-enhanced spin-phonon coupling with a single colour centre

The radiative properties of emitters are inherently linked to their surrounding environment¹. Placing an electromagnetic resonator around emitters can enhance spontaneous emission, as shown by Purcell in the 1940s². This approach is now routinely used in quantum computing and communication to channel photons emitted by atoms into well-defined modes and control atom-photon interactions^3-9. For solid-state emitters, such as colour centres, the host lattice introduces an acoustic environment, allowing excited atoms to relax by emitting phonons^10,11. Here we observe the acoustic Purcell effect by constructing a specially engineered, microwave-frequency nanomechanical resonator around a colour-centre spin qubit in diamond. Using a co-localized optical mode of the structure that strongly couples to the excited state of the colour centre, we perform single-photon-level laser spectroscopy at millikelvin temperatures and observe a 10-fold faster spin relaxation when the spin qubit is tuned into resonance with a 12 GHz acoustic mode. Moreover, we use the colour centre as an atomic-scale probe to measure the broadband phonon spectrum of the nanostructure up to 28 GHz. Our work establishes a new regime of control for quantum defects in solids and paves the way for interconnects between atomic-scale quantum memories¹² and qubits encoded in acoustic and superconducting devices¹³.