Spectral hole burning and its application in microwave photonics

Rabi oscillations with a decay time of 26.7 μs are observed in a system comprising the electron spins in a diamond nitrogen–vacancy centre and a superconducting microwave cavity. Such oscillations are achieved by engineering the spectral hole burning of the spin ensemble. Spectral hole burning, used in inhomogeneously broadened emitters, is a well-established optical 1 technique, with applications from spectroscopy to slow light 2 and frequency combs 3 . In microwave photonics 4 , electron spin ensembles 5 , 6 are candidates for use as quantum memories 7 with potentially long storage times 8 . Here, we demonstrate long-lived collective dark states 9 by spectral hole burning in the microwave regime 10 . The coherence time in our hybrid quantum system (nitrogen–vacancy centres strongly coupled to a superconducting microwave cavity) becomes longer than both the ensemble's free-induction decay and the bare cavity dissipation rate. The hybrid quantum system thus performs better than its individual subcomponents. This opens the way for long-lived quantum multimode memories, solid-state microwave frequency combs, spin squeezed states 11 , optical-to-microwave quantum transducers 12 and novel metamaterials 13 . Beyond these, new cavity quantum electrodynamics experiments will be possible where spin–spin interactions and many-body phenomena 14 are directly accessible.