Spectral hole burning of inhomogeneously broadened ensembles using optical fields is now a well-established technique  that allows one to selectively bleach the absorption spectrum of materials. These spectral holes have been used in a variety of applications ranging from spectroscopy in photochemistry to slow light in photonics, and the storage of quantum information by the creation of optical solid-state frequency combs. Similarly, in microwave photonics hybrid solid-state devices with ensembles of electron spins are considered as a versatile component for future quantum memories with high storage capacities , but their operational capability lag behind those of full optical systems. To demonstrate the potential of electron spin ensembles in circuit-QED (Quantum Electrodynamics) systems, we implement spectral hole burning using a cavity in the microwave domain. This enables us to selectively bleach the ensembles microwave absorption spectrum and thus engineer long-lived collective dark states in a hybrid system, composed of an ensemble of electron spins hosted by nitrogen-vacancy centers in diamond strongly coupled to a superconducting microwave cavity.
We show how to eliminate the ensembles inhomogeneous broadening, which is a severe issue that limits the memory’s storage time. The coherence rates are significantly better than either those given by the ensembles free induction decay or the bare cavity dissipation rate. Our hybrid quantum system truly lives up to the promise of this technology to perform better than its individual subcomponents as is shown in Fig. 1. To further demonstrate the potential of our approach for the selective preparation of "decoherence-free" states, we engineer multiple long-lived dark states , a key step towards a solid-state microwave frequency comb. Our technique allows the advantages given by spectral hole burning using optical fields to come to the solid-state microwave world opening up the way for long-lived quantum memories, spin squeezed states, optical-to-microwave quantum transducers and novel metamaterials. Furthermore, our approach also paves the way for a new class of cavity QED experiments with dense spin ensembles, where dipole spin-spin interactions become important and many-body phenomena are directly accessible on a chip.
|Fig. 1. (Left) Image of the hybrid system composed of a superconducting resonator and NV- electron spin ensemble. (Middle) Cavity response to a weak driving pulse without spectral hole burning (Right) Cavity response to a weak driving pulse after spectral holes are burnt.|