  Ensembles of electron spins in hybrid microwave systems are powerful and
versatile components for future quantum technologies. Quantum memories with
high storage capacities are one such example which require long-lived states
that can be addressed and manipulated coherently within the inhomogeneously
broadened ensemble. This broadening is essential for true multimode memories,
but induces a considerable spin dephasing and together with dissipation from a
cavity interface poses a constraint on the memory's storage time. In this work
we show how to overcome both of these limitations through the engineering of
long-lived dark states in an ensemble of electron spins hosted by
nitrogen-vacancy centres in diamond. By burning narrow spectral holes into a
spin ensemble strongly coupled to a superconducting microwave cavity, we
observe long-lived Rabi oscillations with high visibility and a decay rate that
is a factor of forty smaller than the spin ensemble linewidth and thereby a
factor of more than three below the pure cavity dissipation rate. This
significant reduction lives up to the promise of hybrid devices to perform
better than their individual subcomponents. To demonstrate the potential of our
approach we realise the first step towards a solid-state microwave spin
multiplexer by engineering multiple long-lived dark states. Our results show
that we can fully access the "decoherence free" subspace in our experiment and
selectively prepare protected states by spectral hole burning. This technique
opens up the way for truly long-lived quantum memories, solid-state microwave
frequency combs, optical to microwave quantum transducers and spin squeezed
states. 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 will be directly accessible on a chip.
