  Erbium dopants in crystals exhibit highly coherent optical transitions well
suited for solid-state optical quantum memories operating in the telecom band.
Here we demonstrate coupling of erbium dopant ions in yttrium orthosilicate to
a photonic crystal cavity fabricated directly in the host crystal using focused
ion beam milling. The coupling leads to reduction of the photoluminescence
lifetime and enhancement of the optical depth in microns-long devices, which
will enable on-chip quantum memories.
