  Atomic frequency combs memories that coherently store optical signals are a
key building block for optical quantum computers and quantum networks.
Integrating such memories into compact and chip-scale devices is essential for
scalable quantum technology, but to date most demonstrations have been in bulk
materials or waveguides with large cross-sections, or using fabrication
techniques not easily adaptable to wafer scale processing. We demonstrate
compact chip-integrated atomic frequency comb storage in rare earth doped
thin-film lithium niobate. Our optical memory exhibits a broad storage
bandwidth exceeding 100 MHz, and optical storage time of over 250 ns. The
enhanced optical confinement in this device structure enables three orders of
magnitude reduction in optical power as compared to large ion-diffused
waveguides for the same Rabi frequency. These compact atomic frequency comb
memories pave the way towards scalable, highly efficient, electro-optically
tunable quantum photonic systems that can store and manipulate light on a
compact chip.
