Quantum light-matter interfaces that reversibly map photonic quantum states onto atomic states, are essential components in the quantum engineering toolbox with applications in quantum communication, computing, and quantum-enabled sensing. I present a new platform for on-chip quantum light-matter interfaces based on nanophotonic resonators coupled to rare-earth-ions in crystals. The rare-earth ions exhibit long coherence times on optical transitions, which makes them suitable for optical quantum memories. We demonstrate a high-fidelity nanophotonic quantum memory based on a mesoscopic neodymium ensemble coupled to a photonic crystal cavity. The nanocavity enables >95% spin polarization for efficient initialization of the atomic frequency comb memory, and time-bin-selective readout via enhanced optical Stark shift of the comb frequencies. Our current technology can be readily transferred to Erbium doped devices for telecom memories that can be integrated with silicon photonics. Besides ensemble memories, single rare-earth-ions coupled to nano-resonators can be used as single optically addressable quantum bits where the quantum state is mapped on their Zeeman or hyperfine levels with long coherence time. Our solid-state nano-photonic quantum light-matter interfaces can be integrated with other chip-scale photon source and detector devices for multiplexed quantum and classical information processing at the nodes of quantum networks. I also discuss prospects for integration with superconducting resonators and qubits, which can lead to devices for reversible quantum transduction of optical photons to microwave photons, thus enabling optical interconnects between superconducting quantum computers.