Optical quantum memories are essential elements in quantum networks for long-distance distribution of quantum entanglement. Scalable development of quantum network nodes requires on-chip qubit storage functionality with control of the readout time. 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 through an enhanced optical Stark shift of the comb frequencies. Our solid-state memory is integrable with other chip-scale photon source and detector devices for multiplexed quantum and classical information processing at the network nodes.

We demonstrate optical probing of spectrally resolved single Nd 3+ rare-earth ions in yttrium orthovanadate (YVO4). The ions are coupled to a photonic crystal resonator and show strong enhancement of the optical emission rate via the Purcell effect resulting in near-radiatively-limited single photon emission. The measured high coupling cooperativity between a single photon and the ion allows for the observation of coherent optical Rabi oscillations. This could enable optically controlled spin qubits, quantum logic gates, and spin-photon interfaces for future quantum networks.Rare-earth dopants in solids exhibit long-lived coherence in both the optical and spin degrees of freedom [1, 4]. The effective shielding of 4f electrons leads to optical and radio-frequency transitions with less sensitivity to noise in their crystalline surroundings at cryogenic temperatures. Significant progress in rare-earth based quantum technologies has led to ensemble-based optical quantum memories [1, 3-5] and coherent transducers [7], with promising performance as quantum light-matter interfaces for quantum networks. On the other hand, addressing single ions has remained an outstanding challenge, with the progress hindered by the long optical lifetimes of rare-earth ions and resultant faint photoluminescence (PL). So far, only a few experiments have succeeded in isolating individual praseodymium [8-10], cerium [11][12][13], and erbium [14, 15] ions, though the majority of them did not probe ions via their 4f-4f optical transitions. Recently, several works have demonstrated significant enhancement of spontaneous emissions of rare-earth emitters coupled to a nanophotonic cavity [1,[15][16][17], among which [1, 16] also showed negligible detrimental effect on the coherence properties of ions in nanodevices. These results point at a viable approach to efficiently detect and coherently control individual ions in a chip-scale architecture.Here we demonstrate a nanophotonic platform based on a yttrium orthovanadate (YVO 4 ) photonic crystal nanobeam resonator coupled to spectrally resolved individual neodymium (Nd 3+ ) ions. While the system acts as an ensemble quantum memory when operating at the center of the inhomogeneous line [1], it also enables direct optical addressing of single Nd 3+ in the tails of the inhomogeneous distribution, which show strongly enhanced, near-radiatively-limited single photon emissions. A measured vacuum Rabi frequency of 2π×28.5 MHz signif-icantly exceeds the linewidth of a Nd 3+ ion, allowing for coherent manipulation of spins with optical pulses. Unlike prior experiments [8][9][10][11][12][13], this technique does not hinge on the spectroscopic details of a specific type of ion and can be readily extended to other rare-earths or defect centers. The technique opens up new opportunities for spectroscopy on single ions that are distinct from conventional ensemble measurements, which enables probes for the local nanoscopic environment around individual ions and may lead to new quantum information processing, i...

Quantum memories for light are important components for future long distance quantum networks. We present on-chip quantum storage of telecommunications band light at the single photon level in an ensemble of erbium-167 ions in an yttrium orthosilicate photonic crystal nanobeam resonator. Storage times of up to 10 µs are demonstrated using an all-optical atomic frequency comb protocol in a dilution refrigerator under a magnetic field of 380 mT. We show this quantum storage platform to have high bandwidth, high fidelity, and multimode capacity, and we outline a path towards an efficient erbium-167 quantum memory for light.