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.

Improving the temporal resolution of single photon detectors has an impact on many applications 1 , such as increased data rates and transmission distances for both classical 2 and quantum 3-5 optical communication systems, higher spatial resolution in laser ranging and observation of shorter-lived fluorophores in biomedical imaging 6 . In recent years, superconducting nanowire single-photon detectors 7,8 (SNSPDs) have emerged as the highest efficiency time-resolving single-photon counting detectors available in the near infrared 9 . As the detection mechanism in SNSPDs occurs on picosecond time scales 10 , SNSPDs have been demonstrated with exquisite temporal resolution below 15 ps [11][12][13][14][15] . We reduce this value to 2.7±0.2 ps at 400 nm and 4.6±0.2 ps at 1550 nm, using a specialized niobium nitride (NbN) SNSPD. The observed photon-energy dependence of the temporal resolution and detection latency suggests that intrinsic effects make a significant contribution.Temporal resolution in SNSPDs, commonly referred to as jitter, is characterized by the width of the temporal distribution of signal outputs with respect to the photon arrival times. This statistical distribution is known as the instrument response function (IRF), and its width is commonly evaluated as

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...

We demonstrate a 64-pixel free-space-coupled array of superconducting nanowire single photon detectors optimized for high detection efficiency in the near-infrared range. An integrated, readily scalable, multiplexed readout scheme is employed to reduce the number of readout lines to 16. The cryogenic, optical, and electronic packaging to read out the array, as well as characterization measurements are discussed.Superconducting nanowire single photon detectors (SNSPD) have been shown to have high efficiency, low dark counts, and tens of picosecond timing 1 . SNSPDs have been particularly useful in applications requiring high timing resolution and detection in the near-infrared (λ > 1µm)2 . Until recently small arrays of nanowire detectors for imaging, higher count rates, large collection areas, and photon number resolving detection have been technologically challenging to realize. The recent observation of the saturation of internal detection efficiency at ∼ 40% of the maximum bias current in SNSPDs fabricated from amorphous tungsten silicide (WSi) is key in enabling high-efficiency arrays to be constructed 3,4 . Detectors fabricated from niobium nitride (NbN), for example, have high detection efficiencies only at bias currents close to the critical current 5 . Current "cross talk" between detectors in arrays biased so close to their maximum operating current could cause other detectors to falsely fire when one detector in the array fires. A wide margin in operating bias, or "bias plateau", allows the detectors to be biased at a fraction of their respective critical currents and to remain sensitive to photons, even as other detectors in the array fire. Previously, we demonstrated a 4-pixel WSi SNSPD array with an integrated, scalable multiplexed readout 6 . In this work we extend to a free-space coupled 64-pixel (8 × 8 square) array using a slightly modified version of our multiplexed readout.To date, only a handful of experiments have demonstrated arrays of SNSPDs. Architectures where each detector has its own readout/bias line have been measured 5,7-9 . However, one critical issue to consider when scaling up to larger numbers of elements is the available cooling power of the cryogenic system. Each readout channel adds to the total thermal power budget and can quickly limit achievable base temperatures. Therefore, multiplexing schemes, where the total number of readout channels is kept to a small fraction of the number of array elements, become a necessity. Attempts at multiplexed readout have primarily been limited to single flux quantum (SFQ) logic schemes 10-14 . SFQ-logic-based readout is attractive due to the intrinsic low power consumption but the designs can be quite complex and require additional fabrication steps for the Josephson junctions. Apart from SFQ, an inductive current splitting technique where the firing pixel location was encoded onto the magnitude of the output pulse, has been demonstrated in a 4-pixel linear array 15 .Another key issue in array development is device yield. The traditional m...

We report measurements of time-frequency entangled photon pairs and heralded single photons at 1550 nm wavelengths generated using a microring resonator pumped optically by a diode laser. Along with a high spectral brightness of pair generation, the conventional metrics used to describe performance, such as Coincidences-to-Accidentals Ratio (CAR), conditional self-correlation [g (2) (0)], two-photon energy-time Franson interferometric visibility etc. are shown to reach a highperformance regime not yet achieved by silicon photonics, and attained previously only by crystal, glass and fiber-based pair-generation devices.