Single-crystal diamond, with its unique optical, mechanical and thermal properties, has emerged as a promising material with applications in classical and quantum optics. However, the lack of heteroepitaxial growth and scalable fabrication techniques remains the major limiting factors preventing more wide-spread development and application of diamond photonics. In this work, we overcome this difficulty by adapting angled-etching techniques, previously developed for realization of diamond nanomechanical resonators, to fabricate racetrack resonators and photonic crystal cavities in bulk single-crystal diamond. Our devices feature large optical quality factors, in excess of 10 5 , and operate over a wide wavelength range, spanning visible and telecom. These newly developed high-Q diamond optical nanocavities open the door for a wealth of applications, ranging from nonlinear optics and chemical sensing, to quantum information processing and cavity optomechanics.

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.

Quantum networks based on optically addressable spin qubits promise to enable secure communication, distributed quantum computing, and tests of fundamental physics. Scaling up quantum networks based on solid-state luminescent centers requires coherent spin and optical transitions coupled to photonic resonators. Here we investigate single Yb !"! #$ ions in yttrium orthovanadate coupled to a nanophotonic cavity. These ions possess optical and spin transitions that are first-order insensitive to magnetic field fluctuations, enabling optical linewidths less than 1 MHz and spin coherence times exceeding 30 ms for cavity-coupled ions. The cavity-enhanced optical emission rate facilitates efficient spin initialization and conditional single-shot readoutwith fidelity greater than 95%. These results showcase a solid-state platform based on single coherent rare-earth ions for the future quantum internet. Main text:The distribution of entanglement over long distances using optical quantum networks is an intriguing macroscopic quantum phenomenon with applications in quantum systems for advanced computing and secure communication (1, 2). Solid-state emitters coupled to photonic resonators (3) are promising candidates for implementing quantum light-matter interfaces necessary for scalable quantum networks. A variety of systems have been investigated for this purpose, including quantum dots and defects in diamond or silicon carbide (4-8). So far, the ability to scale up these systems has remained elusive and motivates the development of alternative platforms. A central challenge is identifying emitters that exhibit coherent optical and spin transitions while coupled to photonic cavities that enhance the optical transitions and arXiv:1907.12161v1 [quant-ph] 28 Jul 2019 channel emission into optical fibers. Ensembles of rare-earth ions (REIs) in crystals are known to possess highly coherent 4f-4f optical and spin transitions (9, 10), but only recently have single REIs been isolated (11, 12) and coupled to nanocavities (13, 14). The crucial next steps toward using single REIs for quantum networks are demonstrating long spin coherence and single-shot readout in photonic resonators. Here we demonstrate spin initialization, coherent optical and spin manipulation, and high-fidelity single-shot optical readout of the hyperfine spin state of single Yb !"! #$ ions coupled to a nanophotonic cavity fabricated in an yttrium orthovanadate (YVO) host crystal. The relevant energy level structure of Yb ions are coupled to a photonic crystal cavity with small mode volume ~1( KLM ⁄ ) # and large quality factor (1 × 10 P ) (Fig 1C, D. See SI 1.1). This enhances the emission rate, collection efficiency, and cyclicity of the optical transitions A and E via the Purcell effect (16). The qubit is initialized into |0⟩ / by optical and microwave pumping on F, A,and fe to empty | ⟩ / and |1⟩ / , followed by cavity-enhanced decay into |0⟩ / via E (Fig. 1A). A subsequent microwave pulse applied on / optionally initializes the ion into |1⟩ / . The |1⟩ /

GUIDED ACOUSTIC PHONON MODES IN DIAMOND OPTOMECHANICAL CRYSTALSTo supplement our discussion of the guided acoustic phonon modes supported by diamond optomechanical crystals (OMCs), we present normalized displacement profiles of the nominal unit cell at the Γ (kx = 0) and X (kx = π/a) points of its mechanical bandstructure (originally displayed in Figure 1(c) of the main text). Figures S1 and S2 reveal the guided acoustic modes categorized by even (solid black lines) and odd (dashed blue lines) vector symmetries about the y-axis, respectively, with displacement profiles originating from the indicated band edges shown as insets (three dimensional, top down and cross-section views included). Note, the unit cell lattice constant in the displacement profiles is displayed between the (hx, n, hy, n) and (hx, n+1, hy, n+1) center points, in order to clearly reveal displacement components within the air holes. Mechanical simulations included here and throughout the main text use the full anisotropic elasticity matrix of diamond [1], where (C11, C12, C44) = (1076, 125, 578) GPa. However, due to considerations expanded upon in section 5 of this supplementary material, devices characterized in this work were ultimately fabricated with their x-axis oriented with the in-plane [110] crystallographic direction. Thus, a rotated version of the anisotropic elasticity matrix ensured proper device orientation in our simulations, with guided mode propagation along the x-axis aligned with the [110] crystallographic direction, with the z-axis aligned with [001]. Only a small (< 10 %) change in the guided mode frequencies was observed between simulations with unit cell x-axis alignment to the [100] and [110] in plane crystal directions.While the mechanical bandstructures reveal a rich library of guided acoustic modes in the few to 16 GHz frequency range, only guided modes originating from y-symmetric bands ultimately couple to the optical cavity [2]. Additionally, modes originating from the Γ-point ensure large optomechanical coupling rates in the final design [3]. With this in mind, two modes from the Γ-point of ysymmetric bands enable design of diamond OMCs with large single-photon optomechanical coupling rates, go. Specifically, the Γ-point modes from the 4 th and 7 th y-symmetric bands, referred to as the "flapping" and "swelling" modes, respectively, were both investigated. OPTIMIZED DIAMOND OPTOMECHANICAL CRYSTAL DESIGNAs discussed in the main text, the final diamond OMC design relies on transitioning from a "mirror" region formed by the base unit cell in Figure 1(a) to a "defect" cell, which localizes the target

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.

Optical networks that distribute entanglement among various quantum systems will form a powerful framework for quantum science but are yet to interface with leading quantum hardware such as superconducting qubits. Consequently, these systems remain isolated because microwave links at room temperature are noisy and lossy. Building long distance connectivity requires interfaces that map quantum information between microwave and optical fields. While preliminary microwave-to-optical transducers have been realized, developing efficient, low-noise devices that match superconducting qubit frequencies (gigahertz) and bandwidths (10 kilohertz – 1 megahertz) remains a challenge. Here we demonstrate a proof-of-concept on-chip transducer using trivalent ytterbium-171 ions in yttrium orthovanadate coupled to a nanophotonic waveguide and a microwave transmission line. The device′s miniaturization, material, and zero-magnetic-field operation are important advances for rare-earth ion magneto-optical devices. Further integration with high quality factor microwave and optical resonators will enable efficient transduction and create opportunities toward multi-platform quantum networks.

Ensembles of solid-state optical emitters enable broadband quantum storage and transduction of photonic qubits, with applications in high-rate quantum networks for secure communications and interconnecting future quantum computers. To transfer quantum states using ensembles, rephasing techniques are used to mitigate fast decoherence resulting from inhomogeneous broadening, but these techniques generally limit the bandwidth, efficiency and active times of the quantum interface. Here, we use a dense ensemble of neodymium rare-earth ions strongly coupled to a nanophotonic resonator to demonstrate a significant cavity protection effect at the single-photon level—a technique to suppress ensemble decoherence due to inhomogeneous broadening. The protected Rabi oscillations between the cavity field and the atomic super-radiant state enable ultra-fast transfer of photonic frequency qubits to the ions (∼50 GHz bandwidth) followed by retrieval with 98.7% fidelity. With the prospect of coupling to other long-lived rare-earth spin states, this technique opens the possibilities for broadband, always-ready quantum memories and fast optical-to-microwave transducers.