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Engineering the local density of states with nanophotonic structures is a powerful tool to control light-matter interactions via the Purcell effect. At optical frequencies, control over the electric field density of states is typically used to couple to and manipulate electric dipole transitions. However, it is also possible to engineer the magnetic density of states to control magnetic dipole transitions. In this work, we experimentally demonstrate the optical magnetic Purcell effect using a single rare earth ion coupled to a nanophotonic cavity. We engineer a new single photon emitter, Er$^{3+}$ in MgO, where the electric dipole decay rate is strongly suppressed by the cubic site symmetry, giving rise to a nearly pure magnetic dipole optical transition. This allows the unambiguous determination of a magnetic Purcell factor $P_m=1040 \pm 30$. We further extend this technique to realize a magnetic dipole spin-photon interface, performing optical spin initialization and readout of a single Er$^{3+}$ electron spin. This work demonstrates the fundamental equivalence of electric and magnetic density of states engineering, and provides a new tool for controlling light-matter interactions for a broader class of emitters.

Atomic defects in the solid state are a key component of quantum repeater networks for long-distance quantum communication. Recently, there has been significant interest in rare earth ions, in particular Er$^{3+}$ for its telecom-band optical transition, but their application has been hampered by optical spectral diffusion precluding indistinguishable single photon generation. In this work we implant Er$^{3+}$ into CaWO$_4$, a material that combines a non-polar site symmetry, low decoherence from nuclear spins, and is free of background rare earth ions, to realize significantly reduced optical spectral diffusion. For shallow implanted ions coupled to nanophotonic cavities with large Purcell factor, we observe single-scan optical linewidths of 150 kHz and long-term spectral diffusion of 63 kHz, both close to the Purcell-enhanced radiative linewidth of 21 kHz. This enables the observation of Hong-Ou-Mandel interference between successively emitted photons with high visibility, measured after a 36 km delay line. We also observe spin relaxation times $T_1$ = 3.7 s and $T_2$ > 200 $\mu$s, with the latter limited by paramagnetic impurities in the crystal instead of nuclear spins. This represents a significant step towards the construction of telecom-band quantum repeater networks with single Er$^{3+}$ ions.

Individually addressed Er$^{3+}$ ions in solid-state hosts are promising resources for quantum repeaters, because of their direct emission in the telecom band and compatibility with silicon photonic devices. While the Er$^{3+}$ electron spin provides a spin-photon interface, ancilla nuclear spins could enable multi-qubit registers with longer storage times. In this work, we demonstrate coherent coupling between the electron spin of a single Er$^{3+}$ ion and a single $I=1/2$ nuclear spin in the solid-state host crystal, which is a fortuitously located proton ($^1$H). We control the nuclear spin using dynamical decoupling sequences applied to the electron spin, implementing one- and two-qubit gate operations. Crucially, the nuclear spin coherence time exceeds the electron coherence time by several orders of magnitude, because of its smaller magnetic moment. These results provide a path towards combining long-lived nuclear spin quantum registers with telecom-wavelength emitters for long-distance quantum repeaters.

Integrating atomic quantum memories based on color centers in diamond with on-chip photonic devices would enable entanglement distribution over long distances. However, efforts towards integration have been challenging because color centers can be highly sensitive to their environment, and their properties degrade in nanofabricated structures. Here, we describe a heterogeneously integrated, on-chip, III-V diamond platform designed for neutral silicon vacancy (SiV0) centers in diamond that circumvents the need for etching the diamond substrate. Through evanescent coupling to SiV0 centers near the surface of diamond, the platform will enable Purcell enhancement of SiV0 emission and efficient frequency conversion to the telecommunication C-band. The proposed structures can be realized with readily available fabrication techniques.

Spin-photon interfaces based on solid-state atomic defects have enabled a variety of key applications in quantum information processing. To maximize the light-matter coupling strength, defects are often placed inside nanoscale devices. Efficiently coupling light and microwave radiation into these structures is an experimental challenge, especially in cryogenic or high vacuum environments with limited sample access. In this work, we demonstrate a fiber-based scanning probe that simultaneously couples light into a planar photonic circuit and delivers high power microwaves for driving electron spin transitions. The optical portion achieves 46% one-way coupling efficiency, while the microwave portion supplies an AC magnetic field with strength up to 9 Gauss. The entire probe can be scanned across a large number of devices inside a $^3$He cryostat without free-space optical access. We demonstrate this technique with silicon nanophotonic circuits coupled to single Er$^{3+}$ ions.

Solid-state spin defects are a promising platform for quantum science and technology, having realized demonstrations of a variety of key components for quantum information processing, particularly in the area of quantum networks. An outstanding challenge for building larger-scale quantum systems with solid-state defects is realizing high-fidelity control over multiple defects with nanoscale separations, which is required to realize strong spin-spin interactions for multi-qubit logic and the creation of entangled states. In this work, we experimentally demonstrate an optical frequency-domain multiplexing technique, allowing high-fidelity initialization and single-shot spin measurement of six rare earth (Er$^{3+}$) ions, within the sub-wavelength volume of a single, silicon photonic crystal cavity. We also demonstrate sub-wavelength control over coherent spin rotations using an optical AC Stark shift. The demonstrated approach may be scaled to large numbers of ions with arbitrarily small separation, and is a significant step towards realizing strongly interacting atomic defect arrays with applications to quantum information processing and fundamental studies of many-body dynamics.

Optically-interfaced spins in the solid state are a promising platform for quantum technologies. A crucial component of these systems is high-fidelity, projective measurement of the spin state. In previous work with laser-cooled atoms and ions, and solid-state defects, this has been accomplished using fluorescence on an optical cycling transition; however, cycling transitions are not ubiquitous. In this work, we demonstrate that modifying the electromagnetic environment using an optical cavity can induce a cycling transition in a solid-state atomic defect. By coupling a single Erbium ion defect to a telecom-wavelength silicon nanophotonic device, we enhance the cyclicity of its optical transition by a factor of more than 100, which enables single-shot quantum nondemolition readout of the ion's spin with 94.6% fidelity. We use this readout to probe coherent dynamics and relaxation of the spin. This approach will enable quantum technologies based on a much broader range of atomic defects.

The ability to distribute quantum entanglement over long distances is a vital ingredient for quantum technologies. Single atoms and atom-like defects in solids are ideal quantum light sources and quantum memories to store entanglement. However, a major obstacle to developing long-range quantum networks is the mismatch between typical atomic transition energies in the ultraviolet and visible spectrum, and the low-loss propagation band of optical fibers in the infrared, around 1.5 $\mu$m. A notable exception is the Er$^{3+}$ ion, whose 1.5 $\mu$m transition is exploited in fiber amplifiers that drive modern communications networks. However, an optical interface to single Er$^{3+}$ ions has not yet been achieved because of the low photon emission rate, less than 100 Hz, that results from the electric dipole-forbidden nature of this transition. Here, we demonstrate that the emission rate of single Er$^{3+}$ ions in a solid-state host can be enhanced by a factor of more than 300 using a small mode-volume, low-loss silicon nanophotonic cavity. This enhancement enables the fluorescence from single Er$^{3+}$ ions to be clearly observed for the first time. Tuning the excitation laser over a small frequency range allows dozens of distinct ions to be addressed, and the splitting of the lines in a magnetic field confirms that the optical transitions are coupled to the Er$^{3+}$ ions' spins. These results are a significant step towards long-distance quantum networks and deterministic quantum logic for photons, based on a scalable silicon nanophotonics architecture.

Certain continuous-time quantum walks can be viewed as scattering processes. These processes can perform quantum computations, but it is challenging to design graphs with desired scattering behavior. In this paper, we study and construct momentum switches, graphs that route particles depending on their momenta. We also give an example where there is no exact momentum switch, although we construct an arbitrarily good approximation.