The negatively charged silicon-vacancy (SiV^{-}) color center in diamond has recently emerged as a promising system for quantum photonics. Its symmetry-protected optical transitions enable the creation of indistinguishable emitter arrays and deterministic coupling to nanophotonic devices. Despite this, the longest coherence time associated with its electronic spin achieved to date (∼250 ns) has been limited by coupling to acoustic phonons. We demonstrate coherent control and suppression of phonon-induced dephasing of the SiV^{-} electronic spin coherence by 5 orders of magnitude by operating at temperatures below 500 mK. By aligning the magnetic field along the SiV^{-} symmetry axis, we demonstrate spin-conserving optical transitions and single-shot readout of the SiV^{-} spin with 89% fidelity. Coherent control of the SiV^{-} spin with microwave fields is used to demonstrate a spin coherence time T_{2} of 13 ms and a spin relaxation time T_{1} exceeding 1 s at 100 mK. These results establish the SiV^{-} as a promising solid-state candidate for the realization of quantum networks.
Efficient interfaces of quantum emitters and photons are fundamental to quantum networking. Silicon-vacancy (SiV) centers embedded in diamond nanophotonics are a leading approach due to their excellent optical and spin coherence; however, diamond’s weak nonlinearity and challenging fabrication limit long-term scalability. Here, we realize the first high-cooperativity ( C >1) coupling of individual SiV spin qubits in a scalable, hybrid-integrated gallium phosphide (GaP)-on-diamond nanophotonics platform. We fabricate more than 600 GaP nanophotonic cavities on a diamond substrate with a 99.5% yield. We confirm above-unity cooperativity and measure spin-relaxation times exceeding 0.4 ms at 4 K, which together enable single-shot readout of the SiV electron spin. These results, coupled with GaP’s strong nonlinear properties, establish GaP-on-diamond as a scalable planar platform for quantum networking applications.
Quantum networks require qubits that combine efficient optical access, coherent control, and long-lived quantum memory, but realizing all three in one scalable platform remains a central bottleneck. Diamond color centers are leading candidates, yet widely studied defects retain tradeoffs among these capabilities. Here, we show that transition-metal defects in diamond provide a distinct route beyond these platforms by combining spin-orbit protected ground-state coherence, all-optical control, and near-infrared emission. Using a single nickel-vacancy (NiV
−), we demonstrate an all-optically controlled diamond spin qubit with coherence exceeding one millisecond at 1.65 K, compatible with compact closed-cycle cryogenics. We implement Raman Rabi oscillations and Ramsey interferometry and use all-optical dynamical decoupling to extend coherence from
T2∗ = 371 ns to
T2CPMG−4 = 1.27 ms, establishing NiV
− as a deployable diamond spin-photon interface.