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arXiv:2607.02258

A transition-metal qubit in diamond with all-optical control and millisecond quantum memory

I. M. Morris, T. Alberth, L. Crooks, T. Lühmann, D. J. Twitchen, S. Pezzagna, J. Meijer, S. S. Nicley, J. N. Becker

quant-phcond-mat.otherphysics.atom-phSubmitted Jul 2, 2026• Announced Jul 3, 2026• 0 citations
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Abstract

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 $T_2^*$ = 371 ns to $T_2^{CPMG-4}$ = 1.27 ms, establishing NiV$^-$ as a deployable diamond spin-photon interface.

Comments
16 pages, 10 figures

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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∗T_2^*T2∗​ = 371 ns to T2CPMG−4T_2^{CPMG-4}T2CPMG−4​ = 1.27 ms, establishing NiV−^-− as a deployable diamond spin-photon interface.
Group-IV color centers in diamond are a promising light-matter interface for quantum networking devices.The negatively charged tin-vacancy center (SnV) is particularly interesting, as its large spin-orbit coupling offers strong protection against phonon dephasing and robust cyclicity of its optical transitions toward spin-photon-entanglement schemes.Here, we demonstrate multiaxis coherent control of the SnV spin qubit via an all-optical stimulated Raman drive between the ground and excited states.We use coherent population trapping and optically driven electronic spin resonance to confirm coherent access to the qubit at 1.7 K and obtain spin Rabi oscillations at a rate of Ω=2π ¼ 19.0ð1Þ MHz.All-optical Ramsey interferometry reveals a spin dephasing time of T Ã 2 ¼ 1.3ð3Þ μs, and four-pulse dynamical decoupling already extends the spin-coherence time to T 2 ¼ 0.30ð8Þ ms.Combined with transform-limited photons and integration into photonic nanostructures, our results make the SnV a competitive spin-photon building block for quantum networks.
Solid-state spin defects are a promising platform for quantum networks. A key requirement is to combine long ground-state spin-coherence times with a coherent optical transition for spin-photon entanglement. Here, we investigate the spin and optical coherence of single nitrogen-vacancy (NV) centres in (111)-grown isotopically engineered diamond. Our diamond-growth process yields a precisely controlled 13C^{13}\mathrm{C}13C concentration and low-ppb nitrogen concentrations. Combined with the mitigation of 50 Hz noise using a real-time feedforward scheme and tailored decoupling sequences, this enables record defect-electron-spin coherence times of T2=6.8(1)T_2 = 6.8(1)T2​=6.8(1) ms for a Hahn echo and of T2DD=11.2(8)T_2^{DD} = 11.2(8)T2DD​=11.2(8) s under dynamical decoupling. In addition, we observe coherent optical transitions with a near-lifetime-limited homogeneous linewidth of 16.9(4) MHz and characterize the spectral diffusion dynamics. These results provide new avenues to investigate the incorporation of impurities in diamond and new opportunities for improved spin-qubit control for quantum networks and other quantum technologies.
The negatively charged nickel vacancy center (NiV^{-}) in diamond is a promising spin qubit candidate with predicted inversion symmetry, large ground state spin-orbit splitting to limit phonon-induced decoherence, and emission in the near infrared. Here, we experimentally confirm the proposed geometric and electronic structure of the NiV defect via magneto-optical spectroscopy. We characterize the optical properties and find a Debye-Waller factor of 0.62. Additionally, we engineer charge state stabilized defects using electrical bias in planar all-diamond p-i-p junctions. We measure a vanishing static dipole moment and no spectral diffusion, characteristic of inversion symmetry. Under bias, we observe stable transitions with lifetime-limited linewidths as narrow as 16 MHz and convenient frequency tuning of the emission via a second-order Stark shift. Overall, this Letter provides a pathway toward coherent control of the NiV^{-} and its use as a spin qubit and contributes to a more general understanding of charge dynamics experienced by defects in diamond.
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A central aim of quantum information processing is the efficient entanglement of multiple stationary quantum memories via photons. Among solid-state systems, the nitrogen-vacancy centre in diamond has emerged as an excellent optically addressable memory with second-scale electron spin coherence times. Recently, quantum entanglement and teleportation have been shown between two nitrogen-vacancy memories, but scaling to larger networks requires more efficient spin-photon interfaces such as optical resonators. Here we report such nitrogen-vacancy-nanocavity systems in the strong Purcell regime with optical quality factors approaching 10,000 and electron spin coherence times exceeding 200 μs using a silicon hard-mask fabrication process. This spin-photon interface is integrated with on-chip microwave striplines for coherent spin control, providing an efficient quantum memory for quantum networks. Nitrogen-vacancy centres in diamond have established themselves as excellent candidates for solid-state quantum memories due to their optical addressability and long coherence times. Here, the authors report on a diamond-nanocavity system with improved spin-photon interface performances.
The germanium vacancy in diamond (GeV) is a promising candidate for color center based quantum networking. Yet, like for other group-IV vacancy defects in diamond, achieving fast, high-fidelity qubit operations using traditional magnetic resonance techniques is experimentally challenging due to a weak magnetic dipole and susceptibility to thermally induced decoherence. Here, we perform all-optical control of the GeV and realize Rabi frequencies exceeding ∼ 20 MHz. We do so by driving the two Λ -systems of the GeV simultaneously and apply this to probe the spin coherence ( T 2 * = 224 ± 14 ns, T 2 H = 11.9 ± 0.3 µ s ). Our control scheme is applicable to other color centers and particularly, other group-IV defects for which the scheme may be optimized to improve all-optical control in these systems.