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arXiv:2601.04733

A scalable gallium-phosphide-on-diamond spin-photon interface

Nicholas S. Yama, Chun-Chi Wu, Fariba Hatami, Kai-Mei C. Fu

quant-phphysics.opticsSubmitted Jan 8, 2026• Announced Jan 9, 2026• 0 citations
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Abstract

The efficient interfacing of quantum emitters and photons is fundamental to quantum networking. Quantum defects embedded in integrated nanophotonic circuits are promising for such applications due to the deterministic light-matter interactions of high-cooperativity ($C>1$) cavity quantum electrodynamics and potential for scalable integration with active photonic processing. Silicon-vacancy (SiV) centers embedded in diamond nanophotonic cavities are a leading approach due to their excellent optical and spin coherence, however their long-term scalability is limited by the diamond itself, as its suspended geometry and weak nonlinearity necessitates coupling to a second processing chip. Here we realize the first high-cooperativity coupling of quantum defects to hybrid-integrated nanophotonics in a scalable, planar platform. We integrate more than 600 gallium phosphide (GaP) nanophotonic cavities on a diamond substrate with near-surface SiV centers. We examine a particular device with two strongly coupled SiV centers in detail, confirming above-unity cooperativity via multiple independent measurements. Application of an external magnetic field via a permanent magnet enables optical resolution of the SiV spin transitions from which we determine a spin-relaxation time $T_1>0.4$ ms at 4 K. We utilize the high cooperativity coupling to observe spin-dependent transmission switching and the quantum jumps of the SiV spin via single-shot readout. These results, coupled with GaP's strong nonlinear properties, establish GaP-on-diamond as a scalable planar platform for quantum network applications.

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The efficient interfacing of quantum emitters and photons is fundamental to quantum networking. Quantum defects embedded in integrated nanophotonic circuits are promising for such applications due to the deterministic light-matter interactions of high-cooperativity (C>1C>1C>1) cavity quantum electrodynamics and potential for scalable integration with active photonic processing. Silicon-vacancy (SiV) centers embedded in diamond nanophotonic cavities are a leading approach due to their excellent optical and spin coherence, however their long-term scalability is limited by the diamond itself, as its suspended geometry and weak nonlinearity necessitates coupling to a second processing chip. Here we realize the first high-cooperativity coupling of quantum defects to hybrid-integrated nanophotonics in a scalable, planar platform. We integrate more than 600 gallium phosphide (GaP) nanophotonic cavities on a diamond substrate with near-surface SiV centers. We examine a particular device with two strongly coupled SiV centers in detail, confirming above-unity cooperativity via multiple independent measurements. Application of an external magnetic field via a permanent magnet enables optical resolution of the SiV spin transitions from which we determine a spin-relaxation time T1>0.4T_1>0.4T1​>0.4 ms at 4 K. We utilize the high cooperativity coupling to observe spin-dependent transmission switching and the quantum jumps of the SiV spin via single-shot readout. These results, coupled with GaP's strong nonlinear properties, establish GaP-on-diamond as a scalable planar platform for quantum network applications.
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.
Efficient interfaces between optical photons and quantum bits are fundamental building blocks for quantum networks and large-scale quantum computers. We demonstrate an integrated platform for scalable quantum optics based on color centers coupled to diamond nanodevices. First, we incorporate nitrogen-vacancy (NV) centers in diamond into hybrid nanophotonic crystal cavities. Despite our progress towards coupling the NV to cavity photons, we find that the NV-cavity interface is limited by the imperfections of the color center's properties and its local environment. We approach this challenge first by characterizing and modifying the diamond surface to reduce the environmental noise affecting the NV optical transition. Subsequently, we develop a new approach that makes use of a quantum emitter with intrinsic protection from this noise: the silicon-vacancy (SiV) center in diamond. We demonstrate that SiV centers introduced through ion implantation feature highly coherent optical transitions, even inside nanostructures. By placing SiV centers inside all-diamond photonic crystal cavities, we achieve strong SiV-photon coupling with a cooperativity of greater than 20. Using this platform, we realize a quantum-optical switch controlled by the spin degree of freedom of a single SiV. By measuring intensity correlations of indistinguishable Raman photons emitted into a single waveguide, we observe a quantum interference effect resulting from the superradiant emission of two probabilistically entangled SiV centers. Finally, we demonstrate controllable photon-mediated interactions between the spin degrees of freedom of two cavity-coupled SiV centers. When the optical transitions of the two SiV centers are tuned into resonance, the coupling of each SiV to the common cavity mode results in a coherent interaction between the two SiVs, leading to the observation of super- and sub-radiant two-SiV states.
Efficient interfaces between photons and quantum emitters form the basis for quantum networks and enable optical nonlinearities at the single-photon level. We demonstrate an integrated platform for scalable quantum nanophotonics based on silicon-vacancy (SiV) color centers coupled to diamond nanodevices. By placing SiV centers inside diamond photonic crystal cavities, we realize a quantum-optical switch controlled by a single color center. We control the switch using SiV metastable states and observe optical switching at the single-photon level. Raman transitions are used to realize a single-photon source with a tunable frequency and bandwidth in a diamond waveguide. By measuring intensity correlations of indistinguishable Raman photons emitted into a single waveguide, we observe a quantum interference effect resulting from the superradiant emission of two entangled SiV centers.
The realization of quantum networks critically depends on establishing efficient, coherent light-matter interfaces. Optically active spins in diamond have emerged as promising quantum nodes based on their spin-selective optical transitions, long-lived spin ground states, and potential for integration with nanophotonics. Tin-vacancy (SnV -^{\,\textrm{-}}-) centers in diamond are of particular interest because they exhibit narrow-linewidth emission in nanostructures and possess long spin coherence times at temperatures above 1 K. However, a nanophotonic interface for SnV -^{\,\textrm{-}}- centers has not yet been realized. Here, we report cavity enhancement of the emission of SnV -^{\,\textrm{-}}- centers in diamond. We integrate SnV -^{\,\textrm{-}}- centers into one-dimensional photonic crystal resonators and observe a 40-fold increase in emission intensity. The Purcell factor of the coupled system is 25, resulting in channeling of the majority of photons (90%90\%90%) into the cavity mode. Our results pave the way for the creation of efficient, scalable spin-photon interfaces based on SnV -^{\,\textrm{-}}- centers in diamond.
The construction of a quantum internet, an interconnected network which can distribute and store quantum states, is an essential step towards the development of next generation quantum technologies, including unconditionally secure communication, enhanced metrology, and distributed quantum computing. Development of these networks require functional nodes consisting of stationary registers with the capability of high-fidelity quantum information processing and storage, which efficiently interface with photonic qubits. This thesis outlines one emerging platform for building a quantum network node using the silicon-vacancy color center in diamond (SiV). Although previous work has established the SiV as a superior optical emitter capable of strong light-matter coupling, its feasibility as a quantum memory has remained elusive, primarily due to coupling to thermal phonons. This suggests two branching research directions: one where this coupling is investigated and characterized as a nanoscale sensor, and one where this coupling is supressed in order to realize the SiV as a quantum information processor. This thesis begins by highlighting the extreme phonon sensitivity as a metrological tool, where an ensemble of SiVs are used as an all-optical thermometer, capable of \SI{70}{\milli\kelvin} precision at room temperature. When incorporated into nanodiamonds, the sensing properties deviate by less than 1%1\%1% between nanodiamonds, enabling calibration-free thermometry for sensing and control of complex nanoscale systems. For quantum information tasks, coherent control of the SiV electronic spin qubit is enabled by suppressing phonon-induced dephasing by five orders of magnitude at temperatures below \SI{500}{\milli\kelvin}. By aligning the magnetic field along the silicon-vacancy symmetry axis, optical transitions are highly spin-conserving, allowing for single-shot readout of the qubit with high fidelity. This coherent control is used to demonstrate a spin coherence time T2>T_2 >T2​> \SI{13}{\milli\second} and spin relaxation time T1>T_1 >T1​> \SI{1}{\second} at \SI{100}{\milli\kelvin}. Finally, SiV quantum memories are packaged into a fully integrated quantum network node by incorporating SiV qubits into nanophotonic cavities. The efficient SiV-cavity coupling (with cooperativity C>30C > 30C>30) provides a nearly-deterministic interface between photons and the spin qubit, which enables heralded single-photon storage. Coherent coupling to nearby \cnuc\ nuclear spins (with nearly second-long coherence times) enables multi-qubit registers and paves the way towards large-scale quantum networks.
Photon-mediated interactions between quantum systems are essential for realizing quantum networks and scalable quantum information processing. We demonstrate such interactions between pairs of silicon-vacancy (SiV) color centers strongly coupled to a diamond nanophotonic cavity. When the optical transitions of the two color centers are tuned into resonance, the coupling to the common cavity mode results in a coherent interaction between them, leading to spectrally-resolved superradiant and subradiant states. We use the electronic spin degrees of freedom of the SiV centers to control these optically-mediated interactions. Our experiments pave the way for implementation of cavity-mediated quantum gates between spin qubits and for realization of scalable quantum network nodes.