Quantum Network Nodes Based on Diamond Qubits with an Efficient Nanophotonic Interface

Quantum networks require functional nodes consisting of stationary registers with the capability of high-fidelity quantum processing and storage, which efficiently interface with photons propagating in an optical fiber. We report a significant step towards realization of such nodes using a diamond nanocavity with an embedded silicon-vacancy (SiV) color center and a proximal nuclear spin. Specifically, we show that efficient SiV-cavity coupling (with cooperativity $C>30$) provides a nearly deterministic interface between photons and the electron spin memory, featuring coherence times exceeding 1 ms. Employing coherent microwave control, we demonstrate heralded single photon storage in the long-lived spin memory as well as a universal control over a cavity-coupled two-qubit register consisting of a SiV and a proximal ${}^{13}\mathrm{C}$ nuclear spin with nearly second-long coherence time, laying the groundwork for implementing quantum repeaters.

Figure 1

(a) Schematic of a SiV-nanophotonic quantum register. A diamond nanostructure with embedded SiV centers and ancillary ${}^{13}\mathrm{C}$ nuclei are coupled via a waveguide to a fiber network. Spins are controlled by an on-chip microwave CPW at 0.1 K. (b) Scanning electron micrograph of several devices. The gold CPW is designed to localize microwave fields around the cavity center (green inset).

Figure 2

(a) Level structure of SiV spin-cavity system. The SiV optical transition at 737 nm is coupled to the nanocavity with detuning $\mathrm{\Delta }$. Spin conserving transitions (purple, orange) are split by an external magnetic field (${\mathbf{B}}_{\mathrm{ext}}$), at an angle $\alpha$ with respect to the SiV symmetry axis. Photons at frequency $f_Q$ are preferentially reflected by the cavity when the SiV is in state $|\uparrow ⟩$. Microwave fields at frequency $f_{\uparrow \downarrow }$ coherently drive the qubit states. (b) Spin-dependent reflection spectrum for $B_{\mathrm{ext}}=0.19\mathrm{T}$, $\alpha \approx \pi /2$ at $\mathrm{\Delta }=0.25\kappa$. Probing at the point of maximum contrast ($f_Q$) results in high-fidelity spin-photon correlations and single-shot readout (inset, $F=0.92$). (c) SiV spin coherence time $T_2(N=64)>1.5\mathrm{ms}$ with dynamical decoupling. (inset) Fast microwave Rabi driving of the SiV spin.

Figure 3

(a) Schematic for heralded photon storage. First, the SiV is prepared in a superposition of two spin states, then a photonic qubit is reflected off the device, flipping the spin state between the time bins. Detection of the photon in the $X$ basis ($X_p$) heralds mapping of the qubit onto the spin (green box). The qubit is stored in memory for $2T=20\mu \mathrm{s}$ with dynamical decoupling on the SiV (pink box). (b) Spin-photon storage fidelity. The state $|\pm ⟩=|\downarrow ⟩\pm |\uparrow ⟩$ is mapped onto the SiV, with average fidelity $\mathcal{F}=87(6)\%$.

Figure 4

(a) Schematic of an SiV coupled to nearby ${}^{13}\mathrm{C}$ nuclear spins. Orange (purple) vectors are conditional fields when the SiV is in state $|\uparrow ⟩$ ($|\downarrow ⟩$). (b) $XY8–2$ spin-echo. (Top) envelope for spin-echo shows a $T_2(N=16)=603\mu \mathrm{s}$. $XY8–2$ at early times (Center) exhibits collapses in signal due to interaction with nuclear spins. Single ${}^{13}\mathrm{C}$ cannot be identified at early times (red inset), but can be separated from the bath at long times (green inset). (c) Ramsey measurement on the ${}^{13}\mathrm{C}$ nuclear spin. The nuclear spin precesses at a different Larmor frequency depending on whether the SiV is prepared in $|\uparrow ⟩$ (orange) or $|\downarrow ⟩$ (purple). Coherent oscillations persist for $T_2^{*}>2\mathrm{ms}$ [25]. (d) Spin echo on ${}^{13}\mathrm{C}$, revealing $T_2>0.2\mathrm{s}$ (e) Reconstructed amplitudes for a cnot gate transfer matrix.