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

TL;DR

This work tackles the challenge of scalable spin–photon interfaces for quantum networks by implementing a planar GaP-on-diamond hybrid photonic platform. A 1-D GaP photonic crystal nanobeam on diamond is designed via a two-stage optimization to maximize cooperativity, achieving a simulated $Q_i$ of about $3 imes10^5$ and a mode volume of $V=2(rac{ ilde{ ho}}{ ext{n}})^3$, with an overlap supporting $C>1$ for near-surface SiV centers. The authors demonstrate large-scale fabrication (thousands of devices) and high transfer yield to diamond, and report high-cooperativity coupling to two SiV centers with $g/2π>2.1$ GHz and $rac{oldsymbol{oldsymbol{ extgamma}}}{2 ext{π}}≈100$–$190$ MHz, yielding $C>1$ in multiple independent measurements, as well as spin-dependent transmission switching and single-shot readout with $F≈96 ext%$. The results establish GaP-on-diamond as a scalable, nonlinear, planar platform for quantum networking, with clear pathways to further enhancements in $Q$, dipole overlap, and integrated functionalities such as on-chip frequency conversion and phononics.

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.

Figures (5)

• Figure 1: Cavity design. (A) Schematic of the GaP-on-diamond cavity design. (B) Simulated field profile of the fundamental mode at the center of the nanobeam. (C) Simulated cavity mode overlap versus $z$ at the center of the cavity for an SiV center oriented along a perpendicular $\langle111\rangle$ axis. The gray Gaussian curve indicates the estimated implanted SiV distribution with mean depth $\ev{z}=-20.0$ nm and straggle $\sigma=6.5$ nm. (D) The expected number of coupled SiV with $C>0.1$ (orange), $1$ (red), and $10$ (purple) versus total quality factor $Q$. The calculation assumes an SiV density of $50$ µ m$^{-2}$ and an intrinsic radiative efficiency $\gamma_0/\gamma = 0.07$chakravarthi2023stamp.
• Figure 2: Large-scale device integration. Optical images of (A) the 2-mm diamond chip and (B) a close up of one of the 16 sets of devices. Thru/drop devices are indicated by the red/blue boxes. SEM images of the (C) thru and (D) drop devices. Texture is due to a gold/palladium coating for imaging. (E) Representative transmission spectra at room temperature. The primary resonance is highlighted in gray. (F) Quality factor versus coupling coupling parameter for all measured devices ($N=321$). The color brightness denotes the device set. (G) Distribution of the quality factors and resonance wavelengths at room temperature. Devices within the gray band between $740$--$742$ nm can be tuned onto resonance with SiV centers at cryogenic temperatures.
• Figure 3: Dipole-induced scattering. (A) Illustration of the transmission measurement in a drop device. (B) Transmission and PLE spectra when in near resonance with the $C$ lines. A strong modulation of the transmission is observed for two blue-shifted SiV centers (right: expanded view). (C) Single-shot readout of the SiV charge state by resonant scattering off the DIT peak. Counts are collected in 100-ms bins. (D) Intensity distribution of the single-shot readout. Vibrations cause additional broadening of the distribution beyond shot noise.
• Figure 4: High-cooperativity coupling. (A) High-resolution transmission spectra at the SiV 1 and SiV 2 resonances for different SiV-cavity detunings. Fitting to the model enables determination of the cooperativity. (B) High-resolution PLE scans on SiV 1 and 2 showing broadened linewidths when the cavity is near resonant. (C) Dependence of the DIT transmission dip contrast near resonance as a function of input power. (D) Linewidths of SiV 1 and 2 as the cavity is tuned into and out of resonance. The fit assumes the measured cavity linewidth and is in good agreement with the data.
• Figure 5: An efficient spin-photon interface. (A) States and optical transitions of an SiV center in an applied magnetic field at angle $\alpha$ from the symmetry axis. (B) Transmission and PLE spectra on SiV 1 with clear resolution of the spin conserving transitions $C_2$ and $C_3$. The two weaks peaks on either end are attributed to additional SiV centers. (C) Spin-relaxation curve obtained via PLE measurements on SiV 1, pulse sequence shown in the inset. (D) Spin-dependent switching of the cavity transmission and PLE signal. (E) A typical time trace of the cavity transmission demonstrating single shot readout of the quantum jumps of the SiV spin state (left). The intensity distribution of 15 time traces with a bimodal Poisson fit (right). The dashed line at 30 cts/bin corresponds to the optimal discrimination threshold with fidelity $F=96$%. (F) Distribution of the time between quantum jumps fit to an exponential distribution.