Coherent On-Chip Suppression of Resonant Pump Light for High-Fidelity Diamond Spin–Photon Entanglement

Remote heralded entanglement between group-IV color centers in diamond is a key step toward scalable quantum networks, computation, and sensing. However, Bell-state measurement schemes require strong resonant excitation to prepare the emitters, leading to pump photons that scatter into the collection mode and produce false detections, degrading entanglement fidelity. We develop a reconfigurable photonic integrated circuit (PIC) that both delivers the resonant pump and collects spin-entangled photons, enabling active, on-chip suppression of the resonant pump. Orienting the diamond orthogonal to the pump waveguides minimizes coupling between the pump’s TE mode and the diamond waveguide’s TE mode, and finite-element simulations predict up to 70 dB suppression with optimized waveguide width. Further extinction is achieved by routing two pump paths through an on-chip Mach–Zehnder interferometer (MZI) and tuning their relative phase and amplitude for coherent cancellation, which could in principle futher yield up to 100 dB suppression and enable high-fidelity spin-photon entanglement. Preliminary fiber-based MZI measurements demonstrate 7 dB coherent suppression, marking an early step toward fully integrated, phase-controlled pump cancellation in diamond systems.