Sub‐Shot‐Noise Laser Phase Stabilization via Cavity‐Coupled Mach–Zehnder Interferometer

Laser phase noise fundamentally limits precision across modern optics applications. Existing stabilization protocols suffer a fundamental trade‐off: they are constrained by either shot noise or technical noise, and none simultaneously eliminate amplitude noise while preserving quantum enhancement. We overcome this limitation by injecting squeezed vacuum into a cavity‐coupled Mach–Zehnder interferometer, thereby realizing sub‐shot‐noise phase stabilization with intrinsic immunity to amplitude noise—enabled by the high common‐mode rejection ratio of balanced detection. With –11 dB squeezing, we achieve 6.2 dB quantum‐enhanced suppression at 30 kHz over 5–200 kHz. This architecture provides a practical path to quantum‐compatible feedback for next‐generation ultra‐stable lasers.