Curator's Take
AI Commentary
This article shows a practical upgrade to the work‑horse Pound‑Drever‑Hall technique by using an optimal linear combination of the three interrogation tones, thereby cancelling the notorious systematic errors from residual amplitude modulation and demodulation‑phase drift. By demonstrating error‑immune cavity readout in both optical and microwave domains—and even achieving single‑shot measurement of a superconducting qubit—the work promises more reliable frequency stabilization for quantum processors, sensors, and clock networks that rely on ultra‑high‑Q resonators. The approach dovetails with recent pushes toward hardware‑efficient error mitigation, though its performance will still depend on the quality of tone generation and detector linearity in real‑world deployments.
— Mark Eatherly
Summary
Precise measurement of the resonance frequencies of microwave and optical cavities is a foundational capability across quantum technologies. One of the highest-performance methods currently available is Pound-Drever-Hall (PDH) spectroscopy, which relies on a three-tone interrogation and eliminates sensitivity to path-length fluctuations. However, PDH is known to suffer from systematic offsets due to residual amplitude modulation and demodulation-phase errors. Here we leverage an optimal linear combination of the phases of the three interrogation tones to implement a method for robust cavity metrology which eliminates significant systematic-error sensitivities of PDH. We experimentally demonstrate robust cavity measurements in both the optical and microwave frequency regime, showing insensitivity to sideband imbalance and demodulation phase, as well as demonstrating single-shot readout of a cavity-coupled superconducting qubit.