Pound-Drever-Hall Readout for Superconducting Circuits

The circuit QED architecture for superconducting qubits relies on using the frequency of a microwave resonator to infer the state of a qubit. Currently, this is carried out by using a homodyne or heterodyne interferometer to measure the amplitude or phase of a microwave tone at the resonator frequency. This method, while extremely effective, is inherently sensitive to path length fluctuations in the measurement apparatus and to the phase stability of the microwave generators, necessitating high phase-stability generators and high-performance clock synchronization between both generators and digitizers. The Pound-Drever-Hall (PDH) method, which is common in optical-frequency laser-locking applications [1, 2], uses interference between multiple co-propagating tones to measure the detuning from cavity resonance. Recently it has been shown that PDH can improve measurement speeds in superconducting resonators enabling better characterization of TLS loss [3, 4]. In this talk, we will show that PDH can also benefit other aspects of superconducting circuits. In particular, we will show that the self-phase-referenced nature of PDH dramatically reduces the sensitivity of readout to microwave-generator phase stability.

[1] Drever, R. W. et al. Laser phase and frequency stabilization using an optical resonator. Applied Physics B 31, 97–105 (1983).

[2] Black, E. D. An introduction to pound–drever–hall laser frequency stabilization. American journal of physics 69, 79–87 (2001).

[3] T. Lindström, J. Burnett, M. Oxborrow, A Ya. Tzalenchuk; Pound-locking for characterization of superconducting microresonators. Rev. Sci. Instrum. 1 October 2011; 82 (10): 104706.

[4] John Pitten, Jim Phillips, Brandon Boiko, Josh Mutus, and Corey Rae McRae, “Rapid characterization of superconducting microwave resonators using the Pound-Drever-Hall technique,” APS March Meeting 2023.