Low-power Pound-Drever-Hall measurement of superconductiong resonators

As superconducting qubit coherence times reach an upper bound set by coupling to two-level system defects in amorphous dielectric substrates, rapid material loss characterization is needed to achieve significant improvement. Power sweeps of superconducting microwave resonators are commonly used to distinguish between loss channels induced in superconducting qubits, but the standard method of measurement using a simple vector network analyzer frequency sweep is inefficient and slow --- a full characterization of a single device takes days due to the very low power levels required. The Pound-Drever-Hall (PDH) technique, however, can extract the relevant resonator parameters in a fraction of the time. This technique generates a frequency modulated signal and locks the carrier frequency to resonance, allowing more efficient measurement of Q by varying the modulation frequency. Previous implementations relied on a microwave power diode for square-law detection of the transmitted signal, limiting the dynamic range of the technique. Here, we report on progress towards a low power implementation of PDH with direct, independent detection of the carrier and sidebands, for the goal of faster superconductiong resonator measurements.