Pound-Drever-Hall readout of superconducting lumped element resonators

The most prominent sources of noise in superconducting quantum systems may be
characterised as a system of two level fluctuators. This is a dominant source of
decoherence and linked with flicker noise in SQUIDs and other sensitive devices. In
this work a Pound.Drever.Hall measurement setup has been demonstrated for
readout of superconducting microwave resonators; which are well coupled to the
sources of noise. The setup provides direct measurement of frequency deviations at
resonance. Both qualitative and quantitative characterisation of frequency
fluctuations are performed by analysing frequency domain spectra and the time
domain Allan deviation. Analysis of the latter highlights a strong source of random
walk frequency noise, which is not readily visible in the power spectral density.
Atomic layer deposition (ALD) has been used to selectively deposit an additional
dielectric layer atop some niobium lumped element resonators. Measurements
between 30mK.900mK at low power levels between –80dBm and –100dBm have
been made to compare a bare resonator to one with an additional dielectric. These
measurements show noise to vary with both temperature and power while
highlighting additional frequency fluctuations (noise) in the covered resonators.