Stability of superconducting resonators: Motional narrowing and the role of Landau-Zener driving of two-level defects

Abstract

Frequency instability of superconducting resonators and qubits leads to dephasing and time-varying energy loss and hinders quantum processor tune-up. Its main source is dielectric noise originating in surface oxides. Thorough noise studies are needed to develop a comprehensive understanding and mitigation strategy of these fluctuations. We use a frequency-locked loop to track the resonant frequency jitter of three different resonator typesone niobium nitride superinductor, one aluminum coplanar waveguide, and one aluminum cavity-and we observe notably similar random telegraph signal fluctuations. At low microwave drive power, the resonators exhibit multiple, unstable frequency positions, which, for increasing power, coalesce into one frequency due to motional narrowing caused by sympathetic driving of two-level system defects by the resonator. In all three devices, we identify a dominant fluctuator whose switching amplitude (separation between states) saturates with increasing drive power, but whose characteristic switching rate follows the power law dependence of quasi-classical Landau-Zener transitions.

Fluctuation Spectroscopy of Two-Level Systems in Superconducting Resonators

“…Interestingly, the time series reported in Ref. [25] display a single dominant TLS in all time series; in contrast, our data reveals the presence of an ensemble of Q-TLSs. It is also worth noting that their technique does not make it possible to measure Γ r int , which, in our experiments, allows us to gain further information about the resonators' noise processes.…”

“…Ref. [25] (i.e., higher than ours) and only at ∆ r,p = 0 (i.e., only on resonance). Within these constraints, their PSDs generally indicate a progressive noise reduction with increasing n .…”

“…In the works of Refs. [17,24,25], the authors have measured f r (t) up to approximately 3 h. While their measurement method allows them to access higher frequencies than ours, their lowest frequency is limited to larger than 10 −3 Hz. Additionally, their measurements are performed for n 2 in Refs.…”

“…Recent works have investigated time fluctuations in presence of a pump [17,24,25]. However, these authors only use a resonant pump and carry out measurements over short time periods, on the order of 3 h. Additionally, they only study the low and medium power regimes but not the pump off and high power regimes.…”

Feedback stabilization of the resonant frequency in tunable microwave cavities with single-photon occupancy

“…The scheme to achieve the suppression of intrinsic biasnoise follows the well-established technique of Pound-Drever-Hall (PDH) locking, extensively used in laser optics to stabilize laser sources during cavity reflection measurements [35]. Studies reporting the successful tracking of the resonant frequency fluctuations in superconducting microwave resonators utilizing this technique are also available in the literature [17,36,37]. By carefully calibrating the circuit at each stage to provide maximum signal-to-noise ratio (SNR), we suppress intrinsic 1/fnoise in the resonant frequency fluctuations over a bandwidth of 10 Hz, while driving the cavity at an average of merely a single photon.…”