1/f noise of Josephson-junction-embedded microwave resonators at single photon energies and millikelvin temperatures

TL;DR

This study measures 1/f frequency noise in superconducting aluminum resonators with and without Josephson junctions at millikelvin temperatures, finding the junctions do not add noise and setting limits on qubit dephasing times.

Contribution

It demonstrates that Josephson junctions do not increase frequency noise in superconducting resonators, establishing upper bounds on critical current fluctuations at low temperatures.

Findings

Josephson junctions do not add significant frequency noise.

Upper limit for fractional critical current fluctuations is $10^{-8}$ at 1 Hz.

Potential dephasing times for qubits range from 40 to 1400 microseconds.

Abstract

We present measurements of 1/f frequency noise in both linear and Josephson-junction-embedded superconducting aluminum resonators in the low power, low temperature regime - typical operating conditions for superconducting qubits. The addition of the Josephson junction does not result in additional frequency noise, thereby placing an upper limit for fractional critical current fluctuations of $10^{-8}$ (Hz$^{-1/2}$) at 1 Hz for sub-micron, shadow evaporated junctions. These values imply a minimum dephasing time for a superconducting qubit due to critical current noise of 40 -- 1400 $\mu$s depending on qubit architecture. Occasionally, at temperatures above 50 mK, we observe the activation of individual fluctuators which increase the level of noise significantly and exhibit Lorentzian spectra.