lossy stainless coax. It was coupled to the qubit via an on-chip $1\ {\ensuremath{{\ensuremath{\mathrm{fF}}}}}$ capacitor [$C_{r\!f}$]{} \[see Fig. \[FDevice\](a)\]; while the large impedance mismatch this produced potentially improved the isolation, it did make an independent calibration of the power reaching the qubit difficult. The device was mounted in a superconducting aluminum box that was attached to the mixing chamber of a dilution refrigerator with a base temperature of 20 mK. The refrigerator, located in an rf shielded room, was surrounded by a mu-metal cylinder. In addition, the measurement and bias lines were protected from noise at room temperature by discrete $LC$ filters and copper powder filters at the mixing chamber.

The escape rate of the qubit junction was measured by simultaneously increasing $I_b$ and $I_f$ as described above, while monitoring the voltage on the current bias line of the SQUID. We measured the time interval between the start of the ramps and when the system tunneled to the voltage state. Repeating this procedure many times ($\sim 10^5$) at a rate of about 250 Hz yielded a histogram of switching times, from which the escape rate could be calculated.[@Fulton74a] Decreasing the repetition rate did not yield a significant change in the 