High-Temporal-Resolution Measurements of the Impacts of Ionizing Radiation on Superconducting Qubits

Abstract

We measure the effect of ionizing radiation on superconducting qubits with a timing resolution of 1 $μs$ using microwave kinetic inductance detectors (MKIDs) fabricated on the same substrate. We observe no correlation between two-level system (TLS) scrambling events and ionizing radiation events detected with the MKIDs, suggesting TLS scrambling events may not arise from ionizing radiation and instead the previously reported apparent correlation may be due to events without sufficient energy to trigger our MKIDs. We characterize the fast-time system recovery of transmons following a radiation event, where we observe the recovery of the enhanced qubit relaxation and excitation to be well-described by an exponential recovery to the baseline quasiparticle density, with a characteristic time of $13\pm1\ μ$s, and a peak quasiparticle density at the junction per deposited energy of $240/μm^3/MeV$. The fast recovery is consistent with literature reported values for Nb-based devices with direct injection of 2$Δ_{\text{Al}}$ phonons, demonstrating the recovery is strongly dependent on the proximity of niobium to the junction.

Figures (5)

• Figure 1: (a) Cartoon of the device. The chip is broken into quadrants, with each quadrant having eight qubits surrounding four MKIDs. The red regions indicated devices used to collect data shown in this manuscript. (b) Conditional probabilities between three MKIDs as labeled in (a). The center of MKID 4c is 2.3 mm away from MKID 1d. The conditional matrix is consistent with ionizing radiation events being chip wide, with MKID detection efficiencies of 0.90, 0.89, and 0.50 for MKID 1b, 1d, and 4c, respectively.
• Figure 2: MKID response curves. (a) The MKID magnitude change (top, green) and phase change (bottom, blue) following an ionizing radiation event. The S21 is an average over 1 $\mu$s, and is used as a the trigger for an on-chip ionizing radiation event. The fast recovery characteristic time is 35 $\mu$s, which sets the timescale for resetting the sensor, with a slower secondary recovery time of a couple milliseconds. A matched filter is used to provide the timestamp of an ionizing radiation event used as the reference time for the qubit measurements in Fig. \ref{['fig:Figure3']}. (b) MKID frequency response to temperature. The calibration curve fit allows for estimates of the kinetic induction fraction ($\alpha$ = 0.14), the critical temperature ($T_c$ = 2.2 K), and superconducting gap ($\Delta$ = 340 $\mu$eV). From this data, we estimate a frequency shift of 0.67 Hz/QP, suggesting the MKID needs 25 eV of total energy deposited in the grAl film to register a sufficient signal.
• Figure 3: (a) Response of the transmon $|1\rangle$-state (red) and $|0\rangle$-state (blue) to an ionizing radiation event. By continuously streaming a measurement of an MKID positioned near a transmon, and averaging over 3731 (606) events during 90 (17) hours of data collection, the averaged transmon response for enhanced decay (excitations) can be determined. Our recovery time for the enhanced qubit decay (excitation) following an event has a characteristic time of $13\pm1\ \mu$s ($8.3^{+1.7}_{-1.2}\ \mu$s) in our Nb-based devices. (b) The extracted average quasiparticle density at the junction from $P(1)$. We infer a peak quaisparticle density, ($n_{\rm qp}(\delta\text{t=0})$), is $85^{+9}_{-8} / \mu m^{-3}$, averaging over all detected events.
• Figure 4: TLS dynamics following ionizing radiation events. (a) Time difference of the closest ionizing radiation event that precedes a TLS scrambling event. The blue bars are the observed time difference. The orange line is the expected curve for uncorrelated data. None of the 18,454 ionizing radiation events occurred within 100 ms of any of the 371 TLS scrambling events. (b-d) Example time dynamics of a weakly-coupled (b-c) and strongly-coupled (d) TLS. The orange vertical dashed lines are events detected by the MKID. The blue dots are the data averaged over 100 ms (b,d) and 10 ms (c). The black curve is a further averaged window to provide a visual guide.
• Figure S1: MKID signal vs time relative to event. The MKID $|\text{S21}|$ near an event with 200-ns averaged time-bins, with an 80-$\mu$s-long trace (left) and a zoomed-in 3-$\mu$s-long trace (right). The MKID response shows a rising edge of 600--700 ns, matching the expectation from the measured resonance linewidth (250 kHz).