Real-time detection of correlated quasiparticle tunneling events in a multi-qubit superconducting device

Technical Summary: Real-time detection of correlated quasiparticle tunneling events in a multi-qubit superconducting device

Problem Statement
Quasiparticle (QP) tunneling across Josephson junctions is a significant source of decoherence and correlated errors in superconducting quantum circuits. While recent material improvements have enhanced qubit coherence, the scaling of quantum processors has heightened the need to understand and mitigate QP-induced errors, particularly "bursts" of QPs that create temporally and spatially correlated errors. Such correlations violate the assumptions of quantum error-correction codes like the surface code. These bursts are often attributed to ionizing radiation (cosmic rays, radioactive materials) or phonon stress release, which can simultaneously degrade relaxation times across multiple qubits. A critical gap exists in the ability to detect these events in real-time on individual devices to distinguish between uncorrelated background tunneling and correlated burst events, and to identify their physical origins (e.g., ionizing vs. phonon-only events).

Methodology
The authors developed a real-time detection scheme using two moderately charge-sensitive transmon qubits (QPD1 and QPD2) co-housed on a single silicon chip and coupled to a common coplanar waveguide.

• Device Architecture: The two transmons, separated by 1.5 mm, share a capacitor geometry but have different junction areas. They are fabricated with aluminum Josephson junctions on a silicon substrate with an aluminum ground plane. The device is shielded in an indium-sealed copper enclosure with high-energy radiation drain filters to suppress external high-frequency radiation.
• Detection Mechanism: The system utilizes coherent microwave scattering. A continuous drive tone is applied at the transition frequencies of the qubits, which depend on the charge parity ($P = \pm 1$) of the superconducting islands. When a QP tunnels on or off an island, the charge parity flips, causing an instantaneous shift in the qubit's transition frequency.
• Measurement Protocol: The researchers continuously monitor the transmitted field through the waveguide. By digitally demodulating the signal at the parity-dependent transition frequencies ($\omega^\pm_{01}$), they resolve abrupt transitions between two discrete signal levels corresponding to even and odd charge parity states.
• Data Analysis: Time traces were acquired with a 100 $\mu$s bin size. The authors employed a thresholding protocol to identify switching events and used kernel density estimation (KDE) to distinguish between baseline switching and "burst" intervals (defined as sequences of jumps with inter-event separations $< 5$ ms). They performed cross-correlation analysis on the burst-count time series to determine inter-device correlations.

Key Contributions and Results

1. Real-Time Detection of QP Tunneling: The study successfully demonstrates the real-time detection of individual QP tunneling events with a temporal resolution of tens of microseconds. The background tunneling rates were measured at the single-hertz level ($\Gamma^{(1)}_{slow} \approx 5.28$ s$^{-1}$ and $\Gamma^{(2)}_{slow} \approx 3.61$ s$^{-1}$).
2. Identification of Burst Regimes: The authors identified intermittent "QP bursts" where the tunneling rate increases by approximately three orders of magnitude (up to $\sim 4000$ s$^{-1}$). These bursts have a characteristic lifetime of approximately 7 ms.
3. Correlation Analysis:
   • Uncorrelated Background: Outside of burst intervals, switching events are well-described by independent Poissonian processes with no measurable temporal correlation between the two devices.
   • Correlated Bursts: A significant subset of bursts (47% of QPD1 bursts and 76% of QPD2 bursts) occurs coincidentally in both detectors. Cross-correlation analysis ($R_{12}(\tau)$) shows a pronounced peak at zero delay, confirming temporal correlation across the devices.
   • Uncorrelated Bursts: Some bursts occur in only one detector, indicating that not all burst events are spatially correlated.
4. Offset-Charge-Shifting Events: The authors identified a rare subclass of correlated bursts (occurring at $\sim 0.43$ h$^{-1}$ for coincident events) accompanied by a discrete shift in the effective offset charge ($n_g$). This shift manifests as a displacement of the parity-state clusters in the IQ plane and a change in the parity splitting. These events are consistent with ionizing radiation events that perturb the local electrostatic environment.
5. Distinction of Mechanisms: The results suggest that while some bursts are accompanied by charge shifts (likely ionizing events), many correlated bursts occur without detectable charge shifts. This implies that phonon-mediated pair-breaking processes, which do not alter the charge landscape, also contribute to correlated QP dynamics.

Significance and Claims
The paper establishes a practical and extensible method for identifying quasiparticle bursts in superconducting circuits, distinguishing between their correlations and spatial structure. The authors claim that this approach advances the route to suppressing correlated errors in superconducting quantum processors by providing a diagnostic tool that goes beyond transient coherence-time degradation measurements. By resolving individual tunneling events, the method offers a more sensitive benchmark for evaluating mitigation strategies such as gap engineering and phonon trapping. The work highlights that correlated errors can arise from multiple physical mechanisms (ionizing radiation and phonon stress), necessitating a multi-faceted approach to error mitigation in large-scale quantum processors. The authors note that while their current setup provides spatial resolution between two devices, integrating more detectors could further enhance the spatial mapping of these events.