Qubit error bursts in superconducting quantum processors of Quantum Inspire: quasiparticle pumping and anomalous time dependence

This study investigates qubit error bursts in superconducting processors, confirming their consistency with ionizing radiation while identifying two novel, device-specific signatures in Dolan junction processors: a quasiparticle pumping mechanism that accelerates equilibrium recovery under increased $\pi$-pulsing and an anomalous burst rate surge occurring days after cooldown.

The Gist

Imagine you have a super-advanced, microscopic city made of electricity, where tiny messengers (called qubits) carry information. This city is the heart of a quantum computer. But this city is incredibly fragile. If even a single cosmic ray (a high-energy particle from space) or a tiny bit of natural radioactivity from the building materials hits the city, it causes a "tsunami" of energy that knocks the messengers off their paths. This is called a qubit error burst.

For a long time, scientists knew these tsunamis happened, but they didn't fully understand the local weather patterns inside the city. A new study by researchers at Delft University (Quantum Inspire) looked at two nearly identical cities, Starmon-5 and Starmon-7, to see how they handle these disasters.

Here is what they found, explained with some everyday analogies:

1. The Two Cities and the "Ghost" Problem

The researchers built two quantum processors that looked almost the same from the outside. They were made in the same factory, packaged in the same boxes, and even cooled down in the same type of ultra-cold fridges.

However, they had one secret difference: the Josephson Junctions (the tiny switches that make the qubits work).

• Starmon-5 used "Dolan" junctions (an older, specific design).
• Starmon-7 used "Manhattan" junctions (a newer, cleaner design).

When a cosmic ray hits, it creates a shower of energy that turns into Quasiparticles (QPs). Think of QPs as mischievous ghosts that float around the city, stealing energy from the messengers and causing them to drop their messages (errors).

2. The First Surprise: The "Pump" Effect

The researchers noticed something weird happening only in Starmon-5.

They tried to speed up their testing by sending more "check-up" signals (called $\pi$ pulses) to the qubits. Usually, you'd think checking more often would just take more time. But in Starmon-5, checking more often actually cleared the ghosts away faster.

The Analogy:
Imagine the "Dolan" junctions in Starmon-5 have a built-in trash can (a trap) right next to the main road.

• When the researchers sent a "check-up" pulse, it was like giving the ghosts a gentle shove.
• Because of the specific shape of the Dolan junction, this shove didn't just move the ghosts; it actively pumped them into the trash can, where they got stuck and disappeared.
• Starmon-7 didn't have this trash can. So, no matter how many times they checked, the ghosts just kept floating around, and the errors took much longer to clear up.

This is a huge discovery because it suggests that by changing how we "poke" the qubits, we might be able to actively clean up errors in real-time, but only if the hardware is designed with the right "trash cans."

3. The Second Surprise: The "Surge"

The second discovery was even stranger and only happened in Starmon-5.

Usually, after you cool down a quantum computer, the error rate settles into a steady rhythm. But in Starmon-5, after sitting quietly for weeks, the error rate would suddenly explode.

The Analogy:
Imagine a calm lake. For weeks, the water is still. Then, suddenly, a massive wave crashes in (the Surge).

• This wave happened days or weeks after the computer was turned on.
• The error rate jumped by 10 to 100 times!
• Then, just as mysteriously, the lake didn't just go back to normal; it became super calm. The error rate dropped to almost zero and stayed there for weeks.
• The only way to "reset" the lake and make the waves come back was to warm the computer up and cool it down again (a "thermal cycle").

The scientists don't know why this happens. It's like a clock that runs perfectly for a month, suddenly chimes loudly for an hour, and then stops ticking entirely for the next month. They suspect something inside the chip is slowly changing its state, but they haven't found the trigger yet.

Why Does This Matter?

Quantum computers need to fix their own mistakes (called Quantum Error Correction) to be useful. But if a cosmic ray hits and causes a "tsunami" that knocks out all the messengers at once, the computer can't fix it.

This paper teaches us two big lessons:

1. Design Matters: The way you build the tiny switches (junctions) changes how the computer recovers from disasters. Some designs have built-in "vacuum cleaners" for errors.
2. Time Matters: Errors aren't random; they have strange patterns. If we can predict or understand these "surges," we might be able to pause our calculations during the bad times and only run them during the "super calm" periods.

In short, the researchers found that quantum computers have their own unique "personalities" and hidden behaviors that we are just starting to understand. By studying these quirks, we can build better, more reliable quantum machines for the future.

Technical Summary

Here is a detailed technical summary of the paper "Qubit error bursts in superconducting quantum processors of Quantum Inspire: quasiparticle pumping and anomalous time dependence."

1. Problem Statement

Superconducting quantum processors are susceptible to qubit error bursts triggered by ionizing radiation (cosmogenic muons and local radioactivity). When high-energy particles strike the substrate, they generate phonons and electron-hole pairs. These phonons propagate across the chip, breaking Cooper pairs in the superconducting layers to create quasiparticles (QPs).

• The Mechanism: QPs tunnel across Josephson junctions (JJs), absorbing qubit transition energy and causing relaxation errors ($T_1$).
• The Challenge: These errors are correlated (affecting multiple qubits simultaneously), posing a severe threat to Quantum Error Correction (QEC) and fault-tolerant computing.
• The Gap: While strategies exist to mitigate radiation (e.g., underground facilities, phonon traps), the specific dynamics of error bursts, recovery times, and the impact of different JJ fabrication techniques on QP density and pumping remain under-explored.

2. Methodology

The authors investigated two 5- and 7-qubit transmon processors from the Quantum Inspire platform:

• Devices: Starmon-5 (S-5) and Starmon-7 (S-7).
  • Similarities: Identical design, packaging, and connectivity.
  • Differences: S-5 uses Dolan-bridge JJs; S-7 uses Manhattan-style JJs.
• Experimental Setup:
  • Measurements were performed in two different dilution refrigerators (A and B) to distinguish device-specific characteristics from refrigerator-dependent noise.
  • Detection Scheme: A repeated subcircuit involving $\pi$-pulsing, idling, and readout. A "qubit error" is defined as two consecutive '0' outcomes.
  • Burst Identification: A two-pass post-processing algorithm using template matching to distinguish true radiation-induced bursts (long recovery time, $t_{rec}$) from baseline relaxation/readout errors (short recovery time).
• Variables Tested:
  • Cycle time ($t_{cycle}$) and the rate of $\pi$-pulsing.
  • Presence of lead shielding.
  • Long-term monitoring over weeks/months to observe time-dependent behaviors.

3. Key Contributions & Results

A. Device-Specific Recovery Dynamics

• Recovery Time ($t_{rec}$): The time for the system to return to baseline after a burst is device-specific, not refrigerator-dependent.
  • S-5 (Dolan): $t_{rec} \approx 2$ ms.
  • S-7 (Manhattan): $t_{rec} \approx 250$ $\mu$s.
• Cause: The 8-fold difference is attributed to the geometry of the JJ electrodes. The Dolan-bridge fabrication creates a built-in Quasiparticle (QP) trap, whereas the Manhattan style does not.

B. Quasiparticle Pumping (The "Pumping" Signature)

• Observation: In S-5, increasing the rate of $\pi$-pulsing (either by shortening the cycle time or inserting extra pulses during idle time) significantly shortens the recovery time ($t_{rec}$). This effect was absent in S-7.
• Mechanism:
  • The Dolan-bridge JJ consists of a thin Al layer and a thick Al layer with different superconducting gaps ($\Delta$).
  • When a qubit is in the excited state ($|1\rangle$), a QP on the thick layer can tunnel across the junction, relaxing the qubit to $|0\rangle$.
  • The QP then diffuses and tunnels onto the thick layer on the other side, where it becomes trapped due to the energy gap difference.
  • Repeated $\pi$-pulses re-excite the qubit, driving a continuous cycle of QPs from the active junction area into the trap. This "pumping" clears the active region of QPs faster, reducing error duration.
• Significance: This demonstrates that specific JJ fabrication techniques can actively mitigate radiation-induced errors through pulse-controlled QP pumping.

C. Anomalous Time Dependence (The "Surge")

• Observation: In S-5 only, the error burst rate ($\gamma_{kept}$) exhibits a strange, non-stationary behavior:
  1. Steady State: After cooldown, the rate is stable for days/weeks.
  2. Surge: The rate suddenly spikes by 10–100x for several hours, accompanied by a drop in average $T_1$.
  3. Suppression: The rate then monotonically decreases over days to a value 100x lower than the initial post-cooldown rate.
  4. Reset: This low-rate state persists until a thermal cycle (warming to 4K or room temperature) resets the system.
• Characteristics:
  • The surge is device-specific (observed in S-5, not S-7).
  • It is refrigerator-dependent (occurs in both A and B, but with different onset times and magnitudes).
  • Adding GE varnish to the back of S-5 (to improve thermal contact) appeared to suppress the surge in subsequent cooldowns.
• Mechanism: The physical cause is unknown. It is not linked to radiation flux changes or standard thermalization. The authors hypothesize an on-chip mechanism, possibly related to the redistribution of impurities or stress in the substrate/packaging, but invite further investigation.

D. Radiation Shielding

• Adding a rudimentary lead shield reduced the burst rate by ~21–24% in both devices, confirming the ionizing radiation origin of the bursts.

4. Significance and Implications

1. Fabrication Matters: The study proves that JJ fabrication techniques (Dolan vs. Manhattan) have profound effects on error dynamics. The Dolan-bridge design inadvertently creates a QP trap that can be exploited for active error mitigation via pulse sequences.
2. Active Mitigation Strategy: The discovery of QP pumping suggests a new control strategy: by optimizing pulse sequences, processors could actively "pump" QPs away from sensitive junctions, reducing the duration of correlated error bursts.
3. Anomalous Behavior: The "surge" phenomenon highlights that error rates are not static. Long-term stability in quantum processors may be compromised by unknown on-chip mechanisms that evolve over days. Understanding this is critical for reliable QEC scheduling.
4. Benchmarking: The paper provides a rigorous benchmark for error burst rates in state-of-the-art processors, showing that while rates are comparable to other leading labs, the dynamics (recovery and time-dependence) vary significantly based on device architecture.

Conclusion

This work moves beyond simply measuring error rates to understanding the microscopic dynamics of error bursts. It identifies a novel quasiparticle pumping mechanism inherent to Dolan-bridge junctions that can be harnessed to reduce error duration. Furthermore, it uncovers a mysterious anomalous time dependence (the surge) that challenges current models of radiation-induced errors, suggesting that future quantum processors must account for both fabrication-induced traps and unknown long-term on-chip evolution to achieve fault tolerance.