Researchers have identified the mechanism behind persistent error bursts in superconducting quantum computers, even when protected by gap engineering. The findings are published in “Physical Review X” by Vladislav Kurilovich and colleagues at Google Quantum AI.
Superconducting qubits are vulnerable to ionizing radiation from space or environmental sources. Radiation particles interact with the silicon substrate, generating quasiparticles that disrupt qubit operation. Gap engineering creates an energy barrier in the superconducting material to prevent quasiparticle tunneling.
Despite this defense, sudden widespread errors affecting multiple qubits simultaneously have been observed. The cause was previously unknown.
The team developed a rapid measurement protocol using a 72-qubit Willow processor. They performed repetitive qubit measurements every few microseconds to capture error bursts in real time.
Results showed that while quasiparticles do not tunnel through the energy barrier, they induce qubit frequency shifts of up to 3 MHz. This detuning causes qubits to lose synchronization with the microwave control pulses. The consequence is accumulated phase errors and incorrect shifts in the quantum state.
This mechanism explains a previous observation from Google experiments: a logical error rate (LER) floor, at which error correction no longer improves performance despite additional mitigation efforts.
As a countermeasure, the researchers implemented echo pulses, additional control operations that cancel unwanted phase shifts. This approach reduces system sensitivity to radiation-induced frequency fluctuations.
