Crackling in quantum processor silenced for seconds express
A superconducting qubit is like a finicky musical instrument: until you tune it, you won't get a clean note. But even after tuning, the pitch drifts. The culprits are tiny defects in the material that physicists call two-level systems. They're like invisible cracks that appear and disappear, throwing off the processor's rhythm.
The researchers built a setup based on a field-programmable gate array (FPGA)—a flexible microchip that can be reprogrammed for a task. With it, they measured qubit relaxation with sub-second resolution, catching changes faster than a second. It turned out the picture is the same across different qubits, different labs, and different fabrication processes. Moreover, these fast jumps were directly linked to errors in quantum gates — the basic operations that make up a computation. The link was confirmed using randomized benchmarking: a method that estimates gate fidelity by randomly sampling sequences of gates.
Now it's clear: tuning a quantum computer is not a one-time affair but a continuous battle against fast noise. Returning to the musical analogy: the instrument goes out of tune not in a month but in a couple of seconds, and the tuner has to work nonstop.
Original abstract
as written by the authors arXiv:2608.02086v1
Parasitic two-level-system (TLS) defects are a major source of energy relaxation and temporal instability in superconducting quantum processors. Our sub-second adaptive spectroscopy reveals telegraphic switching of TLSs with a characteristic timescale of a few seconds and spectral diffusion with diffusivity $$D \approx 0.9~\mathrm{MHz}^2/\mathrm{s}$$. These timescales are about $$3 \times 10^2$$ times faster than what is observed in conventional nonadaptive spectroscopy, which typically requires hours of measurement time. We resolve such fast dynamics on a field-programmable gate array (FPGA)-based controller that enables measurement of frequency- and time-resolved relaxations with sub-second temporal resolution in flux-tunable superconducting qubits. We observe similar defect dynamics across multiple qubits in independently fabricated devices measured in different laboratories. We correlate TLS-induced fluctuations with gate-level errors using randomized benchmarking. Our results reveal a previously inaccessible regime of frequency-resolved TLS dynamics and redefine the timescales relevant to TLS-aware characterization and calibration of superconducting quantum processors.
Two-level systems are not an abstraction but real atoms or groups of atoms that can exist in two states, like tiny switches. Their switching saps energy from the qubit, causing it to "forget" its quantum state.
In Neal Stephenson's novel The Diamond Age, nanomachines battle material defects at the atomic level — much like engineers must fight two-level systems in quantum processors.