Description
Cosmic rays and environmental ionizing radiation are emerging as important error mechanisms in quantum technologies, particularly in superconducting circuits where particle-induced quasiparticles, phonons, and correlated relaxation events can produce non-local faults. Similar impulsive energy-deposition processes may also perturb precision oscillators, resonators, and timing systems used for quantum control, synchronization, and low-noise readout. Here we investigate whether cosmic-ray backgrounds leave measurable signatures in the frequency fluctuations of high-performance oven-controlled quartz bulk-acoustic-wave oscillators.
We compare long-duration measurements of two phase-locked 10 MHz OCXOs operated above ground and in a low-muon-background underground environment approximately 1 km below the surface. Standard second-order metrics, including power spectral density and Allan deviation, show no statistically compelling separation between the two environments after removal of spurious harmonic artefacts. In contrast, multi-scale sample entropy, $S_E(\tau)$, and a modified entropy metric, $\tilde{S}_E(\tau)$, reveal a pronounced divergence at longer effective integration times. The underground data exhibit lower entropy, corresponding to increased temporal predictability, while the above-ground data retain stronger non-stationary structure.
These results indicate that the dominant environmental difference is not simply a change in fluctuation amplitude, but a change in the temporal organization of oscillator noise. This is consistent with a reduction of rare, radiation-linked relaxation or impulse-like events underground, although definitive attribution to muons requires future coincidence measurements with particle detectors. More broadly, the work demonstrates that entropy-based diagnostics can reveal non-Gaussian and intermittent error channels that are largely invisible to conventional stability metrics. Such tools may be valuable for identifying radiation-induced correlated errors in quantum processors, cryogenic resonant sensors, and precision timing infrastructure for quantum science and technology.
| I am the presenting author | Yes |
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