  Scalable quantum computing can become a reality with error correction,
provided coherent qubits can be constructed in large arrays. The key premise is
that physical errors can remain both small and sufficiently uncorrelated as
devices scale, so that logical error rates can be exponentially suppressed.
However, energetic impacts from cosmic rays and latent radioactivity violate
both of these assumptions. An impinging particle ionizes the substrate,
radiating high energy phonons that induce a burst of quasiparticles, destroying
qubit coherence throughout the device. High-energy radiation has been
identified as a source of error in pilot superconducting quantum devices, but
lacking a measurement technique able to resolve a single event in detail, the
effect on large scale algorithms and error correction in particular remains an
open question. Elucidating the physics involved requires operating large
numbers of qubits at the same rapid timescales as in error correction, exposing
the event's evolution in time and spread in space. Here, we directly observe
high-energy rays impacting a large-scale quantum processor. We introduce a
rapid space and time-multiplexed measurement method and identify large bursts
of quasiparticles that simultaneously and severely limit the energy coherence
of all qubits, causing chip-wide failure. We track the events from their
initial localised impact to high error rates across the chip. Our results
provide direct insights into the scale and dynamics of these damaging error
bursts in large-scale devices, and highlight the necessity of mitigation to
enable quantum computing to scale.
