Disentangling the sources of ionizing radiation in superconducting qubits

Abstract

Radioactivity was recently discovered as a source of decoherence and correlated errors for the real-world implementation of superconducting quantum processors. In this work, we measure levels of radioactivity present in a typical laboratory environment (from muons, neutrons, and $$\gamma $$ γ -rays emitted by naturally occurring radioactive isotopes) and in the most commonly used materials for the assembly and operation of state-of-the-art superconducting qubits. We present a GEANT-4 based simulation to predict the rate of impacts and the amount of energy released in a qubit chip from each of the mentioned sources. We finally propose mitigation strategies for the operation of next-generation qubits in a radio-pure environment.

Conceptual study of a two-layer silicon pixel detector to tag the passage of muons from cosmic sources through quantum processors

“…of radioactivity in the laboratory environment and further simulation studies of the Fermilab-SQMS/INFN-LNGS Round Robin chip show that about 64% of particle interactions would be due to 𝛾 radiation in an above-the-ground facility, whereas about 36% would be due to muons from cosmic rays, followed by a trace amount of contribution from neutron radiation. In comparison, rates at the 1.4 km-deep LNGS facility are calculated to be almost exclusively due to 𝛾 radiation as the thick rock layer acts as a barrier to muons [10]. Particle interaction rates similar to the ground-level estimates of this study were also reported in ref.…”

“…Strategies considered so far to mitigate the effect of high-energy particles include improving the design of chips with phonon and quasiparticle traps or membrane structures [11][12][13][14][15][16], adding shielding from natural radiation [10], distributing QEC across separate chips [17], and using co-located transition-edge sensors (TES) to detect energy transmission into the qubits from ionizing radiation [18]:…”

“…However, muons from cosmic sources are typically more energetic, and as also noted in ref. [10], the same shielding equipment would be unable to stop most muons.…”

“…While more detailed studies using the Geant4 package [21] could be performed, as done for example in ref. [10], to include further effects from natural radiation sources and secondary particle production from muon scattering, a simple numerical simulation that includes multiple scattering effects and particle propagation over a simplified dilution refrigerator geometry should suffice to assess the efficiency and accuracy of the different choices for pixel sizes and detector layer positions considered in this conceptual study.…”

“…In repetition code studies using solid-state devices, however, it has been shown that this exponential reduction flattens beyond a code distance of 20, down to a level of 10 −6 per correction cycle when using cyclical corrections [4]. Ionizing radiation from high-energy particles, which can come from cosmic rays, or natural radiation from the environment and the materials used in electronics, are understood to be one of the contributors to this flattening [5][6][7][8][9][10].…”