Ionizing Radiation Challenges Quantum Computing, Mitigation Strategies in Development

Superconducting qubit chips, crucial to quantum computing, are vulnerable to errors caused by ionizing radiation. This radiation generates quasiparticle (QP) excitations, reducing qubit coherence and causing errors across a large portion of the qubits in an array. Current error correction schemes struggle to mitigate these errors. Researchers have proposed software-based approaches and hardware modifications to combat this issue. A numerical tool, GEANT4 Condensed Matter Physics (G4CMP), has been developed to simulate the phonon and charge dynamics in dielectric crystals, aiding in the development of strategies to mitigate QP poisoning and optimize qubit array layouts.

What is the Impact of Ionizing Radiation on Superconducting Qubit Chips?

Superconducting qubit chips, which are integral to quantum computing, are susceptible to errors caused by ionizing radiation. This radiation generates quasiparticle (QP) excitations in the qubit electrodes, which temporarily reduce qubit coherence significantly. The many energetic phonons produced by a particle impact travel efficiently throughout the device substrate and generate quasiparticles with high probability, thus causing errors on a large fraction of the qubits in an array simultaneously.

The impact of high-energy particles from background radioactivity or cosmic-ray muons on superconducting qubit arrays presents a significant challenge for implementing a fault-tolerant quantum processor. A typical gamma ray from background radioactive contamination hitting the qubit chip deposits energy of order 100 keV in the Si substrate. This generates a significant number of electron-hole pairs near the impact site, some of which recombine while others travel distances of a few hundred µm before trapping on defects in the Si, thus causing a reconfiguration of the offset-charge environment for any qubits near the impact site.

Such charge jumps can cause rearrangements of the local two-level system defects near a qubit, shifting the qubit frequency and affecting qubit coherence. More importantly, the generation of these electron-hole pairs is accompanied by the emission of many energetic athermal phonons, which travel throughout the entire volume of the substrate. Because these phonons generally have energies well above the superconducting energy gap on the device layer, whenever they scatter within the superconducting electrodes, they will break Cooper pairs and generate nonequilibrium quasiparticles (QPs) with high probability.

How Do Quasiparticles Affect Superconducting Qubits?

Transient elevations in QP density in superconducting qubits reduce coherence and enhance the probability of qubit errors. Because of the spread of pair-breaking phonons throughout much of the chip, these QP-induced errors can be correlated across a significant portion of the processor, which current error correction schemes such as the surface code are unable to mitigate.

Software-based approaches to dealing with these correlated errors, based on modified error-correcting protocols, have been proposed recently. However, due to the added complexity of these schemes, practical approaches for mitigating the QP poisoning in hardware are desirable. While operation in facilities deep underground can shield against cosmic-ray muons, and the use of thick Pb layers can protect from environmental radiation sources outside the cryostat, these approaches are not always practical.

Additionally, non-negligible radioactive contamination can also be present in the cryostat and device packaging itself. Some suppression of quasiparticle bursts in superconducting resonators has been achieved by adding small islands of a lower-gap superconductor onto a chip, which downconvert the energy from the phonons below the superconducting gap of the device.

What are the Strategies for Mitigating Quasiparticle Poisoning?

More recently, some researchers demonstrated the use of thick normal-metal islands on the back side of a multi-qubit chip opposite from the device layer. This resulted in a reduction in the rate of two and threefold correlated errors by two orders of magnitude. Understanding the details of the phonon dynamics and QP generation is crucial for further improvements to strategies for mitigating QP poisoning and developing optimal qubit array layouts.

In recent years, a sophisticated numerical tool, GEANT4 Condensed Matter Physics (G4CMP), has been developed for simulating the phonon and charge dynamics in dielectric crystals at millikelvin temperatures, primarily for designing cryogenic dark matter detectors. Building on the capabilities in GEANT4 for modeling interactions of high-energy particles and matter, the G4CMP package incorporates the relevant solid-state physics for simulating the generation, propagation, and absorption of phonons, as well as the generation and transport of charge carriers.

The researchers describe a comprehensive strategy for the numerical simulation of the phonon and quasiparticle dynamics in the aftermath of an impact. They compare the simulations with experimental measurements of phonon-mediated QP poisoning and demonstrate that their modeling captures the spatial and temporal footprint of the QP poisoning for various configurations of phonon downconversion structures. They thus present a path forward for the operation of superconducting quantum processors in the presence of ionizing radiation.

How Can We Improve the Operation of Superconducting Quantum Processors?

The operation of superconducting quantum processors in the presence of ionizing radiation is a significant challenge. However, through comprehensive strategies for the numerical simulation of the phonon and quasiparticle dynamics in the aftermath of an impact, it is possible to mitigate the effects of ionizing radiation.

By comparing simulations with experimental measurements of phonon-mediated QP poisoning, researchers can demonstrate that their modeling captures the spatial and temporal footprint of the QP poisoning for various configurations of phonon downconversion structures. This provides a path forward for the operation of superconducting quantum processors in the presence of ionizing radiation.

The development of sophisticated numerical tools, such as GEANT4 Condensed Matter Physics (G4CMP), has been instrumental in simulating the phonon and charge dynamics in dielectric crystals at millikelvin temperatures. These tools, which incorporate the relevant solid-state physics for simulating the generation, propagation, and absorption of phonons, as well as the generation and transport of charge carriers, are crucial for further improvements to strategies for mitigating QP poisoning and developing optimal qubit array layouts.