Cosmic Ray Simulation Boosts Quantum Computing

Scientists have developed a new facility linking an electron linear accelerator to a dilution refrigerator, enabling controlled studies of radiation-induced errors in superconducting qubits, a key obstacle to building practical quantum computers. Thomas McJunkin and colleagues at the Johns Hopkins Applied Physics Laboratory have constructed this apparatus to precisely mimic the energy deposition of cosmic-ray muons, a common source of qubit decoherence, and observe resulting changes in multi-qubit superconducting transmon chips. The team can reliably induce and characterise qubit relaxation, excitation and detuning, revealing dependencies on junction placement and superconducting gaps, and offering a pathway towards mitigating these correlated errors in future quantum processors.

Precise electron irradiation characterises and reduces qubit error rates in superconducting circuits

Error rates caused by ionizing radiation represent a significant impediment to the development of scalable quantum computing, but have decreased by a factor of ten thanks to the new CLIQUE facility. Previously, investigations were limited by stochastic radiation sources, necessitating extensive data collection to discern meaningful signals; CLIQUE delivers precisely timed, 18.5 MeV electrons, accurately mimicking cosmic ray muons with sub-10 microsecond precision, enabling rapid characterisation of qubit behaviour. This on-demand radiation source allows isolation of individual events, overcoming the averaging issues inherent in ambient radiation and the limitations of alternative quasiparticle generation techniques such as optical or microwave pulse excitation. The ability to isolate single events is crucial for understanding the fundamental mechanisms of error induction and developing targeted mitigation strategies.

Detailed analysis reveals dependencies of qubit excitation and detuning on junction placement and superconducting gaps, offering pathways to mitigate correlated errors in future processors. Thorough simulations, utilising MCNP modelling, a widely used Monte Carlo N-Particle transport code, confirm that the 18.5 MeV electrons generated by the facility closely replicate the energy deposition characteristics of 400 MeV cosmic-ray muons within a 350μm silicon chip. Both exhibit a most probable energy deposition of 100 keV, a critical parameter influencing the creation of quasiparticle excitations. The facility’s ability to deliver precisely timed radiation, with sub-10 microsecond precision, enabled the rapid collection of data on correlated errors across a multi-qubit transmon chip. Within just 7.4 minutes of data acquisition, characteristic relaxation events were identified, and analysis of 831 individual electron impacts revealed dependencies between qubit excitation, detuning, and the placement of Josephson junctions, the fundamental building blocks of superconducting qubits, as well as the superconducting gaps within them. These gaps, determined by the superconducting material properties, directly influence the energy required to break Cooper pairs and generate quasiparticles. Understanding these relationships is vital for optimising qubit design and resilience.

Controlled electron beam irradiation of superconducting qubits for single-event effect

This work centres on a newly constructed facility, coupling an electron linear accelerator, or linac, to a dilution refrigerator; this specialised cooling system maintains the extremely low temperatures, typically below 20 millikelvins, required for superconducting qubits to operate effectively. Superconductivity, and therefore qubit coherence, is destroyed by the introduction of unpaired electrons, or quasiparticles, which are generated by ionizing radiation. Unlike previous methods relying on unpredictable background radiation, the linac delivers a controlled beam of electrons with 18.5 MeV of kinetic energy, accurately mimicking the impact of cosmic ray muons with sub-10 microsecond precision. Precise timing is important, allowing isolation and study of the effects of individual radiation events on the tiny electronic circuits that form qubits, rather than averaging over many random occurrences. This allows for a more detailed understanding of the error mechanisms at play.

Despite concerns about potential radio-frequency interference from the linac, a type of particle accelerator, these findings remain important for building quantum computers. Detailed control tests, including directing the electron beam away from the qubits and towards a tungsten radiator, demonstrate the observed qubit errors originate from ionizing radiation impacting the system. High-energy photons and charged particles deposit energy into the chip, creating excess quasiparticles near Josephson junctions that increase qubit decoherence, and account for the few remaining errors when the beam is diverted. These quasiparticles disrupt the delicate quantum states of the qubits, leading to errors in computation. The Josephson junction, a crucial component, is particularly susceptible due to its sensitivity to changes in the superconducting order parameter.

The CLIQUE facility offers a controlled environment for studying how ionizing radiation affects superconducting qubits, overcoming limitations of previous stochastic methods. Naren Manjunath from the Perimeter Institute and colleagues now precisely replicate the energy deposited by cosmic ray muons by coupling an electron linear accelerator, or linac, to a dilution refrigerator; these high-energy particles disrupt qubit function. Detailed analysis of resulting qubit behaviour reveals dependencies on the design of Josephson junctions and superconducting gaps, informing strategies to build more durable quantum processors. This capability opens avenues for investigating how different radiation energies and angles impact qubit stability, which is important for advancing practical quantum computation. Future research will focus on exploring shielding techniques, error correction codes tailored to radiation-induced errors, and the development of more robust qubit designs to mitigate the effects of ionizing radiation and pave the way for fault-tolerant quantum computers. The ability to systematically study these effects is a crucial step towards realising the full potential of quantum technology.