Superconducting circuits are the qubits of choice for Google, IBM, and multiple other companies and institutions developing quantum computers. The qubits in the latest devices can sustain their delicate quantum states for more than 100 µs at a time; that longevity enables advances such as modeling chemical reactions. But extending qubit coherence times by orders of magnitude may prove challenging. Antti Vepsäläinen and William Oliver from MIT and colleagues have now investigated one potential threat: environmental radiation from sources such as cosmic rays, which is suspected to trigger decoherence by breaking some of the superconducting material’s Cooper pairs of electrons.

The researchers quantified the reduction of qubits’ coherence times on exposure to ionizing radiation by placing a silicon chip with two superconducting aluminum qubits alongside a disk of radioactive copper-64. The coherence times abruptly dropped before gradually increasing as radiation levels from the copper, with a 12.7-hour half-life, tailed off. In a separate experiment, the researchers shielded the qubits from normal background radiation with 10-cm-thick lead bricks and were able to slightly increase the coherence time of the qubits. Based on the two experiments, the researchers concluded that environmental ionizing radiation would limit those particular qubits to coherence times on the order of milliseconds, just an order of magnitude greater than those in the latest quantum computers.

The findings build on recent research by Laura Cardani of Italy’s National Institute for Nuclear Physics and colleagues, who showed that ambient radioactivity can trigger broken Cooper pairs in superconducting circuits. Cardani’s team subdued the phenomenon by placing the devices in Gran Sasso National Laboratory, a facility that houses neutrino and dark-matter experiments beneath 1.4 km of rock.

The two studies suggest that engineers need to take ionizing radiation into account when designing superconducting circuits for not only quantum computers but also ultrasensitive sensors (see the article by Kelsey Morgan, Physics Today, August 2018, page 28) and Majorana fermion experiments (see the article by Ramón Aguado and Leo Kouwenhoven, Physics Today, June 2020, page 44). Vepsäläinen and colleagues suggest the use of lead shielding or the development of devices that are resistant or less sensitive to broken Cooper pairs. (A. P. Vepsäläinen et al., Nature 584, 551, 2020.)