Cosmic Rays May Soon Stymie Quantum Computing
upstart writes in with an IRC submission for RandomFactor:
[...] The team, working with collaborators at Lincoln Laboratory and PNNL [(Pacific Northwest National Laboratory)], first had to design an experiment to calibrate the impact of known levels of radiation on superconducting qubit performance. To do this, they needed a known radioactive source -- one which became less radioactive slowly enough to assess the impact at essentially constant radiation levels, yet quickly enough to assess a range of radiation levels within a few weeks, down to the level of background radiation.
The group chose to irradiate a foil of high purity copper. When exposed to a high flux of neutrons, copper produces copious amounts of copper-64, an unstable isotope with exactly the desired properties.
"Copper just absorbs neutrons like a sponge," says [MIT physics professor Joseph] Formaggio, who worked with operators at MIT's Nuclear Reactor Laboratory to irradiate two small disks of copper for several minutes. They then placed one of the disks next to the superconducting qubits in a dilution refrigerator in [William] Oliver [associate professor of electrical engineering and computer science and Lincoln Laboratory Fellow at MIT]'s lab on campus. At temperatures about 200 times colder than outer space, they measured the impact of the copper's radioactivity on qubits' coherence while the radioactivity decreased -- down toward environmental background levels.
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