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posted by martyb on Friday August 28 2020, @02:39PM   Printer-friendly
from the ray-of-hope? dept.

Cosmic rays may soon stymie quantum computing: Building quantum computers underground or designing radiation-proof qubits may be needed, researchers find.:

[...] 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.

The radioactivity of the second disk was measured at room temperature as a gauge for the levels hitting the qubit. Through these measurements and related simulations, the team understood the relation between radiation levels and qubit performance, one that could be used to infer the effect of naturally occurring environmental radiation. Based on these measurements, the qubit coherence time would be limited to about 4 milliseconds.

The team then removed the radioactive source and proceeded to demonstrate that shielding the qubits from the environmental radiation improves the coherence time. To do this, the researchers built a 2-ton wall of lead bricks that could be raised and lowered on a scissor lift, to either shield or expose the refrigerator to surrounding radiation.

[...] Every 10 minutes, and over several weeks, students in Oliver's lab alternated pushing a button to either lift or lower the wall, as a detector measured the qubits' integrity, or "relaxation rate," a measure of how the environmental radiation impacts the qubit, with and without the shield. By comparing the two results, they effectively extracted the impact attributed to environmental radiation, confirming the 4 millisecond prediction and demonstrating that shielding improved qubit performance.

Journal Reference:
Antti P. Vepsäläinen, Amir H. Karamlou, John L. Orrell, et al. Impact of ionizing radiation on superconducting qubit coherence [$], Nature (DOI: 10.1038/s41586-020-2619-8)


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  • (Score: 2) by HiThere on Friday August 28 2020, @10:56PM

    by HiThere (866) Subscriber Badge on Friday August 28 2020, @10:56PM (#1043541) Journal

    I think this test is specific to a particular way of implementing qubits. Spin states might be less affected, and distribution of Nitrogen isotopes in diamond would almost certainly not be affected. (Of course, that one's pretty slow to write to.)

    That said, I think what they're talking about is the most common way of implementing qubits. But it doesn't sound like an absolute limit, just another implementation limit.

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