(PhysicsWorld) Quantum computers might in future be shielded by thick layers of lead and even operated deep underground because of their sensitivity to environmental noise. So say physicists in the US, who have found that ionizing radiation significantly limits the coherence time of superconducting qubits. Indeed, they say that minimizing radiation effects will be crucial if general-purpose quantum computers are to be made using superconducting technology.
Earlier this year, physicists in Germany and Italy reported that environmental radioactivity can impair the performance of superconducting resonators. The group, headed by Laura Cardani of the National Institute of Nuclear Physics in Rome and Ioan Pop of the Karlsruhe Institute of Technology, showed that cosmic rays and radioactive impurities can significantly increase the density of broken Cooper pairs, known as quasiparticles, within devices above ground. Conversely, by using a radio-pure set-up within Italy’s Gran Sasso laboratory – located under 1400 m of rock – it was able to reduce the incidence of what are known as quasiparticle bursts by up to a factor of 50.
Recently, William Oliver and colleagues at at the Massachusetts Institute of Technology (MIT) and the Pacific Northwest National Laboratory have taken this research a step forward by measuring and modelling the effect of ionizing radiation on superconducting qubits themselves.
To see how qubits might be shielded from ionizing radiation, the MIT team surrounded seven such qubits (or rather the cryostat used to keep them cool) with 10 cm-thick lead bricks. This is the kind of shielding often used in neutrino and dark-matter experiments. By placing the shield on a scissor lift and periodically raising and lowering it, they were able to establish the effect of the external radiation thereby confirming the coherence limit of about 4 ms. They also found that the shield increased the coherence time by around 20%.

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