Typical superconducting quantum circuits, such as qubits—basic processing units of a quantum computer, must be operated at very low temperatures, of a few 10s of millikelvin, or hundredths of a degree from absolute zero temperature. These temperatures are today easily accessible in modern refrigerators. However, the intrinsic temperature of devices turns out to be much higher because the materials required to make good qubit circuits are by their nature very poor thermal conductors. This thermalization problem becomes more and more acute as the scale and complexity of circuits grow.
Much like water (or liquid nitrogen) cooling is sometimes used to effectively cool down high-performance digital computers, a quantum computer could benefit from similar liquid cooling. But at the very low temperatures that quantum circuits operate, most liquids will have turned into ice. Only two isotopes of Helium, Helium-3 and Helium-4, remain in the liquid form at millikelvin temperatures.
In recent work published in Nature Communications, researchers from the National Physical Laboratory, Royal Holloway University of London, Chalmers University of Technology and Google developed new technology to cool down a quantum circuit to less than a thousand of a degree above absolute zero, almost 100 times lower temperature than achieved before. This was made possible by immersing the circuit in liquid 3 He, chosen for its superior thermal properties.
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