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Typical superconducting quantum circuits, such as the basic processing unit qubits of a quantum computer, must operate at very low temperatures, within a few tens of millikelvins or hundredths of a degree from the temperature of absolute zero. These temperatures are now easily accessible in modern refrigerators. However, the intrinsic temperature of the devices turns out to be much higher because the materials needed to make good qubit circuits are by their nature very poor thermal conductors. This thermalization problem becomes increasingly acute as the scale and complexity of the circuits increases.
Just as water (or liquid nitrogen) cooling is sometimes used to effectively cool high-performance digital computers, a quantum computer could benefit from similar liquid cooling. But at the very low temperatures at which quantum circuits operate, most liquids will have turned to ice. Only two isotopes of helium, helium-3 and helium-4, remain in liquid form at millikelvin temperatures.
In a recent work published in Nature communications, researchers from the National Physical Laboratory, Royal Holloway University in London, Chalmers University of Technology and Google have developed new technology to cool a quantum circuit to less than a thousand degrees above absolute zero, a temperature nearly 100 times lower than the one reached earlier. This was possible by immersing the circuit in a liquid 3He, chosen for its superior thermal properties.
Surprisingly, the researchers observed this 3It couples very strongly to atomic-scale defects in the quantum circuit material and strips excess energy from these defects more than 1,000 times faster than when 3Is not present. Basically, at the same time the 3It does not directly interfere with the circuit itself. Furthermore, research has shown that the noise in the present experiment was also reduced more than 1,000 times at the lowest temperatures using 3It has immersion, compared to extrapolated theoretical predictions.
This mechanism, if optimized, has the potential to significantly improve the coherence of quantum circuits. In particular, it may be able to radically change the behavior of the noisy environment, silence it and further improve the coherence of the circuits. The noise and energy losses caused by these material defects are currently the main obstacle the field is facing towards scaling up to practical quantum computers.
“Improving coherence that is marred by material defects is the biggest challenge quantum computing is facing to grow and build practical quantum computers. Going to these extremely low temperatures allows us to study the fundamental physics of quantum circuits more detailed, and we’re confident that the knowledge gained in this way will help us improve the consistency of circuits even when used at easily achievable temperatures of 10s of mK in typical refrigerators used today,” said NPL’s Sebastian de Graaf.
“Cooling devices to extremely low temperatures is a significant technological challenge. In the present study, we solved many technical challenges to achieve this and our results demonstrate the benefits of doing so on circuit performance. Furthermore, we are confident that this technology can be expanded to cool much larger quantum loops for the benefit of future developments in quantum computing,” said Professor John Saunders, director of the London Low Temperature Laboratory at Royal Holloway University in London.
M. Lucas et al, Quantum bath suppression in a superconducting circuit by immersion cooling, Nature communications (2023). DOI: 10.1038/s41467-023-39249-z
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