Amplifiers used to read out quantum devices inevitably add noise, but phase-sensitive amplifiers can in principle beat the standard quantum limit. Researchers at Aalto University and VTT studied a SQUID-based Josephson parametric amplifier made with VTT's SWAPS fabrication technology, comparing its phase-preserving and phase-sensitive modes at millikelvin temperatures.

To probe the noise directly at low gain, they followed it with a travelling-wave parametric amplifier. In phase-preserving mode the system noise temperature was 351 ± 53 mK at 6 GHz, close to the quantum limit; in phase-sensitive mode it fell to 94 ± 12 mK, well below the standard quantum limit of 288 mK. The work brings together QMAT's Pertti Hakonen group at Aalto and VTT (Visa Vesterinen), with Sorin Paraoanu's group at Aalto.

Figure: Lilja et al., J. Low Temp. Phys. (2026); image from arXiv:2608.25645, CC BY 4.0