Low-noise amplification of weak microwave signals is crucial in quantum information processing. Quantum mechanics sets a standard quantum limit (SQL) of at least half a photon of added noise for phase-preserving amplification of narrowband signals. This paper demonstrates that this limit can be surpassed using nondegenerate parametric amplification of broadband signals, theoretically achieving unity quantum efficiency. Experimentally, using a flux-driven Josephson parametric amplifier and broadband thermal signals, a quantum efficiency of 0.69 ± 0.02 was achieved, exceeding the SQL and promising improvements in detecting ultraweak microwave signals.

M. Renger, S. Pogorzalek, Q. Chen, Y. Nojiri, K. Inomata, Y. Nakamura, M. Partanen, A. Marx, R. Gross, F. Deppe, K. G. Fedorov