The low-noise amplification of weak microwave signals is crucial for countless protocols in quantum information processing. Quantum mechanics sets an ultimate lower limit of half a photon to the added input noise for phase-preserving amplification of narrowband signals, also known as the standard quantum limit (SQL). This limit, which is equivalent to a maximum quantum efficiency of 0.5, can be overcome by employing nondegenerate parametric amplification of broadband signals. We show that, in principle, a maximum quantum efficiency of 1 can be reached. Experimentally, we find a quantum efficiency of 0.69±0.02, well beyond the SQL, by employing a flux-driven Josephson parametric amplifier and broadband thermal signals. We expect that our results allow for fundamental improvements in the detection of ultraweak microwave signals.
@article{arxiv.2011.00914,
title = {Beyond the standard quantum limit of parametric amplification},
author = {M. Renger and S. Pogorzalek and Q. Chen and Y. Nojiri and K. Inomata and Y. Nakamura and M. Partanen and A. Marx and R. Gross and F. Deppe and K. G. Fedorov},
journal= {arXiv preprint arXiv:2011.00914},
year = {2020}
}