npj Quantum Information (Nov 2021)

Beyond the standard quantum limit for parametric amplification of broadband signals

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

DOI
https://doi.org/10.1038/s41534-021-00495-y
Journal volume & issue
Vol. 7, no. 1
pp. 1 – 7

Abstract

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Abstract 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 unity 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.