Beyond the standard quantum limit of parametric amplification*

The low noise amplification of weak microwave signals is crucial for many 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, however, can be circumvented by employing nondegenerate parametric amplification of broadband signals. We show that, in theory, a maximum quantum efficiency of 1 can be reached. Experimentally, we detect the quantum efficiency of 0.69 beyond the SQL of 0.5 by employing a flux-driven Josephson parametric amplifier and broadband thermal signals. Thus, we demonstrate the violation of the SQL for nondegenerate parametric amplification of broadband input signals, a result which can be exploited for improvement of various qubit readout techniques.