Experimental violation of the standard quantum limit for parametric amplification of broadband signals*

Phase-preserving amplification of weak signals is a crucial part of many protocols in microwave quantum information processing, such as quantum teleportation, remote state preparation, or dispersive qubit readout. Flux-driven Josephson parametric amplifiers (JPAs) allow amplification close to the standard quantum limit (SQL), implying a fundamental bound of 1/2 for the maximal quantum efficiency η for amplification of narrowband input signals. We demonstrate that the SQL does not hold for broadband input signals and experimentally find η = 70% with an amplification chain consisting of a JPA and a cryogenic HEMT amplifier. We show that η can reach 100% and experimentally is limited by the Poissonian fluctuations in the JPA pump line. This result can be exploited for multiple applications such as high-efficiency parity measurements of superconducting qubits.