DC-powered broadband quantum-limited microwave amplifier

Fast, high-fidelity, single-shot readout of superconducting qubits in quantum processors demands quantum-limited amplifiers to preserve the optimal signal-to-noise ratio. Typically, quantum-limited amplification is achieved with parametric down-conversion of a strong pump tone, which imposes significant hardware overhead and severely limits scalability. Here, we demonstrate the first dc-powered broadband amplifier operating within 0.2 photons of the quantum limit. Our impedance-engineered inelastic Cooper-pair tunneling amplifier (ICTA)—a voltage-biased superconducting quantum interference device in which Cooper pairs tunnel inelastically by emitting signal-idler photon pairs—operates in reflection, delivering 13 dB of average gain across a 3.5-GHz bandwidth in a single stage. Semiclassical simulations accurately predict the gain and saturation power, enabling further design improvements. By eliminating the pump-tone infrastructure, the broadband ICTA promises to dramatically reduce the hardware complexity of quantum-limited amplification in superconducting quantum processors.