Near-quantum-limited amplification from inelastic Cooper-pair tunnelling

The readout of microwave quantum systems, such as spin or superconducting qubits, requires low-noise amplifiers with added noise as close as possible to the quantum limit. This limit has so far been approached only by parametric amplifiers that exploit nonlinearities in superconducting circuits and are driven by a strong microwave pump tone. However, this microwave drive makes the amplifiers much more difficult to implement and operate than conventional d.c.-powered amplifiers, which currently suffer from much higher noise. Here, we show that a simple d.c.-powered set-up can provide amplification close to the quantum limit. Our amplification scheme is based on the stimulated microwave photon emission accompanying inelastic Cooper-pair tunnelling through a d.c.-biased Josephson junction. The key to the low noise of this approach is a well-defined auxiliary idler mode, which allows for operation analogous to parametric amplifiers.