# Quantum limited amplification from inelastic Cooper pair tunneling

## TL;DR

This paper demonstrates a novel, simple DC-powered microwave amplifier based on inelastic Cooper pair tunneling that approaches the quantum noise limit, offering a potentially easier alternative to traditional parametric amplifiers.

## Contribution

It introduces the first experimental realization of a near-quantum-limited amplifier using inelastic Cooper pair tunneling with a DC bias, avoiding complex microwave pumping.

## Key findings

- Achieves amplification close to the quantum limit.
- Uses inelastic Cooper pair tunneling for microwave photon emission.
- Operates with a simple DC bias, simplifying implementation.

## Abstract

Nature sets fundamental limits regarding how accurate the amplification of analog signals may be. For instance, a linear amplifier unavoidably adds some noise which amounts to half a photon at best. While for most applications much higher noise levels are acceptable, the readout of microwave quantum systems, such as spin or superconducting qubits, requires noise as close as possible to this ultimate limit. To date, it is approached only by parametric amplifiers exploiting non-linearities in superconducting circuits and driven by a strong microwave pump tone. However, this microwave drive makes them much more difficult to implement and operate than conventional DC powered amplifiers, which so far suffer from much higher noise. Here we present the first experimental proof that a simple DC-powered setup allows for amplification close to the quantum limit. Our amplification scheme is based on the stimulated microwave photon emission accompanying inelastic Cooper pair tunneling through a DC-biased Josephson junction, with the key to low noise lying in a well defined auxiliary idler mode, in analogy to parametric amplifiers.