  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.
