Multiplication and detection of photons using inelastic Cooper-pair tunneling in Josephson junctions

Linear amplifiers do not allow couting of photons because of their added noise. Instead one usually uses a photon detector, but in opposition to linear amplifiers, those detectors are non-linear non-unitary devices with a binary outcome. None of these devices is able discriminate photon numbers in itinerant quantum states, an important task in many quantum sensing applications. In this work, we experimentally demonstrate a microwave photon-multiplication scheme based on inelastic tunneling of Cooper pairs through a voltage biased Josephson junction, which multiplies the photon number by an integer factor. It is linear in photon number and unitary by deamplifying phase information by the same factor. We achieve a 3-fold multiplication, with efficiency above 0.7. This device loses phase-information but does not require any dead time or time binning. We expect a fully optimized device based on this scheme to achieve number-resolving measurement of itinerant photons with low dark count which would offer new possibilities in a wide range of quantum sensing and quantum computing applications.