Microwave photon-number multiplication using inelastic Cooper-pair tunneling in Josephson junctions

Linear amplifiers do not allow counting of photons because of their added noise. Instead one usually uses single photon detectors for this task, but contrary to linear amplifiers, these detectors are usually non-linear devices with a binary outcome, mapping all incoming non-zero photon-number states to the same output. In this work we improve our implementation of a linear 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 [1]. Most of the improvements stem from refinements of our nano-fabrication process enabling resonant modes with higher characteristic impedance and optimized frequencies leading to reduced spontaneous emission processes. We expect a fully optimized device based on this scheme to achieve number-resolving measurement of itinerant photon states with low dark count which would offer new possibilities in a wide range of quantum sensing and quantum computing applications.

[1]: Albert, Romain, et al. "Microwave photon-number amplification." arXiv preprint arXiv:2303.03173 (2023).