Microwave photo-multiplication based on inelastic Cooper-pair tunneling

When a Josephson junction is embedded in a low impedance circuit Cooper pair transport is usually elastic and the DC voltage across the junction has to be zero to allow for tunneling through the junction. By coupling a DC voltage-biased junction to a microwave circuit, the light-charge interaction enables inelastic charge transport with photon emission or absorption. The nonlinearity of this light-charge interaction can be tuned via the characteristic impedance of the microwave circuit and makes it possible to design sources of non-classical microwave radiation [Westig17, Grimm18], parametric amplifiers [Jebari18] or in our case, a photon multiplier [Leppäkangas18].

By designing particular high impedance electromagnetic environments, processes involving 3 or more photons can become dominant. In this case, we can show that the energy of a tunneling Cooper pair can be used to convert an incoming single photon state into a n-photon Fock state in a different mode. By cascading two of these multiplication stages followed by linear amplification, this device can discriminate itinerant single photon state from vacuum without dead time.

[Westig17] Phys. Rev. Lett. 119, 137001
[Grimm18] ArXiv:1804.10596
[Jebari18] Nature Electronics volume 1, p223–227
[Leppäkangas18] Phys. Rev. A 97, 013855