Generation of photon pairs at different frequencies: route toward quantum microwave source

The dynamical Coulomb blockade (DCB) is a quantum phenomenon where the tunneling of charge through a tunnel junction is modified by its electromagnetic environment. The sudden charge transfer generates photons in the electromagnetic modes. We coupled a Josephson junction to two resonators at frequencies $\nu_1\neq\nu_2$; when voltage-biased at $2eV=h\nu_1+h\nu_2$, Cooper pairs can tunnel only if two photons, one at each frequency, are simultaneously emitted. We measured the cross-correlations between the emission rates and showed that a Cauchy-Schwarz inequality is violated. This result, in agreement with theoretical prediction made by Leppakangas and coworkers\footnote{J. Lepp\"{a}kangas, G. Johansson, M. Marthaler, and M. Fogelstrom, ``Nonclassical Photon Pair production in a Voltage-Biased Josephson Junction,'' Phys. Rev. Lett. \textbf{110}, 267004-5 (2013).}, reveals the amplitude two-mode squeezing. Our setup is a easy way to produce non-classical microwave radiation from a battery. We believe that this source is a good candidate for producing pairs of entangled photons with high rate (few hundreds of MHz).