  We investigate electromagnetic radiation emitted by a small voltage-biased
Josephson junction connected to a superconducting transmission line. At
frequencies below the well known emission peak at the Josephson frequency
(2eV/h), extra radiation is triggered by quantum fluctuations in the
electromagnetic environment. For weak tunneling couplings and typical ohmic
transmission lines, the corresponding photon flux spectrum is symmetric around
half the Josephson frequency, indicating that the photons are predominately
created in pairs. By establishing an input-output formalism for the microwave
field in the transmission line, we give further evidence for this nonclassical
photon pair production, demonstrating that it violates the classical
Cauchy-Schwarz inequality for two-mode flux cross correlations. In connection
to recent experiments, we also consider a stepped transmission line, where
resonances increase the signal-to-noise ratio.
