title: Nonclassical photon pair production in a voltage-biased Josephson junction.
abstract: 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 transmission line. 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.
authors.affiliation: Microtechnology and Nanoscience, MC2, Chalmers University of Technology, SE-412 96 Göteborg, Sweden.
authors.forename: Juha
authors.initials: J
authors.lastname: Leppäkangas
authors.forename: Göran
authors.initials: G
authors.lastname: Johansson
authors.forename: Michael
authors.initials: M
authors.lastname: Marthaler
authors.forename: Mikael
authors.initials: M
authors.lastname: Fogelström