Nonclassical Photon Pair Production in a Voltage-Biased Josephson Junction

Juha Leppäkangas; Göran Johansson; Michael Marthaler; Mikael Fogelström

doi:10.1103/PhysRevLett.110.267004

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Nonclassical Photon Pair Production in a Voltage-Biased Josephson Junction

Juha Leppäkangas, Göran Johansson, Michael Marthaler, Mikael Fogelström

DOI 10.1103/PhysRevLett.110.267004 · Physical Review Letters

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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.