Input-output description of microwave radiation in the dynamical Coulomb blockade

J. Leppäkangas*, G. Johansson, M. Marthaler, M. Fogelström

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


We study microwave radiation emitted by a small voltage-biased Josephson junction connected to a superconducting transmission line. An input-output formalism for the radiation field is established, using a perturbation expansion in the junction's critical current. Using output field operators solved up to the second order, we estimate the spectral density and the second-order coherence of the emitted field. For typical transmission line impedances and at frequencies below the main emission peak at the Josephson frequency, radiation occurs predominantly due to two-photon emission. This emission is characterized by a high degree of photon bunching if detected symmetrically around half of the Josephson frequency. Strong phase fluctuations in the transmission line make related nonclassical phase-dependent amplitude correlations short lived, and there is no steady-state two-mode squeezing. However, the radiation is shown to violate the classical Cauchy-Schwarz inequality of intensity cross-correlations, demonstrating the nonclassicality of the photon pair production in this region.

Original languageEnglish
Article number015015
JournalNew Journal of Physics
StatePublished - Jan 2014
Externally publishedYes


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