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arXiv:1703.05009 · Physical Review Letters

Emission of Nonclassical Radiation by Inelastic Cooper Pair Tunneling

M. Westig, Björn Kubala, Olivier Parlavecchio, Yury Mukharsky, Carles Altimiras, P. Joyez, D. Vion, P. Roche, D. Esteve, M. Hofheinz, Mircea Trif, Pascal Simon, Joachim Ankerhold, F. Portier

Published Sep 26, 2017• 57 citations
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Abstract

We show that a properly dc-biased Josephson junction in series with two microwave resonators of different frequencies emits photon pairs in the resonators. By measuring auto- and intercorrelations of the power leaking out of the resonators, we demonstrate two-mode amplitude squeezing below the classical limit. This nonclassical microwave light emission is found to be in quantitative agreement with our theoretical predictions, up to an emission rate of 2 billion photon pairs per second.

DOI
10.1103/physrevlett.119.137001

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generates photons in the electromagnetic modes. We coupled a Josephson junction to two resonators at frequencies 1 2; when voltage-biased at 2eV = h 1 + h 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 1 , 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).
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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.