Nonclassical photon pair production in a voltage-biased Josephson junction

TL;DR

This paper demonstrates nonclassical photon pair production from a voltage-biased Josephson junction, revealing quantum fluctuations and pair correlations in microwave emission, with implications for quantum information applications.

Contribution

It introduces an input-output formalism for microwave fields and shows evidence of nonclassical photon pairs violating classical inequalities in Josephson junctions.

Findings

Photon flux spectrum is symmetric around half the Josephson frequency.

Photon pairs are created predominantly in the emission process.

Resonances in stepped transmission lines enhance the signal-to-noise ratio.

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 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.