Generating two continuous entangled microwave beams using a dc-biased Josephson junction

TL;DR

This paper demonstrates experimentally that a dc-biased Josephson junction can generate entangled microwave beams, with coherence and entanglement characterized through second-order coherence functions, aligning well with theoretical predictions.

Contribution

It provides the first experimental verification of entangled microwave beams from a Josephson junction with detailed coherence measurements and theoretical validation.

Findings

Entangled microwave beams emitted at 2.5 billion photon pairs per second.

Coherence times limited by low frequency noise of the dc bias.

Quantitative agreement between experimental results and theory.

Abstract

We show experimentally that a dc-biased Josephson junction in series with two microwave resonators emits entangled beams of microwaves leaking out of the resonators. In the absence of a stationary phase reference for characterizing the entanglement of the outgoing beams, we measure second-order coherence functions for proving entanglement up to an emission rate of 2.5 billion photon pairs per second. The experimental results are found in quantitative agreement with theory, proving that the low frequency noise of the dc bias is the main limitation for the coherence time of the entangled beams. This agreement allows us to evaluate the entropy of entanglement of the resonators, and to identify the improvements that could bring this device closer to a useful bright source of entangled microwaves for quantum-technological applications.