Generating entangled quantum microwaves in a Josephson-photonics device

TL;DR

This paper demonstrates how a Josephson-photonics device can generate and characterize entangled microwave photons, offering a versatile platform for quantum information experiments and entanglement studies.

Contribution

It introduces a method to produce and analyze entangled microwave photons in a Josephson-photonics device, highlighting its tunability and potential for quantum information applications.

Findings

Successful generation of entangled microwave photons.

Demonstration of entanglement witnesses accessible experimentally.

Ability to access various entangled states by tuning parameters.

Abstract

When connecting a voltage-biased Josephson junction in series to several microwave cavities, a Cooper-pair current across the junction gives rise to a continuous emission of strongly correlated photons into the cavity modes. Tuning the bias voltage to the resonance where a single Cooper pair provides the energy to create an additional photon in each of the cavities, we demonstrate the entangling nature of these creation processes by simple witnesses in terms of experimentally accessible observables. To characterize the entanglement properties of the such created quantum states of light to the fullest possible extent, we then proceed to more elaborate entanglement criteria based on the knowledge of the full density matrix and provide a detailed study of bi- and multipartite entanglement. In particular, we illustrate how due to the relatively simple design of these circuits changes of experimental parameters allow one to access a wide variety of entangled states differing, e.g., in the number of entangled parties or the dimension of state space. Such devices, besides their promising potential to act as a highly versatile source of entangled quantum microwaves, may thus represent an excellent natural testbed for classification and quantification schemes developed in quantum information theory.