Observation of broadband entanglement in microwave radiation from a single time-varying boundary condition

TL;DR

This paper demonstrates broadband entanglement in microwave radiation generated by a single, time-varying boundary condition in a transmission line, expanding the frequency range of entangled microwave photon sources.

Contribution

It introduces a novel method of producing broadband microwave entanglement using a modulated SQUID boundary, surpassing narrowband resonator limitations.

Findings

Successful detection of entanglement at two frequencies separated by 0.7 GHz.

Quantitative analysis of noise correlations confirms the presence of entanglement.

The method provides a new way to generate broadband microwave entanglement.

Abstract

Entangled pairs of microwave photons are commonly produced in the narrow frequency band of a resonator, which represents a modified vacuum density of states. We use a broadband, semi-infinite transmission line terminated by a superconducting quantum interference device (SQUID). A weak pump signal modulates the SQUID inductance, resulting in a single time-varying boundary condition. We detect both quadratures of the microwave radiation emitted at two different frequencies separated by 0.7~GHz. We determine the type and purity of entanglement from the noise correlations and an in-situ noise and power calibration.