Demonstration of an effective ultrastrong coupling between two oscillators

TL;DR

This paper demonstrates an effective ultrastrong coupling between two harmonic oscillators using a nonlinear superconducting circuit, revealing mode hybridization and ground state properties through spectroscopic and squeezing measurements.

Contribution

It presents the first experimental realization of ultrastrong coupling between two harmonic oscillators via dual frequency pumping in a superconducting circuit.

Findings

Spectroscopic signature of mode hybridization observed.

Simultaneous single-mode and two-mode squeezing demonstrated.

Effective coupling rate tuned by pump amplitudes.

Abstract

When the coupling rate between two quantum systems becomes as large as their characteristic frequencies, it induces dramatic effects on their dynamics and even on the nature of their ground state. The case of a qubit coupled to a harmonic oscillator in this ultrastrong coupling regime has been investigated theoretically and experimentally. Here, we explore the case of two harmonic oscillators in the ultrastrong coupling regime. Specifically, we realize an analog quantum simulation of this coupled system by dual frequency pumping a nonlinear superconducting circuit. The pump amplitudes directly tune the effective coupling rate. We observe spectroscopic signature of a mode hybridization that is characteristic of the ultrastrong coupling. Further we experimentally demon- strate a key property of the ground state of this simulated ultrastrong coupling between modes by observing simultaneous single-mode and two-mode squeezing of the radiated field below vacuum fluctuations.