Spectral Properties of Two Superconducting Artificial Atoms Coupled to a Resonator in the Ultrastrong Coupling Regime

TL;DR

This paper experimentally explores a superconducting circuit with two flux qubits coupled to a resonator in the ultrastrong regime, revealing unique spectral features and interactions relevant for quantum optics and information.

Contribution

It demonstrates the realization and spectral analysis of a two-qubit ultrastrongly coupled system with novel interaction signatures and light-matter decoupling effects.

Findings

Observation of avoided level crossing indicating exotic interactions.

Identification of spectral asymmetry as a signature of light-matter decoupling.

Experimental validation of a generalized Dicke Hamiltonian with spin-spin interactions.

Abstract

We experimentally investigate a superconducting circuit composed of two flux qubits ultrastrongly coupled to a common LC resonator. Owing to the large anharmonicity of the flux qubits, the system can be described well by a generalized Dicke Hamiltonian containing spin spin interaction terms. In the experimentally measured spectrum, we observed two key phenomena. First, an avoided level crossing provides evidence of the exotic interaction that allows the simultaneous excitation of two artificial atoms by absorbing one photon from the resonator. Second, we identified a pronounced spectral asymmetry that is a clear signature of light matter decoupling. This multi atom ultrastrongly coupled system opens the door to studying novel processes for quantum optics and quantum-information tasks on a chip.