Multi-mode mediated exchange coupling in cavity QED

TL;DR

This paper experimentally investigates how virtual photons in high-Q microwave cavities mediate exchange coupling between distant qubits, revealing the role of higher-harmonic modes and symmetry effects in cavity QED systems.

Contribution

It demonstrates the inverse scaling of qubit coupling with detuning, the mediation by higher-harmonic modes, and the emergence of two-photon transitions and dark states in resonant qubits.

Findings

Coupling strength scales inversely with detuning from the cavity mode.

Higher-harmonic cavity modes enhance the mediated coupling.

Resonant qubits exhibit two-photon transitions and dark states.

Abstract

Microwave cavities with high quality factors enable coherent coupling of distant quantum systems. Virtual photons lead to a transverse exchange interaction between qubits, when they are non-resonant with the cavity but resonant with each other. We experimentally probe the inverse scaling of the inter-qubit coupling with the detuning from a cavity mode and its proportionality to the qubit-cavity interaction strength. We demonstrate that the enhanced coupling at higher frequencies is mediated by multiple higher-harmonic cavity modes. Moreover, in the case of resonant qubits, the symmetry properties of the system lead to an allowed two-photon transition to the doubly excited qubit state and the formation of a dark state.