Directional emission in a giant atom superstrongly coupled to a high-impedance cavity array.

In waveguide quantum electrodynamics, traditional studies often assume that emitters couple at a single point to waveguides with linear dispersion, where photon group velocity is independent of the wave vector. However, recent technological advancements have enabled the creation of discrete site waveguides using microwave photonic crystals or superconducting coupled cavity arrays (CCA), which exhibit a quasi-continuous band structure with a cosine dispersion relation. In addition, artificial atoms can be coupled at multiple points of a mode’s wavelength beyond the standard dipolar approximation, forming giant atoms [1]. In this work, we experimentally study a transmon qubit coupled to multiple cavities of a CCA, made with a high-kinetic inductance metamaterial [2]. The system's geometry allows us to reach the superstrong coupling regime, where the coupling between the CCA modes and the qubit far exceeds the free spectral range of the CCA (g/Δω up to 11). We measure the atomic ratio in the eigenmodes of the system with a precision down to 10^-2 and find, as expected from theory, a low qubit participation in the eigenmodes where g/Δω > 1. Additionally, reflection measurements of the CCA reveal tunable directional behavior in the superstrong coupling regime.



[1] Kockum, A. Frisk. "Quantum optics with giant atoms—the first five years." International Symposium on Mathematics, Quantum Theory, and Cryptography. Vol. 33. Singapore: Springer, 2021.

[2] Jouanny, Vincent, et al. "Band engineering and study of disorder using topology in compact high kinetic inductance cavity arrays." arXiv preprint arXiv:2403.18150 (2024).