Interacting Qubit-Photon Bound States with Superconducting Circuits

Strongly coupling a qubit to a photonic band-edge induces an exponentially localized photonic mode around the qubit, forming a qubit-photon dressed bound state [1]. Photonic crystals are a natural avenue to realize these bound states [2-3], and offer the ability to create one-dimensional chains of bound states with tunable and potentially long-range interactions that preserve the qubits' spatial organization. Here, we demonstrate tunable on-site and inter-bound state interactions in a two qubit microwave crystal. Interaction between bound states is set by photonic overlap, where localization changes with qubit detuning from the band-edge. This mechanism not only yields interaction strengths of 25-250 MHz between the lowest bound state transitions, but also a two-photon, virtual interaction as high as 15 MHz originating from a fourth order process in coupling. The widely tunable, high magnitude, robust interactions demonstrated with this system are promising benchmarks towards realizing new regimes with larger, more complex systems of bound states.
[1] John et al. PRL 64, 2418–2421, 1990
[2] Douglas et al. Nat Pho 9, 326–331, 2015
[3] Liu et al. Nat Phy 13, 48–52, 2017