Observation of qubit-photon bound states in a rectangular waveguide QED system*

Within the stopband, where the density of states is zero, no travelling modes are allowed through the waveguide. However, evanescent modes, exponentially localized around the atoms’ position can give rise to atom-photon bound states as theoretically predicted in Ref. [1] and experimentally observed in a photonic crystal configuration on a chip [2]. Here, we report observation of an atom-photon bound state with a transmon qubit inserted in a three-dimensional rectangular waveguide. Rectangular waveguides naturally exhibit low-frequency cut-off and the atom-photon bound states shows stronger localization compared to a photonic crystal realization [3]. We provide evidence of exponential localization of the atom-photon bound states through spectroscopic measurements, study their radiative emission and coherent interaction.

1. J. Sajeev and J. Wang, Phys. Rev. Lett. 64, 2418 (1990).
2. Y. Liu and A. Houck, Nature Phys. 13, 48 (2017).
3. E. Shahmoon and G. Kurizki, Phys. Rev. A 87, 033831 (2013)