Slowing and storing microwaves in a single superconducting fluxonium artificial atom

Three-level Λ systems provide a versatile platform for quantum optical phenomena such as electromagnetically induced transparency (EIT), slow light, and quantum memory. Such Λ systems have been realized in several quantum hardware platforms, including atomic systems, superconducting artificial atoms, and meta-structures. Previous experiments involving superconducting artificial atoms incorporated coupling to additional degrees of freedom, such as resonators or other superconducting atoms. In this work, we performed an EIT experiment in the microwave frequency range utilizing a single fluxonium qubit within a microwave waveguide. The Λ system consists of two plasmon transitions in combination with one metastable state originating from the fluxon transition. In this configuration, the controlling and probing transitions are strongly coupled to the transmission line, safeguarding the transition between |0⟩ and |1⟩ states and ensuring the fluxonium qubit is close to the sweet spot. Our observations include the manifestation of EIT, a slowdown of light with a delay time of 217 ns, and photon storage. These results highlight the potential as a phase shifter or quantum memory for quantum communication in superconducting circuits.