Experimental characterization of the qubit-mediated coupling between two waveguides

We report the first experimental realization and comprehensive characterization of a system consisting of two spatially separated superconducting waveguides coupled via a single transmon qubit. Since this system enables coherent control and the exchange of microwave photons between waveguides, and also holds the potential to serve as a scalable basic unit in quantum routing architectures, we will henceforth refer to it as a basic cell. The cell was characterized at 10 mK with an average input signal of ≈1 photon at ≈6 GHz, and with the qubit biased to its optimal point to minimize sensitivity to external magnetic fluctuations. By combining steady-state and time-domain measurements, we reconstructed the key parameters of the system, including qubit relaxation and dephasing, waveguide-qubit couplings, and cross-waveguide photon transfer efficiency. The observed performance is consistent with a non-Hermitian Hamiltonian formalism and demonstrates clear limits set by flux bias, temperature, and photon number, in agreement with flux- and temperature-induced dephasing models. Crucially, the cell operates reliably at the low-photon level, and in the high-photon regime we directly observe photon dressing induced by the qubit. These results establish a versatile platform for studying open quantum system phenomena and pave the way for scalable implementations of quantum routing and network nodes.