Basic cell for a quantum microwave router

TL;DR

This paper demonstrates the first experimental realization of a scalable quantum routing cell using superconducting circuits, capable of coherently controlling microwave photons at the single-photon level, crucial for quantum networks.

Contribution

It introduces a novel, scalable quantum routing cell based on a transmon qubit coupled to two waveguides, with detailed characterization and validation at the single-photon level.

Findings

Successful operation at the single-photon level

Consistent with non-Hermitian Hamiltonian models

Identified limits due to flux bias, temperature, and photon number

Abstract

We report the first experimental realization of a scalable basic cell for quantum routing, enabling coherent control and exchange of microwave photons between two spatially separated superconducting waveguides coupled via a single transmon qubit. The cell was characterized at 10 mK with an average input signal of approximately 1 photon at approximately 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 single-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.