Superconducting qubits are a leading candidate for utility-scale quantum computing due to their fast gate speeds and steadily decreasing error rates. The requirement for millikelvin operating temperatures, however, creates a significant scaling bottleneck. Modular architectures using optical fiber links could bridge separate cryogenic nodes, but superconducting circuits do not have coherent optical transitions and microwave-to-optical conversion has not been shown for any non-classical photon state. In this work, we demonstrate the on-demand generation and tomographic reconstruction of itinerant single microwave photons at 8.9 GHz from a superconducting qubit. We upconvert this non-Gaussian state with a transducer added noise below 0.012 quanta and count the converted telecom photons at 193.4 THz with a signal-to-noise ratio of up to 5.1±1.1. We characterize the trade-offs between throughput and noise, and establish a viable path toward heralded entanglement distribution and gate teleportation. Looking ahead, these results empower existing superconducting devices to take a key role in distributed quantum technologies and heterogeneous quantum systems.
@article{arxiv.2602.00928,
title = {Electro-optic conversion of itinerant Fock states},
author = {Thomas Werner and Erfan Riyazi and Samarth Hawaldar and Rishabh Sahu and Georg Arnold and Paul Falthansl-Scheinecker and Jennifer A. Sánchez Naranjo and Dante Loi and Lucky N. Kapoor and Martin Zemlicka and Liu Qiu and Andrei Militaru and Johannes M. Fink},
journal= {arXiv preprint arXiv:2602.00928},
year = {2026}
}