Building photonic links for microwave quantum processors.

Optical photons play unique role in transmitting information over long distances. Photonic links by the optical fiber networks compose the backbone of today's global internet. Such fiber optics can also provide the most cost-effective quantum channels to distribute quantum information across distant stationary nodes in future large-scale quantum networks. This prospect motivates the recent emerging efforts in developing microwave-optical quantum transduction technology to interconnect microwave quantum processors. Various frequency conversion approaches are investigated to efficiently bridge the enormous electromagnetic frequency gap while preserving quantum coherence. Nonetheless, high-fidelity entanglement generation between remote quantum processing units has remained out of reach to date. Here, we summarize the state-of-the-art progresses in quantum transducer engineering and provide the perspectives on the key challenges and opportunities toward optically heralded quantum entanglement distributions.