Imagine the benefits if the entire internet got a game-changing upgrade to speed and security. This is the promise of the quantum internet - an advanced system that uses single photons (particles of light) to operate. Researchers at Tohoku University developed a new photonic router that can direct single and (quantum) entangled photons with unprecedented levels of efficiency. This advancement in quantum optics brings us closer to quantum networks and next-generation photonic quantum technologies becoming an everyday reality.

The findings were published in Advanced Quantum Technologies on September 2, 2025.

Photons are the backbone of many emerging quantum applications, from secure communication to powerful quantum computers. To make these technologies practical, photons must be routed quickly and reliably, without disturbing the delicate quantum states they carry. Polarization, the orientation of a photon's electric field, is one of the most common ways to encode quantum information. However, until now, no device had been able to route photons of arbitrary polarization at telecom wavelengths with both low loss and high fidelity.

The research team led by Professor Fumihiro Kaneda from the Graduate School of Science at Tohoku University has overcome this challenge by developing a novel electro-optic router. This router features a compact, specially designed interferometer where the optical paths form a parallelogram instead of a typical rectangle. This design allows for polarization-maintaining operation of all optical components at a nearly normal angle of incidence.

"It shouldn't be like a game of broken telephone where information gets lost along the way," explains Kaneda. "Essentially, our design makes it so that the signal received is as close as possible to the signal that was sent - photon polarization was preserved with over 99% fidelity."

These advancements resulted in a router with a minimal number of optical components, significantly reducing optical loss and enabling stable, high-quality performance.

"It transmits photons with only 0.06 dB of loss, which is only about 1.3%," explains Kaneda, "Not only is it running efficiently at nanosecond speeds, it is already compatible with the telecommunication networks we use today for the internet. The findings from this research provide a valuable foundation for the development of practical quantum network technologies."

In a world-first demonstration, the team also successfully routed two-photon entangled statesーa crucial resource for quantum sensing and quantum networksーwhile maintaining interference visibility of about 97%. This shows that the router can handle not only individual quantum bits, but also complex multiphoton entanglementーa key requirement for scaling up quantum technologies.

Compared with previous approaches, which often suffered from high loss, noise, or distortion, this new device combines all the critical features needed for practical use: low loss, high speed, noise-free operation, and compatibility with existing telecom fiber networks.