Low-loss router based on an interferometer preserves polarization while directing single photons and entangled pairs.
Quantum information technologies are steadily moving from theory to experiment, with researchers around the world working on the building blocks of secure communication networks and next-generation computing. A new study from Japan reports another critical advance: a low-loss router that can direct both single photons and pairs of entangled photons without damaging the fragile information they carry.
The device, demonstrated by researchers at Tohoku University and Japan’s National Institute of Information and Communications Technology, is the first to combine extremely low signal loss with the ability to preserve polarization — the orientation of light waves — that encodes quantum information.
“Routing of arbitrarily polarized single photons and entangled photons is a crucial technology for scaling up quantum information applications,” the team explains in their study published in Advanced Quantum Technologies.
Why photon routers matter
Quantum communication networks and some prototypes of quantum computers rely on photons to carry information. Unlike classical communication, where beams of light consist of trillions of particles, these systems often use individual photons, which makes them both powerful and fragile. Losing even one can mean losing information, and any noise introduced during transmission can quickly scramble delicate quantum states.
For this reason, researchers need devices that can steer photons along different paths while keeping their properties intact. In particular, polarization is one of the most widely used carriers of quantum information, or qubits. A polarization-preserving router is therefore essential for building quantum memories, long-distance communication channels, and large-scale networks.
As the authors note, “a photonic router for single photons requires low-loss, low-noise, and high-speed operation without disturbing their quantum states.” Achieving this combination has proven difficult. Past attempts either introduced too much loss, worked only for specific polarizations, or required bulky compensators — extra optical elements added to correct distortions in the photon’s polarization — that reduced stability.
A new low-loss design
The Japanese team overcame these hurdles by designing a router based on a compact interferometer — an arrangement that splits light into two paths and then recombines them so that the waves can strengthen or cancel each other depending on the path lengths — combined with specially aligned electro-optic crystals. In quantum optics, this approach is especially valuable because it allows researchers to manipulate photons in precise ways without destroying the fragile information they carry. These elements cancel out unwanted distortions, allowing photons with any polarization to pass through unaltered.
Tests showed that the router introduced a loss of only 0.057 decibels — about 1.3 percent — and switched photon paths in just 3 nanoseconds, all while operating with virtually no added noise.
“Arbitrarily polarized […] single photons are routed with more than 99 percent fidelity to an ideal operation,” the researchers report. They also confirmed that the router worked for entangled photon pairs — photons whose properties remain linked so that measuring one instantly affects the other, no matter how far apart they are — preserving their correlations with an interference visibility of about 97 percent. “This is the first demonstration of actively switching optical paths of multi-photon entanglement, which is encoded into orthogonally polarized states,” they add.
Because the router works in the telecom band — the same range used in today’s fiber optic infrastructure — it can be directly integrated into existing networks. This compatibility is critical for scaling up quantum systems beyond laboratory experiments.
Challenges and next steps
While the results are promising, the researchers acknowledge several challenges remain. One issue is the modest losses that occur when photons are transferred from free space into optical fibers, the channels used to guide light over long distances. Another is that the system’s long-term stability is currently limited to a few hours.
“The stability of the router can be further improved by miniaturizing the setup and by using active phase stabilization techniques,” the authors write, referring to methods that continually adjust the optical paths to keep them balanced despite environmental disturbances. They add that more precise alignment of the electro-optic crystals — the components that control how the photons’ polarization is shifted — could raise fidelity even closer to perfection.
Future work will explore integrating the router with quantum memories and multiplexing techniques that combine many photons into complex states. Such advances could enable universal quantum gates — basic building blocks that can perform any quantum computation — and more efficient entanglement distribution, the process of sharing entangled particles between different locations to create secure quantum communication links. They could also support precision measurements that surpass the limits of classical physics.
“Our demonstrated scheme will improve various fundamental photonic quantum operations,” the team concludes, “contributing to the advancement of a wide range of quantum information applications.”
Source: Pengfei Wang et al, Low‐Loss Polarization‐Maintaining Router for Single and Entangled Photons at a Telecom Wavelength, Advanced Quantum Technologies (2025). DOI: 10.1002/qute.202500355
Feature image by Gerd Altmann via Pixabay