Quantum information technologies, once confined to theoretical realms, are rapidly advancing towards practical applications, driving progress in secure communication networks and next-generation computing. A recent breakthrough from researchers in Japan has yielded a crucial component for this evolving landscape: a novel low-loss router capable of directing both single photons and entangled photon pairs without compromising the fragile quantum information they carry.
The Significance of Polarization-Preserving Routing
Photons – individual particles of light – are the workhorses of many quantum communication and computing prototypes. Unlike classical communication systems that rely on beams of light containing trillions of particles, quantum systems often operate using single photons, a characteristic that confers both power and fragility. The loss of even a single photon can result in lost information, and any noise introduced during transmission can rapidly corrupt these delicate quantum states.
To address this challenge, researchers need routing devices that can steer photons along various paths while meticulously preserving their properties. Polarization – the orientation of light waves – is a particularly important carrier of quantum information, or qubits. Therefore, a router that preserves polarization is essential for building quantum memories, facilitating long-distance communication, and ultimately enabling large-scale quantum networks. As the authors emphasize, an ideal photonic router must operate with low loss, minimal noise, high speed, and without disturbing the photons’ quantum states – a combination that has proven difficult to achieve.
A Novel Low-Loss Design
The Japanese team has overcome previous limitations with a design based on a compact interferometer and specially aligned electro-optic crystals. An interferometer splits light into two paths, then recombines them so that light waves either strengthen or cancel each other out depending on their respective path lengths. This configuration, valuable in quantum optics, allows researchers to precisely manipulate photons without disrupting the information they carry. The carefully aligned electro-optic crystals cancel unwanted distortions, ensuring that photons, regardless of their polarization, pass through unaltered.
Exceptional Performance Metrics
Testing demonstrated remarkable performance: the router introduced a loss of just 0.057 decibels – approximately 1.3 percent – and switched photon paths in a mere 3 nanoseconds, while generating virtually no added noise. Researchers achieved more than 99 percent fidelity when routing arbitrarily polarized single photons. Crucially, the device also maintained the correlations of entangled photon pairs – photons whose properties are linked so that measuring one instantaneously affects the other, regardless of distance – with an interference visibility of about 97 percent. This represents the first demonstration of actively switching optical paths for multi-photon entanglement, particularly those encoded into orthogonally polarized states.
Compatibility with Existing Infrastructure
A key advantage of this new router is its operation within the telecom band—the same range used by today’s fiber optic infrastructure. This compatibility allows for direct integration into existing networks, a critical step in scaling up quantum systems beyond laboratory settings.
Challenges and Future Directions
While the results are highly encouraging, the researchers acknowledge remaining challenges. Photon loss during transfer from free space to optical fibers poses one issue. Moreover, the current system’s stability is limited to a few hours. To improve stability, they suggest miniaturizing the setup and implementing active phase stabilization techniques – methods that continuously adjust the optical paths to compensate for environmental disturbances. Furthermore, more precise alignment of the electro-optic crystals could further refine fidelity.
Future work will explore integrating the router with quantum memories and multiplexing techniques that combine multiple photons into complex states. These advancements could pave the way for universal quantum gates—the fundamental building blocks of quantum computation—and more efficient entanglement distribution, which creates secure quantum communication links. The technology also holds promise for precision measurements surpassing the limits of classical physics.
“Our demonstrated scheme will improve various fundamental photonic quantum operations, contributing to the advancement of a wide range of quantum information applications,” the team concludes.
The research, published in Advanced Quantum Technologies, marks a significant step toward realizing practical quantum information technologies.
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
