Proof-of-principle long-distance Sagnac twin-field quantum key distribution network

Imagine a world where two people can share a secret code that is physically impossible to steal. This is the promise of quantum key distribution, a method of communication that uses individual particles of light to create a secure link. If a thief tries to peek at the message while it travels, the laws of physics ensure the message changes, alerting the sender and receiver that the line is compromised. For decades, scientists have successfully built these secure lines between two distant points. However, the real world is not made of simple point-to-point connections; it is a complex web of cities and towns that need to talk to one another. To make this technology useful for everyone, we must move from connecting two friends to building a network that can connect many, all while keeping the signal strong over hundreds of kilometers of fiber-optic cable.

The challenge in building such a network is that light signals fade as they travel through glass fibers. In the past, extending these secure connections required expensive, delicate equipment to constantly correct the signal, or it was limited to very short distances. A newer approach, known as twin-field quantum key distribution, offers a way to send signals much farther by having two users send their light to a central station to meet in the middle. While this method has shown great promise in the lab, most demonstrations have been limited to just two users or required complex, active systems to stabilize the signal. The question remained: could this work in a true, multi-user network that stretches over long distances without needing constant, high-tech intervention?

A team of researchers has now answered this question by building a working prototype of a three-user quantum network that spans 127 kilometers. Their experiment, described in a recent study, proves that it is possible to generate secure keys between any pair of users in a ring-shaped network using relatively simple and affordable equipment. Instead of relying on complex, active systems to constantly adjust the phase of the light, the team used a clever arrangement called a Sagnac interferometer. This setup allows the light to travel in two directions around a loop, meeting back at the center. Because the light travels both ways through the same fibers, the system naturally cancels out many of the disturbances that usually ruin the signal, such as temperature changes or vibrations. This "plug-and-play" nature means the network can remain stable without the need for active phase stabilization or post-compensation, which are typically required in long-distance experiments.

The researchers set up their network with three users, named Alice, Bob, and Danny, connected in a ring. The total length of the fiber optic cable used was 127 kilometers, composed of four large spools of ultra-low-loss fiber. Between the users and a central station, the distances varied, creating an asymmetric network where some paths were longer than others, much like a real-world city grid. To handle the noise that naturally occurs when light bounces back inside the fiber, the team used a technique called burst patterning. This involves sending the light in short, timed bursts rather than a continuous stream, which helps separate the useful signal from the background noise. They also implemented a system to keep the polarization of the light aligned. Since the orientation of light waves can drift as they travel through the fiber, the team used electronic controllers to constantly monitor and adjust the light, ensuring it remained in the correct state to interfere properly when the two paths met.

Over the course of an hour, the system demonstrated remarkable stability. The researchers measured how well the light waves interfered with each other, a value known as visibility, and found it remained steady at 93 percent. This high level of stability is crucial because it ensures that the secret key can be generated without too many errors. The team successfully generated secure keys for two different pairs of users. In the first scenario, Alice and Bob communicated across a 102-kilometer channel with a total signal loss of 45 decibels. Despite this significant loss, they achieved a secure key rate of 1.398 times 10 to the power of minus 5 bits per pulse. In the second scenario, Alice and Danny communicated across a 77-kilometer channel with 34 decibels of loss, achieving an even higher key rate. The experiment also accounted for the finite amount of data collected, showing that secure keys could be generated even with limited data, a critical requirement for real-world deployment.

What makes this achievement particularly significant is the equipment used. The entire network operated using single-photon avalanche detectors, which are standard, commercially available components, rather than the expensive and complex superconducting detectors often required for long-distance quantum experiments. Furthermore, the system used a single commercial laser and did not require the users to lock their lasers to a specific frequency, a process that usually demands complex hardware. By demonstrating that a multi-user network can function over long fibers without active phase stabilization, the researchers have shown a path toward a more practical and cost-effective future for quantum communication. This work suggests that the dream of a widespread, secure quantum internet is not just a theoretical possibility but an engineering reality that can be built with existing technology. The results indicate that the barriers to scaling quantum networks are being lowered, moving the technology closer to the day when secure communication is a standard feature of our global infrastructure.