Physicists in China have linked two quantum memories across 420 kilometers (261 miles) of optical fiber, more than four times farther than earlier demonstrations. The result sits beyond the point where sending entanglement straight down a fiber hits its basic limits.
The team, from the University of Science and Technology of China, is led by Xi-Yu Luo, Chao-Yang Wang and Ming-Yang Zheng. Their paper is due in Physical Review Letters. “Our experiment provides a test bed of studying quantum network applications beyond metropolitan scale,” they write.
How the link works
The memories are laser-cooled clouds of rubidium atoms. Two units, nicknamed Alice and Bob, sit at opposite ends of the fiber. A third node in the middle, Charlie, receives photons from both. When Charlie sees the right interference pattern, Alice and Bob are entangled, though the two never meet.
Three techniques made the distance workable. None is new alone; the team combined and improved them.
First, the photons that quantum memories naturally emit get absorbed quickly by fiber. The researchers converted them to telecom wavelengths, which travel with much lower loss. Second, tiny temperature shifts and vibrations can scramble the interference entanglement depends on, so they built a stabilization system that corrects for them continuously. Third, they used a single-photon scheme, which needs only one photon to survive the trip instead of two.
Beating the limit
A 2017 paper, now a benchmark in the field, set a ceiling on what a lossy channel can carry without repeaters or memories. It is known as the PLOB bound. Past 320 kilometers, Luo and colleagues report, their system produced entanglement at a higher rate than any direct fiber transmission could. That is not a violation of the bound. It shows a memory-based network can beat the best possible direct link.
The researchers also note the setup can generate entanglement at a high rate over shorter spans of about 100 kilometers, which matters for building quantum repeaters. Paired with storage lasting under a second in an optical lattice, they write, entanglement swapping could stitch many such segments together.
A practical quantum internet is still a long way off. But long-distance entanglement between memories is widely seen as one of its building blocks, and this work shows that block holding up over hundreds of kilometers of fiber.
