Four hundred and twenty kilometers of optical fiber now separate a pair of entangled quantum memories, and they stay linked. A team at the University of Science and Technology of China, led by Xi-Yu Luo, Chao-Yang Wang and Ming-Yang Zheng, announced the result, which beats earlier matter-to-matter demonstrations in fiber by more than a factor of four. The paper is in Physical Review Letters.
The setup is simple to describe. At each end sits a cloud of laser-cooled rubidium atoms that works as a quantum memory. The researchers call the two ends Alice and Bob. Midway along the fiber, a station called Charlie catches one photon from each. When the interference pattern matches, the two distant memories snap into a shared entangled state.
Why fiber is hard
Send a single photon straight down a fiber and it fades quickly. That sets a ceiling on how far a direct link can carry quantum information, a limit known as the PLOB bound, which the entrelligence.com write-up places somewhere beyond 320 kilometers. Storing the state in atoms and meeting in the middle got the team past it. Beyond 320 kilometers, the group reports, entanglement came out at a higher rate than any direct transmission through the same fiber could manage.
Three techniques made that possible. Each existed already, but the team combined and tuned them. Photons from the memories were converted to telecom wavelengths, which fiber absorbs far less. A stabilization system corrected for small temperature changes and vibrations that would otherwise scramble the interference. And a single-photon scheme meant only one photon, not two, had to survive the trip.
Still a lab result
Nobody involved is calling this a network. Four hundred and twenty kilometers in a controlled experiment is a long way from a dependable link between cities, and the rate at which entanglement is produced remains low, according to the entrelligence.com report.
The authors point to repeaters as the next use. They say the experiment also produced entanglement at a high rate over shorter distances, around 100 kilometers, which matters for building quantum repeaters. With storage lasting under a second in an optical lattice, they write, entanglement swapping could join many such segments.
Sources
- Physicists Shatter Quantum Entanglement Distance Record With 420 Kilometers of Optical Fiber
- Chinese Physicists Set a Quantum Entanglement Record at 420 Kilometers
