This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Optical fibers are already the backbone of global communication systems. Recently, however, physicists have started to explore how their functionality could be boosted further by conveying information via entangled quantum particles—potentially enabling instantaneous exchanges of information across vast distances.
Such a system could eventually be the basis of a future 'quantum internet,' offering a level of security and computing power beyond anything possible today. In new research published in Physical Review Letters, a team led by Xi-Yu Luo at the University of Science and Technology of China in Hefei has pushed that vision further than ever, entangling two quantum memories across 420 kilometers (261 miles) of optical fiber—more than four times the previous record. To date, entangled photons have retained their quantum information for as far as 404 kilometers (251 miles) when passing through optical fibers, but pushing past this limit has proven notoriously difficult.
Since glass isn't perfectly transparent, every kilometer of fiber absorbs roughly the same fraction of the photons passing through it. Because this fraction compounds over distance, an individual photon's odds of surviving a very long trip fall off exponentially. In addition, the information-storing capacity of entangled photons has remained largely out of reach beyond much shorter ranges.
Luo's team tackled the problem with an approach named the 'Duan-Lukin-Cirac-Zoller' scheme. Rather than trying to usher a fragile entangled photon pair safely to both ends of the line, each end holds a cloud of atoms cooled to extreme temperatures. Each cloud can be nudged to emit a single photon carrying a trace of that atom cloud's quantum state.
Only one photon needs to make the long journey to a meeting point in the middle, where it is compared with its counterpart. A successful comparison confirms that the two atom clouds are now entangled, even though they never physically touch and each has only ever sent light in one direction. To implement this system, the team converted the atoms' photons to wavelengths used in telecommunications, which travel through fiber with far less loss.
They also built a stabilization system to counter the tiny vibrations and temperature shifts that would otherwise scramble the delicate timing needed for the photons to interfere correctly. With these improvements in place, they verified entanglement between two clouds of atoms separated by an optical fiber some 420 kilometers (261 miles) long. Beyond 320 kilometers (199 miles), their approach beat the best theoretical limit for sending entanglement directly through fiber without any memory involved.
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