Chinese scientists have achieved the quantum entanglement of cold atoms across a record-breaking 420km (260 miles), a breakthrough they say lays the groundwork for intercity quantum networks.
According to a paper published by the peer-reviewed journal Physical Review Letters on August 11, the distance was more than four times previous demonstrations and exceeded the threshold at which direct transmission encounters fundamental physical limits.
“Our experiment provides a test bed for studying quantum-network applications beyond the metropolitan scale,” said the team of physicists, who included Pan Jianwei from the University of Science and Technology of China in Hefei, Anhui province.
The researchers used two quantum memory units, each consisting of laser-cooled clouds of rubidium atoms, 420km apart and connected by an optical fibre link.
The aim was to achieve entanglement – the phenomenon in which two or more particles – often photons – become inextricably linked, sharing a joint quantum state regardless of the physical distance between them.
While scientists have previously succeeded in entangling light particles across vast stretches of empty space and through optical fibre, introducing quantum memory into the equation makes the process more complex.