Entanglement and a quantum internet

US physicists say they have found “the missing link” for a practical quantum internet and, as such, taken a big step forward in the development of long-distance quantum networks.

The answer is a modern version of the old-fashioned idea of a repeater, which allows communication over distance by correcting or compensating for signal loss. We’ve been doing it since the days of the telegraph.

In this case, researchers from Harvard University and the Massachusetts Institute of Technology (MIT) have developed a prototype quantum node they say can catch, store and entangle bits of quantum information.

They describe the process and the outcomes in a paper in the journal Nature.

“This demonstration is a conceptual breakthrough that could extend the longest possible range of quantum networks and potentially enable many new applications in a manner that is impossible with any existing technologies,” says Harvard’s Mikhail Lukin.

Entanglement is the vital bit because it allows information to be perfectly correlated across any distance, removing the risk of eavesdroppers. Thus, it is the foundation for applications such as quantum cryptography – security guaranteed by the laws of quantum physics, as the researchers put it.

Quantum communication over long distances is still affected by conventional photon losses, which is one of the major obstacles for realising large-scale quantum internet, and so classical repeaters can’t be used to fix information loss.

In their place are so-called quantum repeaters, which create a network of entangled particles through which a message can be transmitted.

At each stage of such a network, the repeaters must be able to catch and process quantum bits of quantum information to correct errors and store them long enough for the rest of the network to be ready. 

Until now, the researchers say, that has been impossible for two reasons: single photons are difficult to catch, and quantum information is fragile, making it hard to process and store for long periods.

Lukin and colleagues at Harvard and MIT have been working for many years to harness a system that can perform both of these tasks well – silicon-vacancy colour centres in diamonds.

These centres are tiny defects in a diamond’s atomic structure that can absorb and radiate light, giving rise to a diamond’s brilliant colours.

The researchers integrated an individual colour centre into a nanofabricated diamond cavity, which confines the information-bearing photons and forces them to interact with the single colour centre. 

They then placed the device in a dilution refrigerator, which reaches temperatures close to absolute zero, and sent individual photons through fibre optic cables into the refrigerator, where they were efficiently caught and trapped by the colour centre.

The device can store quantum information for milliseconds, which is long enough for information to be transported over thousands of kilometres. 

“This device combines the three most important elements of a quantum repeater – a long memory, the ability to efficiently catch information off photons, and a way to process it locally,” says Harvard’s Bart Machielse.

“Each of those challenges have been addressed separately but no one device has combined all three.”

The team is now deploying its quantum memories in real, urban fibre-optic links.