Chinese scientists have successfully sent information between entangled particles through sea water, the first time this type of quantum communication has been achieved underwater.
In this proof-of-concept experiment, information was sent across a 3.3-metre (10.8-foot) long tank of seawater, but the researchers predict they should be able to use the same technique to send unhackable communications close to 900 metres (0.55 miles) through open water.
"People have talked about the idea of underwater quantum communication before, but I'm not aware of anyone who has done an experiment like this," Thomas Jennewein from the University of Waterloo in Canada told Devin Powell over at New Scientist.
"An obvious application would be a submarine which wants to remain submerged but communicate in a secure fashion."
This is a big deal, because quantum communication - also known as quantum teleportation - promises to allow people to send messages that are protected from prying eyes by the laws of physics. It's the ultimate encryption.
It's based on the idea of quantum entanglement - that kooky phenomenon Einstein referred to as "spooky at a distance". Basically, quantum entanglement means that two particles become inextricably linked so that whatever happens to one will automatically affect the other, no matter how far apart they are.
Through that mechanism, scientists have already 'teleported' information across vast distances through optical fibre and even open space.
Earlier this year, a separate team of Chinese researchers were able to use quantum entanglement to teleport information to a satellite in Earth's orbit across more than 500 km (311 miles).
But up until now, no one had done the same thing in water, which is notorious for scattering anything we try to beam through it. Just think of shining a laser pointer into the air and into water.
For this experiment, researchers from Shanghai Jiao Tong University took seawater from the Yellow Sea and set it up in a 3 metre tank in the lab.
They then created a pair of entangled photons by shooting a beam of light through a crystal. Whatever the polarisation of one of the photons, its pair would automatically have the opposite polarisation.
These particles were placed at opposite ends of the tank, and the team showed that despite being separated by metres of seawater, they could accurately communicate information between them more than 98 percent of the time.
It's still early days, and not only is it important for other teams to now replicate this result, but it remains to be seen whether the same thing can be done across greater distances, but also in seawater not confined to a tank.
Based on the team's calculations, they predict that it would be possible to achieve quantum communication through open water across a distance of 885 metres (0.55 miles) using photons in the blue-green window.
But New Scientist reports that other groups have calculated a limit of underwater quantum communication of just 120 metres (0.07 miles).
"Because ocean water absorbs light, extending this is going to difficult," Jeffrey Uhlmann, a physicists from the University of Missouri in Columbia, told Powell.
How far we can stretch this underwater quantum communication remains to be seen, but now that researchers have shown it's possible, it's only a matter of time before the limits begin to be pushed.
The research has been published in The Optical Society.