Closing loopholes puts quantum cryptography on surer ground

More evidence that the quantum and macro worlds are very different places comes from researchers who have been sending entangled photons around the Canary Islands.

This is important because it means scientists are probably right about the way quantum entanglement works, which matters enormously if quantum cryptography is to become a viable and useful tool.

The team of physicists, working at the Austrian Academy of Sciences and the University of Vienna, are close to physically demonstrating that without a doubt, quantum mechanics does violate Bell’s Theorem. They have succeeded in closing two of the three so-called loopholes – or experimental problems – associated with the famous Bell Tests.

Bell’s Theorem is a statistical proof that makes predictions for classical systems. But it allows that its predictions may be violated under quantum mechanics.

So far, tests of Bell’s theorem using entangled particles have shown overwhelmingly that Einstein's 'spooky action at a distance' is possible in the quantum realm. But there have been three major problems with the experiments that might affect the validity of the findings. The researchers have simultaneously overcome two of these, and suggest that emerging technologies could allow the third to be tackled as well.

From PhysOrg.com: These three loopholes can involve locality (if there is not a large enough distance separating the two objects at the time of measurement), the freedom to choose any measurement settings (so measurement settings may be influenced by hidden variables, or vice versa), and fair sampling (a small fraction of observed objects may not accurately represent all objects due to detection inefficiencies).

"The question of whether nature can be understood in terms of classical concepts and explained by local realism is one of the deepest in physics,” coauthor Johannes Kofler told PhysOrg.com. “Getting Bell tests as loophole-free as possible and confirming quantum mechanics is therefore an extremely important task. From a technological perspective, certain protocols of quantum cryptography (which is entering the market at the moment) are based on entanglement and violation of Bell’s inequality. This so-called ‘unconditional security’ must in practice take care of the loopholes in Bell tests."

Much more at PhysOrg.com here.