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Physics + MathPhysics & Math

Definitive Quantum Entanglement Test Could Secure the Future of Cryptography

A new experiment demonstrates for the first time that "spooky action at a distance" is a real quantum phenomenon.

ByAllison EckNOVA NextNOVA Next
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These crystals are used for storing entangled photons.

Einstein was right about a lot of things, but this isn’t one of them.

A new experiment demonstrates for the first time that his aversion to quantum entanglement may have been misguided—and that, in fact, “spooky action at a distance” is a real quantum phenomenon, not just a product of scientists’ wishful musings.

According to quantum mechanics, particles can be in different “states” at once, and when they’re observed, those superimposed states collapse into just one. When particles are “entangled,” their states are connected across space: when the one particle is measured, the other particle’s properties become frozen as well. Einstein wasn’t a fan of this idea because it suggested that communication between particles could travel instantaneously—i.e., faster than the speed of light. He countered the argument by saying that quantum particles have hidden variables that make them correlated even before a state-collapsing measurement occurs.

But in the 1960s, a physicist named John Bell claimed that such “hidden variables” can account for only some of entanglement’s weirdness—not all of it.

Here’s Zeeya Merali, writing for Nature News:

The first Bell test was carried out in 1981, by Alain Aspect’s team at the Institute of Optics in Palaiseau, France. Many more have been performed since, always coming down on the side of spookiness—but each of those experiments has had loopholes that meant that physicists have never been able to fully close the door on Einstein’s view. Experiments that use entangled photons are prone to the ‘detection loophole’: not all photons produced in the experiment are detected, and sometimes as many as 80% are lost. Experimenters therefore have to assume that the properties of the photons they capture are representative of the entire set.

To get around the detection loophole, physicists often use particles that are easier to keep track of than photons, such as atoms. But it is tough to separate distant atoms apart without destroying their entanglement. This opens the ‘communication loophole’: if the entangled atoms are too close together, then, in principle, measurements made on one could affect the other without violating the speed-of-light limit.

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It’s a central paradox that physicists have now solved. A new paper published to the arXiv reports that a team, led by Ronald Hanson of Delft University of Technology, used a technique called entanglement swapping to compensate for the flaws that both light (via photons) and matter (via atoms) present. They placed two unentangled electrons in diamond crystals and positioned them about 0.8 miles apart. Both of these electrons were entangled with their own “partner” photon—and the partner photons were placed somewhere else. Then, those two photons were entangled with each other. The result was that the partner electrons, which hadn’t moved, became entangled with each other, as well. The scientists were able to replace this 245 times, all the while avoiding the messiness of detection and communication loopholes.

Understanding the true nature of entanglement could help us secure the future of quantum cryptography, since companies already employ the laws of quantum mechanics to protect against hackers. The problem, though, is that without a reliable Bell test, the detection loophole leaves systems vulnerable to spies. But Anton Zeilinger, a physicist at the Vienna Centre for Quantum Science and Technology, told Nature News that this new experiment “is the final proof that quantum cryptography can be unconditionally secure.” While practical steps toward that end may be more difficult than we bargained for, this new study shows that it’s possible.

Play NOVA’s Cybersecurity Lab to thwart attackers, craft code, and defeat malicious hackers. Also, explore whether quantum mechanics is less “spooky” than all the hype would have us believe, and see whether quantum entanglement could make teleportation a reality.

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