Quantum physics allows you to share secret information that has been mathematically proven safe from prying eyes by spies. However, quantum key distribution demonstrations have been based on the assumption that quantum particle creation and measurement devices must be perfect. Hidden flaws could enable a sneaky snooper to gain unnoticed access to the security system.
Three teams of researchers now have the capability to secure quantum communication, without the need for any prior verification that the devices can be trusted. The method, called device-independent quant key distribution, is based upon quantum entanglement. This mysterious relationship between particles ties their properties even when they are separated over long distances.
A secret code or key is used in everyday communication such as sending credit card numbers via the internet. It is used to scramble the information so that only the person who has the key can read it. There’s one problem: How can a distant sender and recipient share the key while protecting it from being intercepted by others?
Quantum physics allows you to share keys by sending a series of quantum particles (photons) and taking measurements. The key can only be accessed by the two users if they compare their notes. Once established, the secret keys can be used to encrypt sensitive information. Standard internet security, by contrast, rests on a weak foundation of math problems that are hard for computers to solve today. This could make it vulnerable to new technology such as quantum computers.
Quantum communication is often accompanied by a catch. Valerio Scarani, a quantum physicist at the National University of Singapore, says that there can’t be a glitch that is unforeseeable. He says that, for example, you might imagine your device emitting one photon, but it emits two. These flaws could mean that the mathematical proof for security is no longer valid. Even though your transmission appears secure, a hacker could find your secret key.
These flaws can be eliminated by a device-independent quantum key distribution. This method is based on a quantum technique called a Bell test that involves measuring entangled particles. These tests can show that quantum mechanics does indeed have “spooky”, namely non-locality. This is the notion that one particle’s measurements can be correlated with another particle. 2015 saw the first “loophole-free” Bell test, which proved beyond doubt quantum physics’ counterintuitive nature was real.
“The Bell test acts basically as a guarantee,” Jean-Daniel Bancal, CEA Saclay France, says. The test would be failed if the device was not working correctly.
Bancal and his colleagues used strontium atoms that were electrically charged, entangled, and separated by approximately two meters. The devices behaved properly according to the measurements of the ions. Researchers also created a secret key.
Quantum communication is typically used for long-distance dispatches. It would be much easier to just walk across the room and share a secret with someone only two meters away. Scarani and his colleagues looked at entangled rubidium molecules 400m apart. Researchers report that had the necessary setup to generate a secret key. However, the team did not follow the entire process: Due to the extra distance, it would have taken several months to produce a key.
Researchers discovered that entangled photons were more common than atoms and ions in their third study. It was published in the July 29 Physical Review Letters. Wen-Zhao Liu, a physicist at the University of Science and Technology of China (Hefei), and his colleagues demonstrated that keys can be generated from distances of up to 220 meters. Liu says this is especially difficult for photons because photons can be lost during transmission and detection.
Loophole-free Bell testing is not an easy feat. These techniques are even harder, according to Krister Shalm of the National Institute of Standards and Technology, Boulder, Colo. Shalm also wrote a perspective on the same issue of nature.
This means the technique won’t be practical soon, according to Nicolas Gisin, a physicist at the University of Geneva. He was not involved in the research.
Gisin still believes that device-independent, quantum key distribution is “a completely fascinating idea.” Bell tests were intended to answer a philosophical query about the nature and meaning of reality — whether quantum Physics is as strange as it seems. He said, “To see that it now becomes a device that enables another,” “This is beauty.”