Quantum entanglement has made covert communication much more secure.

Quantum physics allows secret information to be shared in a way that is mathematically verified to be safe from the prying eyes of spies. However, until recently, demonstrations of the approach, known as quantum key distribution, have relied on a presumption: the instruments used to make and quantify quantum particles must be known to be faultless. Hidden flaws could allow an unnoticed snoop to breach the security.

Three research teams have now proved the ability to undertake secure quantum transmission without first confirming that the devices are error-free. The technology, known as device-independent quantum key distribution, is based on quantum entanglement, a strange interaction between particles that links their properties even when they are separated by large distances.

A secret code, or key, is used to garble information in ordinary communication, such as the transfer of credit card details over the internet, so that it can only be read by someone else who has the key. But there’s a problem: how can a remote sender and receiver share that key while ensuring that no one else has intercepted it along the way?

Quantum physics allows you to communicate keys by sending a series of quantum particles, such as light particles called photons, and measuring them. Users can ensure that no one else has intercepted the key by comparing notes. Once established, those secret keys can be used to encrypt sensitive information (SN: 12/13/17). In comparison, traditional internet security is built on a flimsy foundation of arithmetic problems that are difficult for today’s computers to answer and may be vulnerable to new technology, specifically quantum computers (SN: 6/29/17).

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However, there is usually a catch to quantum communication. “There can’t be any unexpected glitches,” says National University of Singapore quantum physicist Valerio Scarani. For example, assume your equipment is designed to emit one photon but, unbeknownst to you, it emits two photons. If there are any such flaws, the mathematical proof of security is no longer valid. Even if the transmission appears safe, a hacker could intercept your private key.

Such issues can be avoided with device-independent quantum key distribution. The procedure is based on a quantum technique called a Bell test, which involves measuring entangled particles. Quantum physics contains some “spooky” characteristics, such as the idea that one particle’s observations may be linked to another’s in a faraway location. The first “loophole-free” Bell tests were completed in 2015, thus establishing the existence of quantum physics’ paradoxical character. According to Jean-Daniel Bancal of CEA Saclay in France, “the Bell test simply works as a guarantee.” A malfunctioning gadget would fail the test, implying that “the device is functioning properly.”

The strontium atoms in Bancal and colleagues’ investigation were entangled, electrically charged, and separated by about two meters. To verify that their devices were working, researchers analyzed the ions and produced a secret key, according to an article published in Nature on July 28th.

Quantum communication is typically used for long-distance communications. (It would be easier to simply walk across the room to discuss a secret with someone two meters away.) Scarani and coworkers investigated entangled rubidium atoms separated by 400 meters. The researchers write in the same issue of Nature that the arrangement was capable of producing a secret key. However, the crew did not follow the process completely: the extra distance meant that generating a key would have taken months.

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Researchers in the third study used coupled photons instead of atoms or ions to explore the effects of magnetic fields on atoms and ions. For the first time, key generation has been shown at distances up to 220 meters by Wen-Zhao Liu of Hefei University of Science and Technology and colleagues. According to Liu, photons are particularly challenging since photons are frequently lost during transmission and detection.

Loophole-free Bell tests are difficult enough, but new techniques make it considerably more difficult, according to physicist Krister Shalm of the National Institute of Standards and Technology in Boulder, Colo. “The prerequisites for this experiment are so insanely high that being able to demonstrate some of these skills is just a tremendous achievement,” adds Shalm, who authored a perspective in the same edition of Nature.

That implies the technology will not be used in practice anytime soon, according to physicist Nicolas Gisin of the University of Geneva, who was not involved in the research. Nonetheless, Gisin describes device-independent quantum key distribution as “a tremendously exciting idea.” The Bell tests were created to answer a philosophical inquiry regarding the nature of reality: if quantum physics is as strange as it appears. “The beauty is seeing that this has now become a tool that enables something else,” he explains.

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