Stealth communication is now more secure thanks to quantum entanglement.
Quantum physics provides a way to share secret information that is mathematically proven to be safe from the prying eyes of spies. But until now, demonstrations of the technique, called quantum key distribution, have been based on one assumption: The devices used to create and measure quantum particles must be known to be flawless. Hidden flaws could allow a sneaky eavesdropper to penetrate security unnoticed.
Now, three teams of researchers have demonstrated the ability to perform secure quantum communications without prior confirmation that the devices are foolproof. The method, called device-independent quantum key distribution, is based on quantum entanglement, a mysterious relationship between particles that links their properties even when they are separated by long distances.
In everyday communication, such as transmitting credit card numbers over the Internet, a secret code or key is used to distort information so that it can only be read by someone else with the key. But there is a dilemma: how can a distant sender and receiver share that key while making sure no one else has intercepted it along the way?
Quantum physics provides a way to share keys by transmitting a series of quantum particles, such as light particles called photons, and making measurements on them. By comparing notes, users can be sure that no one else has intercepted the key. Those secret keysonce established, it can be used to encrypt sensitive information (Serial Number: 12/13/17). By comparison, standard Internet security is based on a relatively unstable foundation of mathematical problems that are difficult for today’s computers to solve, which could be vulnerable to new technology, namely quantum computers (Serial number: 06/29/17).
But quantum communication usually has a catch. “There can be no unforeseen technical problem,” says quantum physicist Valerio Scarani of the National University of Singapore. For example, he says, imagine your device is supposed to emit one photon but, unbeknownst to you, it emits two photons. Any such failure would mean that the mathematical proof of security no longer holds. A hacker could detect his secret key, even though the transmission appears secure.
Device-independent quantum key distribution can rule out such flaws. The method is based on a quantum technique known as Bell’s test, which involves measurements of entangled particles. Such tests may show that quantum mechanics really does have “spooky” properties, namely nonlocality, the idea that measurements of one particle can correlate with those of a distant particle. In 2015, researchers conducted the first “gapless” Bell tests, which certified without a doubt that the counterintuitive nature of quantum physics is real (Serial number: 12/15/15).
“The Bell test basically acts as a guarantee,” says Jean-Daniel Bancal of CEA Saclay in France. A faulty device would fail the test, so “we can infer that the device is working properly.”
In their study, Bancal and colleagues used electrically charged, entangled strontium atoms separated by about two meters. Measurements of those ions confirmed that their devices were behaving correctly, and the researchers generated a secret keyreports the team on July 28 Nature.
Quantum communication is generally intended for long-distance shipping. (To share a secret with someone six feet away, it would be easier to just walk across the room.) So Scarani and his colleagues studied entangled rubidium atoms from 400 meters away. Startup had what it took to produce a secret key, the researchers report in the same issue of Nature. But the team didn’t follow the process through to the end: the extra distance meant producing a key would have taken months.
In the third study, published July 29 Physical Review Lettersresearchers entangled photons discussed instead of atoms or ions. Physicist Wen-Zhao Liu of the University of Science and Technology of China in Hefei and his colleagues also demonstrated the ability to generate keys at distances of up to 220 meters. This is particularly difficult to do with photons, Liu says, because photons are often lost in the process of transmission and detection.
Gapless Bell tests are no easy task, and these techniques are even more challenging, says physicist Krister Shalm of the National Institute of Standards and Technology in Boulder, Colorado. “The requirements for this experiment are so absurdly high that it’s just an impressive achievement to be able to demonstrate some of these capabilities,” says Shalm, who wrote a perspective on the same topic of Nature.
That means the technique won’t be of practical use any time soon, says physicist Nicolas Gisin of the University of Geneva, who was not involved in the research.
Still, device-independent quantum key distribution is “a totally fascinating idea,” says Gisin. Bell’s proofs were designed to answer a philosophical question about the nature of reality: is quantum physics really as weird as it seems? “Seeing this now become a tool that enables something else,” she says, “this is the beauty.”