Unbreakable quantum messages can now be sent by air and will be beamed into space shortly.
Researchers from the University of Science and Technology in China (USTC) worked out in 2018 how to secretly share ‘quantum keys’ between satellites and ground stations in orbit, such as Live Science previously reported. That made the connection between the Chinese Micius satellite and three ground locations with which it communicates in Europe and Asia by far the largest secure quantum network in the world. But the quantum secrecy tool Micius originally used had a few leaks, forcing scientists to develop a more advanced form of quantum encryption known as meter-independent quantum key distribution (MDI-QKD). Now, for the first time, those same researchers have run MDI-QKD wirelessly, through a city in China, without any fiber optic intervention. And they are getting ready to send MDI-QKD to Micius.
“The results of the Chinese group [are] very interesting for the quantum communications community, ”said Daniel Oblak, a quantum communications researcher at the University of Calgary in Ontario who did not participate in the experiment.
It opens the door, he said, to practical quantum-encrypted networks that rely on both satellites and fiber optic cables working together, something not possible with current technology.
Related: 12 Amazing Quantum Physics Experiments
Quantum Safe Messages
All the secure data you’ve ever sent from your phone – instructions to your bank via a mobile app, for example, or Whatsapp messages with your mom – has been broadcast over vast distances filled with potential hackers. But all the snoopers who listened in probably couldn’t understand that information, because it was translated into gibberish that could only be deciphered with a secure key, basically a long string of numbers. That sequence of numbers is scrambled with the information it protects, and only someone who knows the sequence can decode it.
However, those systems are not perfect, vulnerable to attacks from anyone who listened in when the key was shared. They also generally do not use long enough number strings to be perfectly secure, even against someone who did not listen to the key, according to the book by Belgian cryptographer Gilles Van Assche ‘Quantum Cryptography and Secret-Key Distillation’ (Cambridge University Press, 2006) .
Therefore, in the 1980s, researchers developed a theoretical method for using quantum mechanics. They found that secure keys could be encoded in the quantum properties of individual particles and secretly exchanged back and forth. The advantage of this “quantum key distribution” (QKD) is that quantum physics dictates that observing a particle irretrievably changes it. So any spy trying to intercept the quantum key could be immediately detected by the changes in the particles.
Securing the Quantum Vault
In recent years, when researchers started building prototypes of quantum key distribution networks using photons (particles of light), a major flaw surfaced in the system: ‘Side channel attacks’ could allow copies of a quantum key directly from the receiver. siphoning, a study published in 2012 in the journal Physical Review Letters found it.
So researchers developed MDI-QKD, calling it in that 2012 paper “a simple solution to remove all (existing and yet to be discovered) side channels from detectors.”
In MDI-QKD, both the sender and the recipient of a message send their quantum key photons (and decoys too) to a third party at the same time. Each photon contains a piece of information: one or one zero. The third party does not have to be safe and cannot read the information that the photons transfer.
“All it can tell is the relationship between the [photons]”says Wolfgang Tittel, an expert in quantum communication at QuTech, a collaboration between Delft University of Technology in the Netherlands and the Netherlands Organization for Applied Science Research. It can just say” whether they are the same or different. “
When both the sender and receiver send a one or a zero, they get a message from the relay that they sent the same bit. If they send different numbers, the relay reports that they sent different numbers. A hacker spying on the relay could only see whether the photons were the same or different, but not whether they represented a one or a zero.
“But of course the people who sent the states know what they sent, so they know what the other person sent,” Tittle told Live Science.
All those ones and zeros add up to a secure quantum key, and there’s no way a hacker can tell what it is.
But MDI-QKD has its own challenges, said Tittel, who was not involved in this latest experiment. It requires both photons to arrive at the relay exactly at the same time.
“We found that this is difficult because of changes in the temperature of the device,” he said, which can mess up the timing.
And that is the use of special fiber optic cables. To send photons through the sky, atmospheric turbulence has to be taken into account, making timing even more unpredictable.
That’s why the new experiment is so impressive, Tittel said. While China has been doing QKD with Micius as standard since 2018, until now no one had figured out how the more shatterproof encryption system could be run over long distances without fiber optic cables to carry the photons back and forth.
In the new study, the researchers sent an MDI-QKD secure key across 19.2 kilometers of open air between two buildings in Hefei city. To ensure that the photons arrived at the relay exactly at the same time, they developed algorithms that allowed the transmitter and receiver equipment to account for the fluctuations in that area of atmosphere.
Getting MDI-QKD into space requires more troubleshooting, including better algorithms that can account for the even greater distances.
“The second challenge we hope to overcome is related to the movement of satellites,” said Qiang Zhang, one of the authors of the paper. told Phys.org.
A moving target changes the behavior of photons in ways that must be accounted for very precisely to understand the signal.
Tittel said the movement of the satellite makes MDI-QKD “very difficult,” but it is likely that the USTC team will get it done.
When they do, they have developed a quantum network that cannot be cracked by any known method of code breaking. It would be the most secure long-distance communication network in the world.
Originally published on Live Science.