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Unbreakable quantum messages can now be sent by air and will soon be broadcast in space.
Researchers at the China University of Science and Technology (USTC) discovered in 2018 how to secretly share “quantum keys” between orbiting satellites and ground stations, such as Previously reported Live Science. This makes the connection between the Chinese satellite Micius and three terrestrial sites with which it communicates in Europe and Asia the largest secure quantum network in the world. But the quantum secrecy tool originally used by Micius had some leaks, forcing scientists to develop a more advanced form of quantum encryption known as Measurement Device Independent Quantum Key Distribution (MDI-QKD). Now, these same researchers have, for the first time, set up the wireless MDI-QKD, in a city in China, without any optical fibers. And they are preparing 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 work on the experiment.
This opens the door, he said, to convenient quantum-encrypted networks relying on both satellites and fiber-optic cables operating in tandem, which is not possible with current technology.
Related: 12 amazing quantum physics experiments
Quantum secure messages
Every secure piece of data you’ve sent from your phone – instructions to your bank through a mobile app, for example, or Whatsapp messages with your mom – has been broadcast huge distances full of potential hackers. But any nosy listening probably couldn’t make sense of this information as it was turned into gibberish that could only be deciphered with a secure key, essentially a long string of numbers. This string of numbers is scrambled with the information it protects, and only someone who knows the string can decipher them.
These systems are not perfect though, vulnerable to attacks from anyone who listened when the key was shared. They also usually don’t use strings of numbers long enough to be perfectly secure even against someone who hasn’t listened to the key, according to Belgian cryptographer Gilles Van Assche’s book “Quantum Cryptography and Secret-Key Distillation” (Cambridge University Press, 2006).
So, in the 1980s, researchers developed a theoretical method to generate secure keys using Quantum mechanics. They understood that secure keys could be encoded in the quantum properties of individual particles, and secretly traded back and forth. The advantage of this “quantum key distribution” (QKD) is that quantum physics dictates that the very act of observing a particle changes it irreparably. So any spies who tried to intercept the quantum key could be immediately detected by the changes in the particles.
Securing the quantum vault
In recent years, as researchers have begun to build prototypes of quantum key distribution networks using photons (particles of light), a significant flaw has been discovered in the system – “side channel attacks” could siphon off data. copies of a quantum key directly from the receiver, a study published in 2012 in the journal Physical examination letters found.
The researchers therefore developed MDI-QKD, calling it in this 2012 article “a simple solution to remove all side channels of the detector (existing and yet to be discovered)”.
In MDI-QKD, the sender and receiver of a message send their quantum key photons at the same time (along with their decoys) to a third party. Each photon contains a single bit of information: a one or a zero. The third party does not need to be secure and it cannot read the information conveyed by the photons.
“All he can say is the relationship between [photons]Said 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 Scientific Research. It can simply say “if they are the same or different”.
When the sender and receiver send a one or a zero, they receive a message from the relay indicating that they sent the same bit. If they send different numbers, the relay broadcasts that they sent different numbers. A hacker spying on the relay could only tell if the photons were the same or different, but not if 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,” Tittel told Live Science.
All of 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. This requires that the two photons arrive at the relay at exactly the same time.
“We found this to be difficult due to the temperature changes in the device,” he said, which can disrupt the timing.
And it uses dedicated fiber optic cables. Sending photons into the air requires taking into account atmospheric turbulence, which makes the timing even more unpredictable.
This is why the new experience is so impressive, Tittel said. While China has been doing standard QKD with Micius since 2018, no one has until now figured out how to make the most unbreakable encryption system over long distances without fiber optic cables to carry photons back and forth.
In the new study, researchers sent an MDI-QKD secure key 19.2 kilometers of open air between two buildings in Hefei town. To make sure that the photons arrive at the relay at exactly the same time, they developed algorithms that allowed the transmitter and receiver devices to account for fluctuations in this expanse of atmosphere.
Introducing MDI-QKD to space will require more problem solving, including better algorithms that can account for the even greater distances involved.
“The second challenge that we hope to overcome is related to the movement of satellites,” said Qiang Zhang, one of the authors of the article, says Phys.org.
A moving target alters the behavior of photons in a way that must be taken into account very precisely to make sense of the signal.
Tittel said the movement of the satellite makes MDI-QKD “very difficult,” but it is plausible that the USTC team will succeed.
If they do, they will have developed an unbreakable quantum network by any known method of coding. It would be the safest long distance communications network in the world.
Originally posted on Live Science.
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