Unbreakable quantum messages can now be sent through the air and will soon be broadcast in space.
Researchers at the University of Science and Technology in China (USTC) worked out in 2018 how to secretly share “quantum keys” between satellites and ground stations, such as Live Science reported earlier. This has made the connection between the Chinese Micius satellite and three ground sites with which it communicates in Europe and Asia by far the largest secure quantum network in the world. But the quantum secret tool Micius originally used had some leaks, and scientists had to develop a more advanced form of quantum encryption, known as measuring instrument-independent quantum key distribution (MDI-QKD). The same researchers pulled MDI-QKD wirelessly for the first time in a city in China, without involving fiber optics. And they are getting ready to send MDI-QKD to Micius.
“The results by the Chinese group [are] very interesting to the quantum communication community, ”said Daniel Oblak, a quantum communication researcher at the University of Calgary in Ontario, who did not work on the experiment.
This opens the door to practical quantum-encrypted networks that rely on both satellites and fiber-optic cables working together, something that is not possible with current technology.
Related: 12 beautiful quantum physics experiments
Quantum Safe Messaging
Every bit of secure data you have ever sent from your phone – instructions to your bank via a mobile app, for example, or Whatsapp messages with your mother – has been broadcast over large distances full of potential hackers. But any tuck shop that listens to this could not possibly make the information meaningful, as it has been turned into a chatter that can only be decrypted with a secure key, basically a long series of numbers. That string of numbers is scratched with the information it protects, and only someone who knows the string can delete it.
However, these systems are not perfect, vulnerable to attacks from anyone who listened when the key was shared. According to the book “Quantum Cryptography and Secret-Key Distillation” (Cambridge University Press), they usually do not use long enough numbers to even be safe against someone who did not listen to the key. , 2006).
Thus, in the 1980s, researchers developed a theoretical method of obtaining secure keys using quantum mechanics. They discovered 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 determines that the observation of a particle changes it irreparably. Any spies trying to intercept the quantum key could therefore be detected immediately by the changes in the particles.
Securing the quantum vault
When researchers began building prototype distribution networks for quantum keys using photons (light particles) over the past few years, a major flaw appeared in the system – ‘Side attacks’ could sift copies of a quantum key directly from the receiver,’ a study published in the journal 2012 Physical overview letters found.
Thus, researchers developed MDI-QKD and called it in the 2012 article “a simple solution to remove all (existing and yet to be discovered) channels from the detector side.”
In MDI-QKD, the sender and recipient of a message send their quantum key photons (as well as decoys) simultaneously to a third party. Each photon contains a single piece of information: one or a zero. The third party does not have to be safe and cannot read the information transmitted by the photons.
“All it can see is the connection between the [photons], “said Wolfgang Tittel, a quantum communications expert with QuTech, a collaboration between the Delft University of Technology in the Netherlands and the Dutch Organization for Applied Scientific Research. It can only say ‘whether it is the same or different.’
When both the sender and the receiver send one or zero, they get a message from the relay that they have sent the same piece. If they send different numbers, send the relay that they send different numbers. A hacker spying on the relay could only see if the photons were the same or different, but not if it represented one or a zero.
“But of course, the people who sent the statements know what they sent, so they know what the other person sent,” Title told WordsSideKick.
All the units and zeros form a secure quantum key, and a hacker cannot know what it is.
But MDI-QKD has its own challenges, says Tittel, who were not involved in this latest experiment. This requires that both photons arrive at the relay at exactly the same time.
“We have found it difficult due to changes in the temperature of the device,” he said, which could upset the timing.
And that’s using dedicated fiber optic cables. To send photons through the air, atmospheric turbulence must be taken into account, which makes timing even more unpredictable.
That’s why the new experiment is so impressive, Tittel said. While China has been doing standard QKD with Micius since 2018, so far no one has determined how to do the more unbreakable long-distance encryption system without fiber optic cables to carry the photons back and forth.
In the new study, the researchers sent a safe key from MDI-QKD over 19.2 kilometers of open air between two buildings in the city of Hefei. To ensure that the photons arrive at the relay at exactly the same time, they developed algorithms that enabled the transmitter and receiver devices to account for the fluctuations in the atmosphere.
Getting MDI-QKD into space will require more problem-solving, including better algorithms that can offset 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 article. tell Phys.org.
A moving target changes the behavior of photons in ways that must be taken into account very accurately to make the signal meaningful.
Title said the movement of the satellite makes MDI-QKD ‘very difficult’, but that it is likely that the USTC team will possibly pull it off.
If they do, they will develop a quantum network that is unrepairable by any known method of codebreaking. It would be the most secure long distance communication network in the world.
Originally published on Live Science.