Could quantum entanglement be used in encryption

Tangled light particles : How quantum communication can make chats tap-proof

Exchanges via messenger services or e-mail seem secure to many as soon as the word "encrypted" appears. But the data lock can be cracked depending on the version, warn experts. According to the current state of knowledge, only quantum communication is really safe.

After various proofs of the secure exchange of data in experiments over land and via satellite, the next step is to put it into practice. In a few years, a tap-proof communication network for authorities and businesses is to be created in Germany and Europe, and later also for private individuals.

The latter in particular often bring the “I-have-nothing-to-hide” argument. Experts like Hannes Hübel warn against careless handling of personal information: "A fingerprint or iris scan is unique and cannot be changed," says the researcher from the Austrian Institute of Technology (AIT).

“Such data must not only be secure today, but also for the next few decades.” He alludes to so-called “Store now, decrypt later” attacks, in which intelligence services try to save the exchange of encrypted data in order to later include them large computing power or to crack with future computer systems.

“The encryption algorithms used today are becoming increasingly insecure because they could be solved by quantum computers or supercomputers,” says Hübel. Only quantum-based security is really safe. The data is also encrypted here, but the key itself is generated and transmitted using quantum technology.

Particles of light can blow up hacks

To do this, the technicians use the phenomenon of quantum entanglement: The states of two photons (light particles) are "merged" with one another. If one of them is influenced, for example by a measurement, you “notice” this in the other photon, even if the two are several kilometers apart.

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In terms of data transfer, this means that if the sender and receiver use entangled photons for a key and a third person tries to read the key, this would be revealed immediately. Only when the key arrives undisturbed can both partners be sure that no stranger is tapping into the line. To be on the safe side, the key is changed as often as possible so that the unwanted eavesdropper would have to start over each time.

In theory this is simple, in practice - as is so often the case - complicated. Hübel is working with researchers from Austria, Germany, Belgium and Denmark in the EU project “Uniqorn” with industry on processes to make optical technologies available for quantum communication.

China competes with Europe in quantum research

"Europe still has a leading position in this research field, in tough competition with China, but ahead of the USA, which concentrates more on quantum computing," says Hübel. Now it is important to transfer the basic knowledge into ready-to-use products.

"Apparatus that were previously arranged on laboratory tables should be miniaturized in such a way that they fit into small, manageable components and the technology should of course also become cheaper," he says. The Fraunhofer Heinrich Hertz Institute (HHI) in Berlin is also involved.

Here, a four-square-centimeter platform was developed that, thanks to microlens technology, accommodates components that are normally arranged on a DIN A4 sheet of paper, reports Moritz Kleinert from the HHI. This includes a crystal that generates the entangled photons, a photon detector and the necessary circuits.

Quantum Internet is not far away

In "eight to ten years", Kleinert estimates, these chips could be installed in the gray boxes on the side of the road. "Where the fiber optic network is connected with DSL connections, they could help to transmit data securely using quantum communication."

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However, he does not see any encryption directly on private connections in the foreseeable future. “Often they do not have a fiber optic connection that is needed for this, and the need will hardly be there.” The situation is different when it comes to data transmission from companies or authorities.

This is where the "Qunet" initiative comes in, which has been funded by the Federal Ministry of Research since 2019 with 125 million euros for seven years. It is one of the favorite topics of Minister Anja Karliczek (CDU). "I would like to bring Germany to the top of the world in the field of 'quantum internet'", she said at the beginning of December when the first results of "Qunet" were presented.

Quantum key possible via air connection

To this end, the Max Planck Society, the Fraunhofer Society and the German Aerospace Center have developed the basis for new communication standards. Specifically, a quantum-encrypted data transmission was to be demonstrated for the first time between two federal authorities in Bonn.

However, due to the pandemic, the public screening had to be postponed. A free-beam connection was planned in which the quantum key is not transmitted over a fiber optic cable, but through the air from a transmitter to the receiving telescope. "We have tested the technology several times in our company and, for example, held a video conference with quantum encryption," says Andreas Tünnermann, head of the Fraunhofer Institute for Applied Optics and Precision Mechanics (IOF) in Jena.

Among other things, a source has been developed that can generate a large number of photon pairs and a telescope that registers a large number of the transmitted photons and shows hardly any losses. The free jet sections for the tests were between a few tens of meters and just under two kilometers, the researcher reports.

However, it was not the video sequences themselves that were transmitted; that would have been too much data. "The images and sounds are encrypted as usual and sent over conventional fiber optics, so I have the full bandwidth," says Tünnermann. "The free jet connection is only used to securely transfer the keys."

Obstacles would disrupt the connection

And there are the expected difficulties. First of all, a line of sight must be possible, there must be no house or tree in the way. Then the approval authorities, which are not very familiar with the new technology, have to give their approval. The laser beam in the near infrared range is expanded from a few millimeters to around 20 centimeters in diameter, the power density decreases significantly, so that the process is "eye-safe", assure the Jena researchers.

Fog, bright sunlight and rain can also impair transmission, explains Tünnermann. Research is being carried out here to make the system more robust. “In reality, it will be hybrid networks that combine free-beam and fiber optic technology for quantum communication, at least within metropolitan areas,” he says.

In order to transmit quantum states over greater distances in the country or across borders, researchers at the German Aerospace Center are working on free-jet connections via aircraft or satellite. Experts at the Max Planck Institute for the Science of Light (MPL) in Erlangen are researching how the overall system of secure data transmission can best be set up. In addition, the technology must be certified, which is why the Federal Office for Information Security is involved.

So it will still be some time before the quantum-based Germany network. Fraunhofer researcher Tünnermann is convinced that individual applications could come soon, citing data centers of banks or insurance companies as examples.

“These permanently reflect your data,” he explains. Companies would like to use quantum keys to secure these lines. "In this case, commercial use could be possible in two or three years."

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