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Danish world record: First quantum-secured fibre-optic link

Anders Eldrup, chairman of the Innovation Fund Denmark, spoke with Natasha Friis Saxberg, CEO of the Danish ICT Industry Association, over a quantum-encrypted connection. Illustration: Kyv Kyvsgaard/DTU Electro

There is a certain sense of nervousness and tension in the tightly packed laboratory, where everyone’s eyes are fixed on a black screen.

It is Tuesday noon, and we are at DTU Electro in Lyngby, where quantum researchers from the Technical University of Denmark (DTU) and the University of Copenhagen will shortly switch on the first quantum-encrypted connection based on a single-photon source.

The video link must be transmitted over an 18-kilometre-long existing fibre-optic connection between DTU and the University of Copenhagen.

A few seconds later, the first direct images appear from the Niels Bohr Institute in Copenhagen.

Anders Eldrup, chairman of the Innovation Fund Denmark, who is currently located at DTU, can start his conversation with Natasha Friis Saxberg, CEO of the Danish ICT Industry Association, who is located at the Niels Bohr Institute.

The whole session takes about five minutes.

“So we have established the first ever quantum-encrypted and therefore fully secure connection between two different locations,” Anders Eldrup says.

The conversation revolves—naturally enough—around quantum encryption, and the whole purpose of today’s demonstration is to show how far the Danish quantum research community has actually come in providing practical and usable quantum encryption.

“It is the first demonstration in the world to use a deterministic single-photon source of appropriate quality and stability to support a key. The advantage of using a single light particle is that it cannot be shared without it being visible to all parties,” explains Leif Katsuo Oxenløwe, professor and leader of the SPOC research centre at DTU Electro.

To replace encryption algorithms

Traditional algorithm-based encryption is based on computational complexity.

The more complex the algorithm that generates a key, the harder it is to crack.

An experimental setup is still required to receive and decode the single-photon source and convert the signal into electrical signals in the form of 0s and 1s. Illustration: Laurids Hovgaard

But that type of encryption technology will in the future be susceptible to being cracked by a quantum computer, so the race to develop new encryption methods is on—with quantum encryption as an obvious candidate.

“Quantum encryption can provably resist all attacks, because any attempt at eavesdropping will be revealed,” says Peter Lodahl, professor at the Niels Bohr Institute, head of the project, and leader of the Hybrid Quantum Networks (Hy-Q) research centre.

Old encryption principles can now be realized

The ideas behind quantum encryption are not new.

Already in the 1960s, Claude Shannon set out four principles behind the one-time pad encryption technique.

The encryption key must be completely random, it must never be reused, outside hackers must not be able to intercept it, and it must be at least as long as the message itself.

“We haven’t been able to practically implement that until today,” Leif Katsuo Oxenløwe says.

But advances in quantum technology now make it possible with a single-photon source in a fibre-optic network.

The encryption keys are encoded in the polarization of the photon, which can flip vertically(1) or horizontally(0) and diagonally by either +45 degrees(1) or -45 degrees(0).

The two parties to the conversation then compare how the single photon is modulated to check if the keys match.

Here you can see three minutes of the conversation between Anders Eldrup, chairman of the Innovation Fund Denmark, and Natasha Friis Saxberg, CEO of the Danish ICT Industry Association:

The single-photon source itself was made at the Niels Bohr Institute, while the encryption was developed at DTU Electro.

At the Niels Bohr Institute, Professor Peter Lodahl has succeeded in “taming the light” in an optical chip which, among other things, is frozen at temperatures close to absolute zero. Once this is done, a single photon can be emitted at a time from the optical chip.

This summer, researchers from DTU Physics succeeded in transferring data using the same method between two of Danske Bank’s data centres simulated on two computers.

This time, the quantum-encrypted connection was achieved at a distance of 18 kilometres.

Keys are exchanged over a regular fibre-optic connection between the Niels Bohr Institute in Copenhagen and DTU in Lyngby. When the optical signal reaches DTU, it is picked up by a photodetector. The single photon is then converted into an electron, which is subsequently amplified into a measurable electronic signal. Illustration: Laurids Hovgaard

Any attempt to intercept the system would require intercepting a photon, which is indivisible. Therefore, every attempt at eavesdropping will be detected.

After a few minutes of conversation, postdoctoral fellow at DTU Electro Catarina Vigilar shows how the connection is quickly cut off if intruders are detected.

“Eavesdropper detected” is displayed in red letters on the screen.

When the modularization of the photon changes, the error rate of the signal increases and the connection is automatically disconnected.

Along the way, there is a loss of 9.6 dB due to noise. Had the fibre been in a controlled laboratory environment, it could have been sent much further.

Today, commercial solutions that are similar to the demonstration at DTU are available. However, the commercial solutions do not use real single-photon sources, as is the case at DTU/Niels Bohr, but they instead attenuate the light pulse in the optical fibres so that they approach a single photon.

However, there is still a statistical probability of not reaching the single photon level.

Secret communication requires encryption and secure keys. Current techniques will not be able to withstand a quantum computer attack, but quantum technology is not only a tool that can be used against encryption; it can also be used as a fully secure defence mechanism. In 1984, Charles H. Bennett from IBM and Gilles Brassard from the Université de Montréal developed the first protocol for quantum cryptography, which is today called BB84. More advanced protocols exist today, but BB84 nicely illustrates the main principles of quantum encryption. Illustration: MI Grafik

Collecting keys

Because the signal is transmitted over a common single-photon fibre connection, the highest key exchange rate is 2.6 kbit/s.

While that in itself is a world record for the highest key rate with quantum distributed keys, it is not enough by itself to establish a stable video connection. Therefore, researchers at DTU and the Niels Bohr Institute have accumulated a large number of encryption keys, which are exchanged in advance.

However, the speed can be increased if more experimental fibre-optic cables are used.

“We have reached over 100 Mbit/s when using special multicore fibres,” Leif Katsuo Oxenløwe says.

The demonstration carried out on Thursday stems from the four-year research project Fire-Q, a collaboration between the Technical University of Denmark (DTU), Aarhus University, the University of Copenhagen and four smaller companies—Sparrow Quantum, SiPhotonIC, nanoPHAB, Swabian Instruments, which are located in Denmark, the Netherlands, and Germany respectively—with support from the Innovation Fund Denmark.