Quantum Encryption

100 kilometres of quantum-encrypted transfer

Researchers at DTU have taken a big step towards securing information against hacking. They have succeeded in using quantum encryption to securely transfer information 100 kilometres via fibre optic cable - roughly equivalent to the distance between Oxford and London.

The record is held by DTU researchers Adnan A.E. Hajomer (front left), Nitin Jain, Ulrik L. Andersen, and Ivan Derkach (back left), Hou-Man Chin, Tobias Gehring. Photo: DTU.


Quantum Key Distribution was developed as a concept in 1984 by Bennett and Brassard, while the Canadian physicist and computer pioneer Artur Ekert and his colleagues carried out the first practical implementation of QKD in 1992. Their contribution has been crucial for developing modern QKD protocols, a set of rules, procedures, or conventions that determine how a device should perform a task.

Quantum Key Distribution (QKD) is based on a fundamental uncertainty in copying photons in a quantum state. Photons are the quantum mechanical particles that light consists of.

Photons in a quantum state carry a fundamental uncertainty, meaning it is not possible with certainty to know whether the photon is one or several photons collected in the given state, also called coherent photons. This prevents a hacker from measuring the number of photons, making it impossible to make an exact copy of a state.

They also carry a fundamental randomness because photons are in multiple states simultaneously, also called superposition. The superposition of photons collapses into a random state when the measurement occurs. This makes it impossible to measure precisely which phase they are in while in superposition.

Together, it becomes nearly impossible for a hacker to copy a key without introducing errors, and the system will know if a hacker is trying to break in and can shut down immediately. In other words, it becomes impossible for a hacker to first steal the key and then to avoid the door locking as he tries to put the key in the lock.

Continuous Variable Quantum Key Distribution (CV QKD) focuses on measuring the smooth properties of quantum states in photons. It can be compared to conveying information in a stream of all the nuances of colours instead of conveying information step by step in each colour.

Works via existing infrastructure

The Continuous Variable Quantum Key Distribution (CV QKD) technology can be integrated into the existing internet infrastructure.

"The advantage of using this technology is that we can build a system that resembles what optical communication already relies on," Tobias Gehring says.

The backbone of the internet is optical communication. It works by sending data via infrared light running through optical fibres. They function as light guides laid in cables, ensuring we can send data worldwide. Data can be sent faster and over longer distances via fibre optic cables, and light signals are less susceptible to interference, which is called noise in technical terms.

"It is a standard technology that has been used for a long time. So, you don't need to invent anything new to be able to use it to distribute quantum keys, and it can make implementation significantly cheaper. And we can operate at room temperature," explains Tobias Gehring, adding:

"But CV QKD technology works best over shorter distances. Our task is to increase the distance. And the 100 kilometres is a big step in the right direction."


Optical fibers are the backbone of the internet. 100 kilometers of optical fibers may not seem like much, but it's what it takes to connect oxford with London. Photo: DTU

Noise, Errors, and Assistance from Machine Learning

The researchers succeeded in increasing the distance by addressing three factors that limit their system in exchanging the quantum-encrypted keys over longer distances:

Machine learning provided earlier measurements of the disturbances affecting the system. Noise, as these disturbances are called, can arise, for example, from electromagnetic radiation, which can distort or destroy the quantum states being transmitted. The earlier detection of the noise made it possible to reduce its corresponding effect more effectively.

Furthermore, the researchers have become better at correcting errors that can occur along the way, which can be caused by noise, interference, or imperfections in the hardware.

"In our upcoming work, we will use the technology to establish a secure communication network between Danish ministries to secure their communication. We will also attempt to generate secret keys between, for example, Copenhagen and Odense to enable companies with branches in both cities to establish quantum-safe communication," Tobias Gehring says.




The Innovation Fund Denmark, the Danish National Research Foundation, the European Union's Horizon Europe research and innovation program, the Carlsberg Foundation, and the Czech Science Foundation support the project. The research group comprises Adnan A.E. Hajomer, Nitin Jain, Hou-Man Chin, Ivan Derkach, Ulrik L. Andersen, and Tobias Gehring. 

The Danish Quantum Communication Infrastructure (QCI.DK) targets the first deployment of Danish quantum communication technologies in a versatile network supporting real-life Quantum Key Distribution applications.


Quantum technology is an area of rapid growth. Researchers at DTU are focusing on three areas of technology: Quantum communication and data security; ultra sensitive quantum sensors; and the development of quantum computers. This is done through both basic research and development of technologies that can be used by businesses and government alike, which are both showing strong interest in the field.

Read more in our special topic about quantum technology.