Researchers at the Technical University of Denmark (DTU) have successfully applied quantum encryption to securely transmit information over a fiber optic cable over a distance of 100 kilometers (about the same distance as Oxford and London).

Scientists at the Technical University of Denmark (DTU) have achieved a breakthrough in secure communications by distributing a quantum-secure key using Continuous Quantum Key Distribution (CV QKD). This team broke a new record by making the technique effective at an unprecedented distance of 100 kilometers, the longest distance achieved with CV QKD. The advantage of the method is that it can be applied to existing internet infrastructure.

Quantum computers threaten existing encryption algorithms that protect data transfers from eavesdropping and surveillance. They’re not strong enough to break them yet, but it’s only a matter of time. If a quantum computer manages to develop the most secure algorithms, it leaves the door open to all data connected over the internet. This accelerated the development of a new encryption method based on the principles of quantum physics.

But to be successful, researchers must overcome one of the challenges of quantum mechanics: ensuring coherence over long distances. Continuously variable quantum key distribution has so far worked best over short distances.

“We have made a wide range of improvements, especially regarding photon loss. In this published experiment Science DevelopmentsWith , we securely distributed a quantum encrypted key over 100 kilometers over fiber optic cable. This is a record distance with this method,” says DTU Associate Professor Tobias Göring, who together with a group of DTU researchers aim to disseminate quantum encrypted information worldwide via the internet.

Secret keys to quantum states of light

“When data needs to be sent from A to B, it needs to be protected. Encryption combines data with a secure key shared between the sender and receiver, allowing both to access the data. A third party must not be able to determine the key during transmission; otherwise the encryption will be compromised. Therefore, key exchange is essential for data encryption.

Quantum Key Distribution (QKD) is an advanced technology that researchers are working on for significant change. The technology enables the exchange of cryptographic keys using light from quantum mechanical particles called photons.

When a sender sends information encoded in photons, the quantum mechanical properties of the photons are used to create a unique key for the sender and receiver. Attempts by others to measure or observe photons in their quantum state will instantly change their state. Therefore, it is physically possible to measure light simply by distorting the signal.

“It is impossible to make a copy of the quantum state, as when copying an A4 format page – if you try, it will be a poor-quality copy. This ensures that the key cannot be copied. This can protect critical infrastructure such as medical records and the financial sector from being hacked,” explains Tobias Goering.

Works over existing infrastructure

Continuously Variable Quantum Key Distribution (CV QKD) technology can be integrated into existing Internet infrastructure.

“The advantage of using this technology is that we can create a system that is similar to the system optical communications currently use.”

The basis of the Internet is optical communication. It works by sending data through infrared light passing through optical fibers. They work like optical fibers laid in cables, allowing data to be sent anywhere in the world. Data can be sent faster and over longer distances over fiber optic cables, and light signals are less susceptible to interference, known in technical terms as noise.

“This 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 much cheaper. And we can operate at room temperature,” explains Tobias Goering.

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

Noise, errors, and help from machine learning

The researchers were able to increase the distance by addressing three factors that limit their system’s ability to share quantum encrypted keys over greater distances:

Machine learning provided preliminary measurements of failures affecting the system. Noise, as these distortions are called, can arise, for example, from electromagnetic radiation that can disrupt or destroy transmitted quantum states. Early detection of noise made it possible to more effectively reduce the corresponding impact.

Researchers have also gotten better at fixing errors that could be caused by noise, interference, or hardware defects during operation.

“In our future work, we will use this technology to establish a secure communication network between Danish ministries that will secure their communications. We will also try to create secret keys, for example, between Copenhagen and Odense, so that companies with branches in both cities can establish a quantum-secure connection,” says Tobias Göring.