Researchers at DTU have successfully distributed a quantum-secure key using a method called Continuous Variable Quantum Key Distribution (CV QKD). The researchers have managed to make the method work over a record 100 km distance—the longest distance ever achieved using the CV QKD method. The advantage of the method is that it can be applied to the existing Internet infrastructure.
Quantum computers threaten existing algorithm-based encryptions, which currently secure data transfers against eavesdropping and surveillance. They are not yet powerful enough to break them, but it's a matter of time. If a quantum computer succeeds in figuring out the most secure algorithms, it leaves an open door to all data connected via the internet. This has accelerated the development of a new encryption method based on the principles of quantum physics.
But to succeed, researchers must overcome one of the challenges of quantum mechanics - ensuring consistency over longer distances. Continuous Variable Quantum Key Distribution has so far worked best over short distances.
"We have achieved a wide range of improvements, especially regarding the loss of photons along the way. In this experiment, published in Science Advances, we securely distributed a quantum-encrypted key 100 kilometres via fibre optic cable. This is a record distance with this method," says Tobias Gehring, a professor at DTU, who, together with a group of researchers at DTU, aims to be able to distribute quantum-encrypted information around the world via the internet.
Secret keys from quantum states of light
"When data needs to be sent from A to B, it must be protected. Encryption combines data with a secure key distributed between sender and receiver so both can access the data. A third party must not be able to figure out the key while it is being transmitted; otherwise, the encryption will be compromised. Key exchange is, therefore, essential in encrypting data," explains Tobias Gehring.
Quantum Key Distribution (QKD) is an advanced technology that researchers are working on for crucial exchanges. The technology ensures the exchange of cryptographic keys by using light from quantum mechanical particles called photons.
When a sender sends information encoded in photons, the quantum mechanical properties of the photons are exploited to create a unique key for the sender and receiver. Attempts by others to measure or observe photons in a quantum state will instantly change their state. Therefore, it is physically only possible to measure light by disturbing the signal.
"It is impossible to make a copy of a quantum state, as when making a copy of an A4 sheet - if you try, it will be an inferior copy. That's what ensures that it is not possible to copy the key. This can protect critical infrastructure such as health records and the financial sector from being hacked," explains Tobias Gehring.
