April 13, 2024

100 kilometers of quantum encrypted transfer

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The 100-kilometer fiber optic cable through which a team of DTU researchers successfully distributed a quantum encryption key securely. Credit: DTU

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The 100-kilometer fiber optic cable through which a team of DTU researchers successfully distributed a quantum encryption key securely. Credit: DTU

DTU researchers have successfully distributed a secure quantum key using a method called continuous variable quantum key distribution (CV QKD). The researchers managed to make the method work over a record distance of 100 km – the greatest distance ever achieved using the CV QKD method. The advantage of the method is that it can be applied to existing Internet infrastructure.

Quantum computers threaten existing algorithm-based cryptography, which currently protects data transfers from eavesdropping and surveillance. They are not yet powerful enough to break them, but it is a matter of time. If a quantum computer can discover the most secure algorithms, it will leave an open door for all data connected via the Internet. This 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. Until now, the continuous distribution of variable quantum keys has worked best over short distances.

“We achieved a wide range of improvements, particularly with regard to the loss of photons along the way. In this experiment, published in Science Advances, we securely distribute a quantum encryption key over 100 kilometers via fiber optic cable. This is a record distance with this method”, says Tobias Gehring, associate professor at DTU, who, together with a group of DTU researchers, aims to be able to distribute quantum encrypted information worldwide via the Internet.

Secret keys to the 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 the sender and recipient so that both can access the data. A third party must not be able to discover the key while it is being transmitted; otherwise, the encryption will be compromised. Key exchange is therefore essential in data encryption.

Quantum key distribution (QKD) is an advanced technology that researchers are working on for crucial exchanges. The technology ensures 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 exploited to create a key unique to the sender and receiver. Attempts by others to measure or observe photons in a quantum state will instantly change its 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, like when you make a copy of an A4 sheet of paper – if you try, it will be an inferior copy. critical infrastructures such as health records and the financial sector are hacked,” explains Gehring.

Works through existing infrastructure

CV QKD technology can be integrated into existing Internet infrastructure.

“The advantage of using this technology is that we can build a system that resembles what optical communication already uses.”

The backbone of the Internet is optical communication. It works by sending data via infrared light that passes through optical fibers. They work like light guides placed in cables, ensuring we can send data around the world. Data can be sent faster and over greater distances via fiber optic cables, and light signals are less susceptible to interference, which is called noise in technical terms.

“It’s a standard technology that has been used for a long time. Therefore, it is not necessary to invent anything new to be able to use it to distribute quantum keys, and this can make implementation significantly cheaper. room temperature,” explains Gehring. “But CV QKD technology works best over shorter distances. Our task is to increase the distance. And the 100 kilometers is a big step in the right direction.”

Long-distance continuous variable quantum key distribution (CV-QKD) system. Credit: Science Advances (2024). DOI: 10.1126/sciadv.adi9474

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Long-distance continuous variable quantum key distribution (CV-QKD) system. Credit: Science Advances (2024). DOI: 10.1126/sciadv.adi9474

Machine Learning Noise, Errors, and Assistance

The researchers were able to increase the distance by addressing three factors that limit their system in exchanging quantum cryptographic keys over longer distances:

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

Additionally, researchers have gotten better at correcting errors that may occur along the way, which could be caused by noise, interference, or imperfections in the hardware.

“In our next work, we will use technology to establish a secure communication network between Danish ministries to protect their communication. We will also try to generate secret keys between, for example, Copenhagen and Odense to allow companies with branches in both cities to establish secure quantum communication,” says Gehring.

We don’t know exactly what happens – yet

QKD was developed as a concept in 1984 by Bennett and Brassard, while Canadian physicist and computing pioneer Artur Ekert and his colleagues performed the first practical implementation of QKD in 1992. Their contribution was crucial to the development of modern QKD protocols, a set of rules, procedures, or conventions that determine how a device should perform a task.

QKD is based on a fundamental uncertainty in copying photons into a quantum state. Photons are the quantum mechanical particles that make up light.

Photons in a quantum state carry fundamental uncertainty, meaning it is not possible to know for sure whether the photon is one or multiple photons collected in a 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 takes place. This makes it impossible to accurately measure which phase they are in during 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 prevent the door from locking while trying to get the key into the lock.

CV QKD focuses on measuring the soft properties of quantum states in photons. It can be compared to transmitting information in a stream of all color nuances, rather than transmitting information step by step in each color.

More information:
Adnan AE Hajomer et al, Long-distance continuous variable quantum key distribution in 100 km fiber with local local oscillator, Science Advances (2024). DOI: 10.1126/sciadv.adi9474

Diary information:
Science Advances

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