Quantum cryptography: This air-filled fiber optic cable can transport un-hackable keys, say researchers

Hollow core fiber is filled only with air, but the technology's applications for security could be very promising.
Written by Daphne Leprince-Ringuet, Contributor

Hollow core fiber has a hollow center filled with air, which runs the entire length of the cable and is encased in a ring of glass.   

Image: BT / Lumenisity

A new type of optical fiber filled with nothing but thin air has been found to be particularly effective for carrying out quantum key distribution (QKD), a security protocol that is in principle un-hackable and could play a key role in protecting sensitive data against ever-more sophisticated cyberattacks. 

BT experimented with QKD over a six-kilometer-long cable of hollow core fiber, a technology that it has been working on for the past few months as an alternative to traditional fiber optic cables.  

Optical fiber is typically made of solid strands of glass that carry information by channeling light signals emitted by laser transmitters. Hollow core fiber, on the other hand, has a hollow center filled with air, which runs the entire length of the cable and is encased in a ring of glass. 

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It turns out that this configuration is better suited to QKD, because it reduces the possibility that different signals interfere with each other and spoil the whole process. 

QKD works in a similar way to traditional cryptography: data is encoded into an unreadable message thanks to a cryptography key that the recipient needs to decrypt the information. The method works by encoding the cryptography key onto a quantum particle (or qubit) that is sent to the other person, who measures the qubit in order to obtain the key value. 

This approach is particularly interesting to security researchers because it is based on the laws of quantum physics, which dictate that qubits collapse as soon as they are measured. This means that if a third-party eavesdrops on the exchange and measures the qubits to figure out the cryptography key, they would inevitably leave behind a sign that they have intruded.  

Cryptographers, therefore, call QKD "provably" secure. The method is expected to bring an additional level of safety to data exchanges, especially as hackers develop better tools to crack existing security protocols. 

The technology is nascent, and researchers are looking at various ways to carry out QKD; but one of the most established approaches consists of using optic-fiber cables to send both the qubits that are loaded with the cryptography key, and the actual encrypted message. 

But when using traditional optical fiber, which is made of glass, the effectiveness of the protocol is limited. This is because the light signals that carry information are likely to spread their wavelengths when travelling through glass, an effect called "crosstalk" that causes channels of light to leak into other channels. 

For this reason, the encrypted message cannot be sent through the same cable as the qubits, which are exceptionally fragile and susceptible to the noise caused by crosstalk. The whole process, says BT, is comparable to trying to have a whispered conversation next to an orchestra. 

This is where hollow core fiber could make a big difference. In an air-filled channel, light signals don't scatter as much, and less crosstalk occurs between channels. In other words, there can be a clear separation between the encrypted data stream and the faint quantum signal that carries the encryption key – even if they are both travelling over the same fiber. 

Ultimately, therefore, hollow core fiber could be a more efficient candidate for QKD – an "all-in-one" solution that requires less infrastructure to be built. 

"We know now that if we were to put hollow core fiber in, it could enable us to put quantum channels potentially anywhere we like, without having to worry," Catherine White, a researcher at BT, tells ZDNet. "Whereas with standard fiber, we either have to assign separate fibers for the QKD system or we have to be really careful not to have too much on classical power when doing the planning." 

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What's more, in previous trials of the technology, BT has also demonstrated that sending light signals through an air-filled core is much faster than through glass: according to the company, hollow core fiber enables data to travel up to 50% faster than in traditional optical cables.  

This means that the technology could also significantly reduce latency in the transmission of data. "This trial shows us the material we can work with, and it has wonderful properties like low latency and low scattering," says White. 

BT's trial remains limited: the experiment didn't go so far as exchanging actual encrypted data, and instead looked at the behavior of the quantum particle when it was sent alongside a high-power classical channel, in this case a light signal. The success of the trial, says White, lies in the fact that both channels remained healthy, which wouldn't be the case with standard fiber.  

"We were just proving key exchange, not testing encryption in this case," says White. 

But parameters from the trial, such as quantum bit error rate, indicate that the system effectively generated a key that could be used to protect data, continued the researcher. Experiments are now underway to apply the configuration to the exchange of data.

The next challenge will be to find out whether the technology can be scaled up. BT trialed QKD on a six-kilometer-long cable – still far off other experiments with the protocol in which researchers have managed to deliver quantum particles over hundreds of kilometers.  

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Earlier this year, for example, researchers from Toshiba Europe's Cambridge Research Laboratory demonstrated QKD on optical fibers exceeding 600 kilometers in length. 

White explains that, for all its low-latency and low-scattering properties, the hollow core fiber used in BT's trial is not low-loss, which is a crucial property to extend the reach of QKD. Researchers, however, are working on fine-tuning the material to improve its performance in that respect. 

"Findings show that, when tuning the fiber for particular wavelengths, we are able to have astoundingly low loss," says White. "This is very promising and we will see further developments. It does mean that hollow core fiber could potentially help reach longer reaches of QKD than we've seen."

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