Optical fibers have revolutionized telecommunications. The move from the transmission of electrical signals down conductive copper wires to light-based communication in fibers has meant that it has become possible to transfer data at unparalleled speeds with improved data integrity over even greater distances.
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Now, all around the world are kilometers of optical fiber that run through the ocean floors and across land to transmit our data. Some of these fiber-optic networks are nearly 30,000 km in length and the data transfer capabilities of optical fibers have seen nearly a thousand-fold increase in performance over the last twenty years.1
While the initial financial outlay for fiber optic cables is greater than more conventional copper wires and there are more stringent manufacturing demands, the technology is widely regarded as superior. As well as the significantly greater bandwidth of information that can be transferred through such cables, the reduced signal loss rates also mean that fewer amplifiers and signal boosters are required. This is significantly more convenient when laying long cable lengths as it reduces the overall amount of infrastructure that is necessary.
Researchers are moving to the next step in light-based information transfer using quantum encryption. Researchers from Toshiba Europe Limited and the University of Leeds have recently demonstrated the longest distance communications transfer encrypted using the twin-field quantum key distribution protocol.2 Managing a transmission distance of over 600 km, the new technical developments that allowed the team to achieve this result are a key step to making real-world communications using quantum encryption possible.
Fiber Structure
In a standard optical fiber, a series of light pulses are transmitted through the fiber’s core. The sequence of pulses is used to encode the information to be sent and cladding around the core of the fiber helps to control the number of reflections and transmission distance as well as to avoid ‘spreading’ the light as it is transmitted.
The information is decoded at the receiving end to recover the transmitted information, but what happens if there is someone trying to eavesdrop on this information or intercept the packets of light as they are transmitted?
Interception of data transmission in optical fibers can be done through physical splicing or tapping of the cables, or through interception of data transmitted along the fiber.3 While fiber optics are more secure than copper wires as the information is not radiated in the form of an electromagnetic field, data security is still a concern.
Usually, most data are encrypted before transmission using some type of algorithm that scrambles the data into a non-human readable format. It can then be decrypted by the receiver. Most of the encryption algorithms rely on mathematically ‘hard’ problems – calculations that are easy to do with the correct, cryptographic key, but otherwise require an unfeasible amount of resources to complete.
However, as computational power continues to scale and quantum computing architectures start to reach larger numbers of qubits, how long current encryption methods remain secure is in question. This means new security architectures are needed.
Quantum Encryption
The encryption scheme used to achieve more than 600 km communication distances was twin-field quantum key distribution.2 Quantum key distribution methods use a randomly generated shared secret key to encrypt and decrypt messages. Any attempt to eavesdrop on the communication is essentially a type of measurement on the system. For quantum systems, this results in a detectable change in the system so the eavesdropper can be detected.
One challenge with quantum key distribution is that every photon that is transmitted down the optical fiber only has a small probability of reaching the end – a significant number are scattered on the way. There is an inherent limit to the number of secure key bits that can be delivered over a line with the limited transmission.4
Normally, the way to overcome the loss of photons and information would be to use some kind of repeater for the signal but at present, there is no technology that can be used as a full quantum repeater. Instead, with twin-field quantum key distribution, the interference between two fields with related but non-identical phases can be used. This enhances the number of secure bits that can be transferred over a distance for a given transmission, effectively increasing the range over which this quantum encryption method can be used.
With the addition of dual-band phase stabilization to ensure that the phase relationship between interfering fields is preserved, which increased the phase stability by a factor of four, the team enhanced the secret key rate from the millibit-per-second range to the bit-per-second range for the longest fiber lengths tested.
Reaching 1 bit per second secret key refresh rates means that key refresh rates are now sufficient to reach encryption protocol standards such as the advance encryption standard. With this new type of ‘quantum repeater’, this development is another step closer to the concept of a quantum internet.
References and Further Reading
- Winzer, P. E. J. W., Eilson, D. A. T. N., Hraplyvy, A. N. R. C., Labs, N. B., & Road, H. (2018). Fiber-optic transmission and networking : the previous 20 and the next 20 years. Optics Express, 26(18), 24190–24239. https://doi.org/10.1364/OE.26.024190
- Pittaluga, M., Minder, M., Lucamarini, M., Sanzaro, M., Woodward, R. I., Li, M., Yuan, Z., & Shields, A. J. (2021). 600-km repeater-like quantum communications with dual-band stabilization. Nature Photonics, 15, 530-535. https://doi.org/10.1038/s41566-021-00811-0
- Iqbal, M. Z., Fathallah, H., & Belhadj, N. (2011). Optical Fiber Tapping : Methods and Precautions. 8th International Conference on High-Capacity Optical Networks and Emerging Technologies, figure 2, 164–168. https://doi.org/10.1109/HONET.2011.6149809
- Pirandola, S., Laurenza, R., Ottaviani, C., & Banchi, L. (2017). Fundamental limits of repeaterless quantum communications. Nature Communications. https://doi.org/10.1038/ncomms15043
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