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Cyber security: new method to boost efficiency of quantum cryptography

Physics Systems and data security
The world champions in fast data transmission use their knowledge and advanced equipment from record attempts to develop more efficient quantum cryptography.

Once the quantum computer becomes a reality, it will make most of the encryption used today to secure the information we send over the Internet insecure.

With its (expected) vast computing power, it will take the quantum computer very little time to solve the mathematical algorithms for generating the decryption keys that are used to decode encrypted information.

The answer to this challenge is quantum cryptography.

With quantum cryptography, the decryption key is based on quantum mechanical principles that guarantee complete randomness when creating the key, making it impossible to recreate for others.

The distribution of the quantum encryption key between sender and receiver (from A to B, often referred to as Alice and Bob) is known as QKD (quantum key distribution). In QKD, individual light particles—photons—carry the encryption key. When using single photons, unauthorized persons cannot intercept the key since it is quickly discovered if photons are missing, and the transmission can be stopped.

In QKD, the quantum-mechanical properties of light are exploited. One of the properties is that it is possible for a single photon to be in several places at the same time.

The researchers at the basic research centre SPOC (Silicon Photonics for Optical Communications) at DTU Fotonik utilizes this to send more information per photon.

This means that instead of a single photon carrying only a single bit (a one or a zero), it can carry several bits.

From records to quantum cryptography 

The SPOC centre is on home turf when it comes to communication effectiveness in the form of sending more information per photon.

On several occasions, the research group set world records in the transfer of data, for example in 2014, when they succeeded in sending 43 terabits per second (Tbit/s) through an optical fibre, and again in 2016, when this was extended to 661 Tbit/s.

The record was possible because the researchers used optical fibres (glass threads) with multiple cores for the light to travel in—also called a multicore fibre.

In 2015, the research group wondered whether they could transfer their knowledge and advanced technological equipment from their record-setting data transfers to the field of quantum cryptography.

“We found that the common way to make QKD corresponds to the way we used photons for data transfer many years ago, where one photon only represents one bit. Therefore, we were curious about whether we could transfer our new methods of data transmission to the field of quantum cryptology,” says Professor Leif Katsuo Oxenløwe, whose basic research centre therefore began experimenting with using optical fibres with more cores (multicore fibres) to make quantum encryption keys.

Takes advantage of optical multi-core fibres

By sending the photon into a multi-core fibre, you can take advantage of the spatial dimension that occurs because there are several paths through the fibre.

In the first experiment, SPOCs researchers used four cores in a multicore optical fibre, the Professor explains:

“When we use four cores, the photon has four potential routes through the fibre. By exploiting that the photon can be in up to four different places at the same time, we can increase the amount of information per photon. In the first experiment, we have shown that we can control which cores the photon uses, and we can even get it to select multiple cores at the same time.”

The experiment shows that with this new method, SPOC researchers can double the bit rate of quantum encryption keys, and in 2017, the research group published the first results in Nature's online partner journal ‘npj Quantum Information’.

More detailed and longer keys

The advantage of quantum encryption keys with higher bit rates is that keys become ‘longer’.

To ensure that it is completely unbreakable, a key must be as long as (have as many bits as) the information you wish to encrypt.

SPOCs solution is therefore an important step towards creating completely secure keys that fit with the very high bitrates circulating the Internet today.

The research group is currently experimenting with using optical fibres with even more cores for achieving even higher bitrates on quantum encryption keys.

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