Success in the Field of Quantum Cryptography

30 Sep 2021

Researcher Matej Pivoluska from the Institute of Computer Science chose the field of quantum cryptography for his research, where he has currently achieved significant success. Together with his international team, he has succeeded in developing a new quantum cryptography protocol that enables higher data rates and is much more noise-resistant.​

Quantum cryptography is one of the most promising quantum technologies of our time. Why? Exactly the same information is generated at two different locations, and the laws of quantum physics guarantee that no third party can intercept this information. This creates a code, called a key, with which information can be perfectly encrypted

Matej Pivoluska from ICS, together with colleagues from the Atom Institute TU Wien, developed a new type of quantum cryptography protocol, which has now been tested in practice in cooperation with Chinese research groups: While up to now one normally used photons that can be in two different states, the situation here is more complicated: Eight different paths can be taken by each of the photons. As the team has now been able to show, this makes the generation of the quantum cryptographic key faster and significantly more robust against interference. The results have now been published in the scientific journal "Physical Review Letters".

Two States, Two Dimensions

There are many ways of using photons to transmit information. Often, experiments focus on photons’ polarization. Polarization property of photons is concerned about, whether they oscillate horizontally or vertically - or whether they are in a quantum-mechanical superposition state in which, in a sense, they assume both states simultaneously. Similar to how you can describe a point on a two-dimensional plane with two coordinates, the state of the photon can be represented as a point in a two-dimensional space.

But a photon can also carry information independently of the direction of polarization. One can, for example, use the information about which path the photon is currently travelling on. This is exactly what has now been exploited: A laser beam generates photon pairs in a special kind of crystal. There are eight different points in the crystal where this can happen. Depending on the point at which the photon pair was created, each of the two photons can move along eight different paths - or along several paths at the same time, which is also permitted according to the laws of quantum theory.

These two photons can be directed to completely different places and analyzed there. One of the eight possibilities is measured, completely at random - but as the two photons are quantum-physically entangled, the same result is always obtained at both places. Whoever is standing at the first measuring device knows what another person is currently detecting at the second measuring device - and no one else in the universe can get hold of this information.

Eight States, Eight Dimensions

The fact that eight possible paths are used here, and not two different polarization directions makes a big difference. The space of possible quantum states becomes much larger. The photon can no longer be described by a point in two dimensions, mathematically it now exists in eight dimensions. This has several advantages.

First, it allows more information to be generated – at 8307 bits per second and over 2.5 bits per photon pair, a new record has been set in entanglement-based quantum cryptography key generation.

And secondly, it can be shown that this makes the process less susceptible to interference. Protection against interference is a crucial property of the new protocol, as no quantum system can be perfectly shielded from disturbances. Contact with disturbances can cause photons to lose their entanglement very easily. This results in drastic decrease of key generation rate. Higher-dimensional quantum states, however, are less likely to lose their quantum entanglement even in the presence of disturbances.

Moreover, sophisticated quantum error-correction mechanisms can be used to compensate for the influence of external perturbations. In the experiments, additional light was switched on in the laboratory to deliberately cause disturbances - and the protocol still worked, but only if eight different paths were used. The research team have compared this to a mere two-dimensional encoding, which could no longer be used to generate cryptographic key at such a noise level.

RNDr. Matej Pivoluska, Ph.D.

Researcher from the IT Infrastructure Division, a graduate of the Faculty of Informatics at MU in the field of theoretical informatics. He is focused on the study of quantum cryptography protocols and the use of quantum computers in practice.

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