New quantum transmission protocol has higher data transmission rate, robustness against interference

Quantum cryptography is one of the most promising quantum technologies of our time: exactly the same information is generated in two different places, and the laws of quantum physics ensure that no third party can intercept this information. This creates a code with which the information can be perfectly encrypted.

The team of Professor Marcus Huber from the Atomic Institute of TU Wien has developed a new type of quantum cryptography protocol, which has now been tested in practice in cooperation with Chinese research groups: while so far we normally used photons which 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 also much more robust against interference. The results are now published in the scientific journal Physical examination letters.

Two states, two dimensions

“There are many different ways to use photons to transmit information,” says Marcus Huber. “Often the experiments focus on the polarization of their photons. For example, if they oscillate horizontally or vertically, or if they are in a state of superposition of quantum mechanics in which, in a sense, they assume both states simultaneously. describe a point on a two-dimensional plane with two coordinates, the state of the photon can be represented as a point in two-dimensional space.

But a photon can also convey information independently of the direction of polarization. One can, for example, use the information on the path on which the photon is currently moving. This is exactly what has now been exploited: “A laser beam generates pairs of photons in a particular type of crystal. There are eight different points in the crystal where this can happen, ”says Marcus Huber. Depending on the point at which the photon pair was created, each of the two photons can travel along eight different paths – or along multiple paths at the same time, which is also allowed under the laws of quantum theory.

These two photons can be directed to completely different places and be analyzed there. One of the eight possibilities is measured, completely at random, but since the two photons are entangled physically quantically, the same result is always obtained in both places. Anyone in front of the first measuring device knows what another person is currently detecting on the second measuring device – and no one else in the universe can get that information.

Eight states, eight dimensions

“The fact that we are using eight possible paths here, and not two different polarization directions as is usually the case, makes a big difference”, explains Marcus Huber. “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 more than 2.5 bits per photon pair, a new record has been set in the generation of quantum cryptography keys based on the entanglement. And secondly, it can be shown that it makes the process less susceptible to interference.

“With all quantum technologies, we have to face the problem of decoherence”, explains Marcus Huber. “No quantum system can be perfectly protected from disturbances. But if it comes into contact with disturbances, then it can very easily lose its quantum properties: the quantum entanglements are destroyed.” Higher dimensional quantum states, however, are less likely to lose entanglement even in the presence of disturbances.

In addition, sophisticated quantum error correction mechanisms can be used to compensate for the influence of external disturbances. “In the experiments, an extra light was turned on in the lab to deliberately cause disruption – and the protocol still worked,” says Marcus Huber. “But only if we actually used eight different paths. We were able to show that with simple two-dimensional encoding, a cryptographic key can no longer be generated in this case.”

In principle, it should be possible to further improve the new, faster and more reliable quantum cryptography protocol by using additional degrees of freedom or an even greater number of different paths. “However, this not only increases the space of possible states, but it also becomes increasingly difficult at some point to read the states correctly,” says Marcus Huber. “We seem to have found a good compromise here, at least in the range of what is currently technically possible.”