New quantum transmission protocol has greater information transmission fee, robustness towards interference

Quantum cryptography is among the most promising quantum applied sciences of our time: Exactly the identical info is generated at two totally different places, and the legal guidelines of quantum physics assure that no third get together can intercept this info. This creates a code with which info could be completely encrypted.

The staff of Prof. Marcus Huber from the Atomic Institute of TU Wien developed a brand new kind of quantum cryptography protocol, which has now been examined in observe in cooperation with Chinese analysis teams: While so far one usually used photons that may be in two totally different states, the scenario right here is extra sophisticated: Eight totally different paths could be taken by every of the photons. As the staff has now been capable of present, this makes the technology of the quantum cryptographic key quicker and likewise considerably extra sturdy towards interference. The outcomes have now been printed within the scientific journal Physical Review Letters.

Two states, two dimensions

“There are many alternative methods of utilizing photons to transmit information,” says Marcus Huber. “Often, experiments focus on their photons’ polarization. For example, 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 even carry info independently of the course of polarization. One can, for instance, use the details about which path the photon is at present touring on. This is precisely 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,” explains Marcus Huber. Depending on the purpose at which the photon pair was created, every of the 2 photons can transfer alongside eight totally different paths—or alongside a number of paths on the identical time, which can also be permitted in accordance with the legal guidelines of quantum concept.

These two photons could be directed to utterly totally different locations and analyzed there. One of the eight potentialities is measured, utterly at random—however as the 2 photons are quantum-physically entangled, the identical result’s at all times obtained at each locations. Whoever is standing on the first measuring gadget is aware of what one other particular person is at present detecting on the second measuring gadget—and nobody else within the universe can pay money for this info.

Eight states, eight dimensions

“The fact that we use eight possible paths here, and not two different polarization directions as it is usually the case, makes a big difference,” says 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 a number of benefits: First, it permits extra info to be generated: At 8307 bits per second and over 2.5 bits per photon pair, a brand new report has been set in entanglement-based quantum cryptography key technology. And secondly, it may be proven that this makes the method much less inclined to interference.

“With all quantum technologies, you have to deal with the problem of decoherence,” says Marcus Huber. “No quantum system can be perfectly shielded from disturbances. But if it comes into contact with disturbances, then it can lose its quantum properties very easily: The quantum entanglements are destroyed.” Higher-dimensional quantum states, nevertheless, are much less prone to lose their entanglement even within the presence of disturbances.

Moreover, subtle quantum error-correction mechanisms can be utilized to compensate for the affect of exterior perturbations. “In the experiments, additional light was switched on in the laboratory to deliberately cause disturbances—and the protocol still worked,” says Marcus Huber. “But only if we actually used eight different paths. We were able to show that with a mere two-dimensional encoding a cryptographic key can no longer be generated in this case.”

In precept, it needs to be doable to enhance the brand new, quicker and extra dependable quantum cryptography protocol additional through the use of extra levels of freedom or an excellent bigger variety of totally different paths. “However, this not only increases the space of possible states, it also becomes increasingly difficult at some point to read out the states correctly,” says Marcus Huber. “We seem to have found a good compromise here, at least within the range of what is currently technically possible.”