Progress in the Invisible TU Vienna and Current Research in Quantum Physics & Co.

Together with a research team from China, TU Wien is pushing the boundaries of quantum computing and has discovered a quantum switch using ion bombardment. And created the world's smallest QR code.

Topic 1: A new type of quantum computing is becoming possible! Thanks to a collaboration between TU Wien and research groups from China. Using a new type of quantum gate, it has now been possible to carry out logical computing operations with photons that are in a combination of four different states. This, it is emphasized, is an important milestone for optical quantum computers, which opens up completely new opportunities. The work has now been presented in the journal "Nature Photonics". The basic idea of a quantum computer is actually simple, they say: While a classical computer can only work with the states 0 and 1, quantum physics also allows any combination of these states.

A quantum bit (qubit) can therefore be in the states 0 and 1 at the same time. This enables algorithms that can solve some tasks much faster than a conventional computer, as the researchers explain. However, in principle, such state superpositions can also be created from more than two components. A quantum particle can not only be in two states, but in many states at the same time. In this case, we no longer speak of a qubit, but of a qudbit. This brings decisive advantages for quantum calculations. They have now succeeded in developing a scheme for processing more complicated quantum states. And the colleagues from China managed to implement this scheme experimentally. The result was the novel quantum gate, which, as they emphasize, has revolutionary potential applications.

Quantum Physics Enters the Fourth Dimension

Until now, quantum experiments with photons have normally measured the polarization of photons - i.e. light quanta—a property that can provide two different measurement results. In terms of quantum physics, however, the photon can also be in a combination of these two states—similar to the way you can move north and east at the same time if you are walking in a north-easterly direction, the Viennese explain. "However, we are now using photons in a fundamentally different way," says Nicolai Friis from the Atomic Institute at TU Wien. Instead of polarization, we look at the waveform of the photon.

They could theoretically assume an infinite number of different states that correspond to different angular momentum values. Friis and his team finally developed a method that works with two such photons. Both particles can therefore exist in combinations of many different waveforms. However, through sophisticated manipulation, both photons can be put into a common state—this is known as quantum entanglement. Or such an entangled state can be deliberately split back into two independent states. This is what a quantum computer can ultimately do. The decision was made to work with four states. Friis sticks to the directional example: "In a sense, you are moving in a four-dimensional space and can work with any combination of these four states."

You Can See Exactly Whether Everything Went According to Plan

To make this possible, not only a new theoretical protocol was necessary! It was also necessary to improve technology and experimental precision, as Friis goes on to explain. Now, Hui-Tian Wang's team in China has achieved great success in this area. "For the first time, we have succeeded in producing a logical quantum gate that works with two photons, each of which is in a combination of four different states," says Friis happily. The experts can therefore entangle the two photons.

And it is also a heralded protocol! Friis explains that this means that after the quantum operation, it is possible to determine exactly whether everything worked or not. If not, you can simply repeat the operation. This is exactly what is needed for quantum operations in practice. The new approach should make quantum information technology better, more efficient and more stable in many areas. The advantage: fewer particles are needed to store the same amount of quantum information! Prof. Marcus Huber, also from the Atomic Institute at TU Wien, adds: "This has great advantages in terms of the reliability of quantum operations! The new work literally opens up new dimensions for quantum technology."