What is quantum information

"A new record track"

The amazing phenomena of quantum physics offer the potential to revolutionize many areas of technology. Research is already being carried out on the quantum physical version of the Internet in order to connect quantum systems with one another. In an interview with Welt der Physik, Josef Schupp from the Institute for Quantum Optics and Quantum Information and the University of Innsbruck explains how information can be exchanged in such a network and what challenges there are still.

World of physics: what is the quantum internet?

Josef Schupp: Just as the internet links classic computers with one another, the quantum internet links various quantum systems with one another. These quantum systems can be very different - from individual atoms to cold gases to solids, many systems are conceivable. The aim is to develop a network of quantum systems that can communicate with each other via the quantum internet.

What are the advantages of the quantum internet compared to the classic internet?

With the quantum Internet, on the one hand, completely tap-proof message transmission is possible. Every eavesdropping attempt leaves traces in the quantum network - a listener is immediately noticeable. On the other hand, quantum measuring devices such as atomic clocks can be connected to one another. GPS systems based on atomic time measurement could function even more precisely in this way.

What information can the quantum systems exchange with each other?

A quantum network can transmit the same type of information as a classic network, only in a different way. The storage units of a quantum network are so-called qubits - the basic units of quantum information. While classic bits take on either the value zero or one, a qubit can also be in an overlapping state of the two values. A measurement of the qubit would give the value zero with a certain probability and the value one with a certain probability. These difficult to imagine properties of quantum physics open up new possibilities in information processing that would be inconceivable with classical systems.

And how does the exchange of information between quantum systems work?

Classic bits are transmitted via comparatively strong electrical or optical pulses. However, quantum information cannot be transferred in this way because the quantum properties are lost in classical systems. However, a phenomenon in quantum physics is ideal for connecting quantum systems - so-called entanglement. Two entangled particles share a common state and can no longer be described independently of one another. If you change the state of one particle, the state of the other particle changes immediately - theoretically over any distance. However, this alone does not transfer any information. Classic communication is also required for this, which is why the exchange of information cannot take place faster than the speed of light.

Josef Schupp and his colleagues before their experiment

You have now taken an important step on the way to the quantum Internet. Can you briefly introduce the new experiment?

Our quantum system consists of a calcium atom that we have entangled with a photon with the help of laser pulses. We sent this photon through a fiber optic cable. However, the entanglement is very fragile and the photon itself can quickly be lost on its way through the fiber optic cable. An optical resonator helps us create entangled atom-photon pairs with very high efficiency, and a crystal changes the wavelength of the photon so that it is much more robust for transmission through the optical fiber. In the end, the photon moved fifty kilometers through the fiber optic cable without losing its entanglement with the calcium atom. In fact, we have set a new record track with it.

Which steps are still missing to a quantum network?

With our experiment we have shown that matter particles and light particles can remain entangled over long distances. A quantum network should, however, connect at least two end systems - in our case two atoms - with one another. We already have an idea how we can achieve this: With a second experimental setup, we get another atom that is entangled with a photon. These photons could then be used to create an entanglement between the two atoms using certain measurements. We are currently working on the interlinking of two experiments that are located about half a kilometer from each other in different buildings. Our goal for the coming years is to connect two atoms over a distance of one hundred kilometers. Regional quantum networks are then conceivable in ten to twenty years.