Carlos Sabín
Ramón y Cajal Researcher Department of Theoretical Physics UAM Madrid
In a quantum communications network, information could be transmitted using a phenomenon called quantum "teleportation", whereby the state of one quantum bit (cubit) can be transported to another without needing to be known at any time, which is not possible outside of quantum physics.
In order to be able to do teleportation, there must be very strong correlations between the parts (the famous quantum entanglement, which is behind all modern quantum technologies). But quantum entanglement is difficult to achieve and very fragile. Ideally, in a network we would want to transmit information between any pair of nodes, no matter how far apart they are. But how can we achieve entanglement between far apart nodes if each node only interacts with the nodes that are closest to it?
To do this, we have to use the phenomenon of "entanglement exchange", which is the same as teleportation, but now what is being transported is precisely an entangled state of two cubits. Thus, if we intertwine node A with node B and also node B with a third node C, the "intertwining exchange" makes A and C intertwine, even though they have never interacted with each other. And, in principle, we could go on like this with any number of nodes. The experimental problem is that quantum entanglement is very fragile, and it is very difficult to carry out this process without losing information.
Both teleportation and "swapping" have been known theoretically since the 1990s and have been realised experimentally in various quantum systems. But in the specific case of systems that can form a quantum communication network, only teleportation between two neighbouring nodes in the network has been achieved. Until now, an experimental team at the Delft University of Technology in the Netherlands has succeeded in achieving entanglement exchange as well.
This could be interpreted as a first (very preliminary) step towards a quantum communication network. However, it should be noted that the great experimental difficulty means that the quality of information transmission is still very low. This can be measured by calculating the so-called "fidelity" of the transmitted state, i.e. the resemblance between the actual final state of the cubit and the state we wanted to transmit.
Ideally, this fidelity should be 100 %. If it is above 66.6%, we know that the process is impossible without using quantum physics. In the experiment we achieve on average a fidelity of 70 %, but in some states it drops to 65 %. This is enough to show that the process is quantum (at least on average), but obviously still far from any possible technological application, since the state obtained is 30 % different from the original one. Much work remains to be done to improve these percentages and to extend the experiment to more distant nodes in the network.