A new technological development has shown for the first time that quantum information can be reliably teleported between network nodes that are not directly connected to each other, a further step towards a scalable quantum internet.
A significant step towards building a super secure quantum internet has been made by researchers at Delft University of Technology in the Netherlands.
The breakthrough is based on a feature of quantum mechanics known as quantum entanglement, one of the most puzzling phenomena of elementary particles.
When two particles, such as atoms, photons, or electrons, become entangled, they experience an inexplicable bond that remains even if the particles are separated on opposite sides of the universe.
This possibility has made possible another no less disconcerting phenomenon, known as quantum teleportationwhich allows the transmission of a quantum state from one location to another thanks to quantum entanglement.
Carlos Sabina researcher at the Institute of Fundamental Physics of the CSIC, explains that quantum entanglement, despite its potential for communications, is somewhat difficult to achieve and maintain in time and space.
It has been necessary to use imagination to be able to take advantage of its full potential: not only has it been possible to maintain quantum entanglement over great distances, even in space, but the formula has been found so that the nodes of a communication network can use the quantum entanglement, even though they are very far apart.
The formula devised for this quantum lattice system is called “interlace exchange”: It is the same as teleportation, but a little more sophisticated. The difference is important, because the quantum entanglement that enables teleportation, by its very nature, is two particle thingwhile the entanglement exchange incorporates a third particle that has not experienced the connection of the first two: it’s a thing of three.
It works like this: if we intertwine node A with node B and also node B with a third node C, we get that A and C are also intertwinedeven though they have never interacted with each other.
playing with diamonds
This is called entanglement exchange and this is what the new research has achieved: it has teleported between node A and node B, with an extra node, C, acting as an intermediary.
To achieve this, the protagonists of this feat, Sophie Hermans, ronald hanson and his collaborators have used spin qubits (units of quantum information) embedded in diamonds and connected by fiber optic links.
Sabín clarifies that both teleportation and “exchange” have been theoretically known since the 1990s, and have been carried out experimentally in various quantum systems.
But in the specific case of systems that can form a quantum communications network, so far only teleportation between two nearby nodes of the network had been achieved.
The new research has transcended this limitation and managed to carry out the entanglement exchange to connect distant nodes to each other, which could be interpreted as a first (very preliminary) step towards a scalable quantum communication network, explains Sabín.
However, he adds, it must be taken into account that the great experimental difficulty means that the quality of the information transmission is still very low. This can be measured by calculating the so-called “fidelity” of the transmitted state, that is, the resemblance between the actual final state of the qubit and the state we wanted to transmit.
Ideally, that fidelity should be 100%. If it is above 66.6% we know that the process is impossible without using quantum physics. In the experiment, a fidelity of 70% is achieved on average, 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 very far from any possible technological application, since the state that is obtained is 30% different from the original. Much work remains to be done to improve these percentages and be able to extend the experiment to more distant nodes in the network, concludes Sabín.
Miguel Angel Martin-DelgadoProfessor of Theoretical Physics and coordinator of QUITEMAD (Quantum Information Technologies Madrid), points out that the use of intermediary nodes in teleportation will allow quantum entanglement to be transported over great distances and prevent it from deteriorating along the way.
This is the basis of the so-called quantum repeatersthe quantum version of the electromagnetic signal repeaters that make radio, television and, more recently, mobile communications possible.
Martín-Delgado adds that quantum teleportation will allow in the future to unify quantum communications with quantum computing, taking the form of a quantum internet: The nodes will be quantum computers that will communicate with each other through quantum links using teleportation.
He agrees with Sabín that the current experiment is a necessary advance, but it is still insufficient to go through all the possibilities that will open up to us when we manage to further master quantum teleportation in a practical and routine way, and in particular, manage to teleport information over sufficiently large distances. .
Qubit teleportation between non-neighbouring nodes in a quantum network. SLN Hermans et al. Nature volume 605, pages663–668 (2022). DOIhttps://doi.org/10.1038/s41586-022-04697-y