Probing Entanglement in a Many-Body–Localized System

April 26, 2019
artist's representation of a many-body-localized system

Imagine two people on opposite sides of a long line of people who want to pass a note to each other. In our classical world, the note has to be handed over from one person to the next until, eventually, it reaches the other end. However, in the quantum world, any object can be at multiple locations at the same time. This means that a "quantum note" can be found at a distant location, although it has never been passed through the "people" in between. The probability of this process, however, dramatically decreases with distance, therefore it becomes highly unlikely for the "quantum note" to make the entire trip. Here another important feature comes into play: interactions. When two "quantum notes" appear in the same place they can copy each other's information prior to returning to their origin. Now, if each person on either side of the line has such an interacting quantum note they will be able to communicate with each other without anyone in between ever noticing.

A team of researchers from the group of Prof. Markus Greiner has now realized such a situation in a many-body-localized system -- a novel, exotic type of quantum matter*. There, particles remain forever fixed at their initial positions; however, like the quantum notes, they have a small probability to be found elsewhere. After very long evolution times, the team detected the buildup of entanglement (a quantum analogue of information) even between the most distant particles of the system, although no particle motion was present. Instead, the entanglement was solely formed by the virtual exchange of particles. These results provide a direct proof that quantum information has been transferred without particle movement. In future, the researchers want to harness this effect for new quantum information devices and quantum thermodynamical engines.

*See A.Lukin, M. Rispoli, R. Schittko, M.E. Tai, A.M. Kaufman, S. Choi, V. Khemani, J. Léonard, and M. Greine, "Probing entanglement in a many-body–localized system," Science 364 (19 Apr 2019) DOI:10.1126/science.aau0818.