Researchers at the Max Planck Institute of Quantum Optics (MPQ) are claiming a world first with a demonstration of a quantum switching network. The Institute reports data being exchanged successfully "with high efficiency and fidelity" between two quantum nodes installed in two separate labs, connected by a 60-metre long optical fibre.
This sounds straightforward enough, but is more impressive when you in mind how fragile quantum information is. If errors are to be kept out, and all the data retained, every element of a quantum network has to be under perfect control.
The breakthrough is in developing stationary nodes "that allow for the reversible exchange of quantum information", the Institute explains in its announcement. The experimental set up consists of two coupled single-atom nodes that are able to exchange single photons, thus communicating quantum information.
The MPQ explains that while atoms are the smallest possible node, and photons the best option as messengers, exchanging information between the two is not straightforward, and cannot be done with the atom in free space.
The trick, then, is to trap the atom in an optical cavity – two highly reflective mirrors positioned close together, facing each other. A photon entering the cavity bounces back and forth thousands of times. This makes its interaction with the captured atom much stronger, and making the atom more likely to absorb it coherently.
From the announcement: "The first experimental challenge was to quasi-permanently trap the atom in the cavity. This was achieved via fine-tuned laser beams, meaning the least disturbance of the atom. In the next step, the physicists achieved controlled emission of single photons from the trapped atom. Finally, they could prove that the single-atom-cavity system represents a perfect interface for storing the information encoded in a single photon, and they were able to transfer it onto a second single photon after a certain storage time."
MPQ says this work is a stepping stone on the way to a larger-scale quantum network. The work is published in the April 12 edition of Nature.