Communications networks are vital for our day-to-day lives, and now the first prototype of a quantum one has been developed based on interfaces between single atoms and photons. For a quantum network to be useful, the exchange of quantum information must be reversible. This is difficult because quantum information is very fragile, and the no-cloning theorem prevents the copying of an arbitrary known quantum state. A breakthrough in solving this problem has been achieved by a group led by professor Gerhard Rempe of the Max Planck Institute of Quantum Optics (MPQ). This team has developed the first elementary quantum network for secure communications, a step toward a high-speed quantum Internet. Quantum networks differ from classical ones in that a classical bit is binary: it can represent either a 0 or a 1. A quantum bit (qubit) can represent a superposition of both at the same time. Information can be transmitted, bit by bit, from one atom to a second atom by mapping its quantum state onto individual photons. The photons travel through a fiber optic cable and are absorbed by the second atom. The second atom can then send information back to the first, or act as a hub to any number of networked atoms. Single atoms form the nodes of an elementary quantum network in which quantum information is transmitted by the controlled exchange of single photons. (Image: Andreas Neuzner, MPQ) The network was based on a proposal by professor Ignacio Cirac, director at the MPQ and leader of the theoretical team, and consists of two coupled single-atom nodes that transfer information through exchanging photons. The atoms are embedded in optical cavities composed of highly reflecting mirrors. When the atom emits photons, they can be directed and controlled in very specific ways. The challenge lay in trapping the atom in the cavity, which was accomplished by using finely tuned lasers without disturbing the atom. From this, they could prove that they could control the emission of the atom, store information on a specific photon and transfer it to another photon after a storage time. The two nodes are in two different laboratories and separated by a distance of 21 meters via a 60-m optical fiber. “We were able to prove that the quantum states can be transferred much better than possible with any classical network. In fact, we demonstrate the feasibility of the theoretical approach developed by professor Cirac,” said Dr. Stephan Ritter, leader of the experimental team. The group also was able to entangle the atoms, which allowed them to share information with each other no matter how far the distance between them. “We have realized the first prototype of a quantum network,” Ritter said. “We achieve reversible exchange of quantum information between the nodes ... we can generate remote entanglement between the two nodes and keep it for about 100 microseconds, whereas the generation of the entanglement takes only about one microsecond. Entanglement of two systems separated by a large distance is a fascinating phenomenon in itself. However, it could also serve as a resource for the teleportation of quantum information. One day, this might not only make it possible to communicate quantum information over very large distances, but might enable an entire quantum Internet.” The research is featured in the April 12 issue of Nature. For more information, visit: www.mpq.mpg.de