The physicists have set up the first, elementary quantum network. It consists of two coupled single-atom nodes that communicate quantum information via the coherent exchange of single photons.
These quantum networks would differ profoundly from their classical counterparts: Besides giving insights into fundamental questions in physics, they could also have applications in secure communication and the simulation of complex many-body systems, or they could be used for distributed quantum computing. One prerequisite for functional quantum networks are stationary nodes that allow for the reversible exchange of quantum information.
Quantum networks exhibit peculiar properties not found in their classical counterparts. This is due to the fundamentally different behaviour of the exchanged information: while a classical bit represents either 1 or 0, a quantum bit can take both values at the same time, a phenomenon called “coherent superposition.”
In order to achieve entanglement between the two network nodes, the polarization of the single photon emitted by atom A is now entangled with the atomic quantum state. Once the photon gets absorbed, this entanglement gets transferred onto atom B. In fact, this is the first time that entanglement has been created between massive particles separated by such a large distance, making it the world’s “largest” quantum system with massive particles.
"We have realized the first prototype of a quantum network," Stephan Ritter concludes. "We achieve reversible exchange of quantum information between the nodes. Furthermore, 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."