Two single-atom nodes have been used to send, receive and store quantum information using photons, a quantum information-sharing milestone. “We have realized the first prototype of a quantum network,” said Dr. Stephan Ritter of Max Planck Institute of Quantum Optics (MPQ). For a quantum network to be useful, the exchange of information must be reversible. This is difficult because quantum information is very fragile and cannot be cloned. A breakthrough in solving this problem was achieved by researchers led by professor Gerhard Rempe of MPQ. Their research appeared in the April 12 issue of Nature (doi: 10.1038/nature11023). Unlike classical bits, which are binary, a quantum bit (qubit) can represent a superposition of both a 1 and a 0 at the same time. Information can be transmitted, qubit by qubit, from one atom to another by mapping its quantum state onto individual photons. The photons travel through a fiber optic cable and are stored in the second atom. The second atom can then send the information back to the first, or act as a hub to any number of networked atoms. The network 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. Photons emitted by the atoms can be directed and controlled in very specific ways. Single atoms form the nodes of an elementary quantum network in which quantum information is transmitted by the controlled exchange of single photons. Courtesy of Andreas Neuzner, MPQ. One challenge lay in trapping the atom in the cavity, which was accomplished by using finely tuned lasers without disturbing the atom. From this, the scientists proved 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 in the experiment were in two labs, separated by a distance of 21 m and connected via a 60-m optical fiber. “We are convinced that much larger distances are possible,” Ritter told Photonics Spectra. “One ultimate limit, however, is the attenuation in the optical fiber. If the distance becomes so large that the probability for the photon to arrive at the other node becomes small, efficient quantum state transfer or remote entanglement becomes impossible.” When asked what challenges lie ahead to achieve a quantum Internet, Ritter said that it would depend on what one would expect from such an Internet. “I think there is no clear definition yet,” he said. “What is obvious is that, for a large-scale quantum network, one will need more than two nodes. Our approach certainly supports this, as our network nodes are universal. Nevertheless, this is a technological challenge, considering that the lasers, optics and electronics for controlling one network node currently fill a whole laboratory. We plan on improving all characteristics of our single-atom network nodes.” A cavity-based quantum network. In the envisaged architecture (top), many single-atom nodes are connected by single-photon links. Here, the scientists explore the universal properties of a system produced by connecting two nodes (middle; A and B) within this configuration. Details of the nodes and connections are shown in the lower part of the figure. The two identical nodes are located in independent labs connected by a 60-m optical fiber (1). Each node consists of a single rubidium atom (2) in an optical dipole trap at the center of a high-finesse optical cavity (3). Quantum state transfer between the atoms and remote entanglement can be achieved via exchange of a single photon (4), with the quantum information encoded in the internal state of the atom and the polarization of the photon. Both the production of a photon (node A) and its storage (node B) are achieved via a coherent and reversible stimulated Raman adiabatic passage. Courtesy of Dr. Stephan Ritter, MPQ. Besides improving the characteristics of their network nodes, the scientists hope to increase storage time. “We would like to increase the storage time by several orders of magnitude by transferring the atomic qubit to magnetic-field-insensitive ‘clock states,’ ” Ritter said. “Currently, the storage time is mainly limited by residual magnetic field fluctuations.” The scientists also are working on elements of a quantum repeater scheme, which they hope will enable the transfer of quantum information over very large distances. “Entanglement of two systems separated by a large distance is a fascinating phenomenon in itself,” Ritter said. “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.”