In an ordinary computer network, data in the form of binary numbers are transferred from one machine (node) to another via some sort of electronic signal, either electrical or optical. The success of this transfer comes when the recipient has precisely the same set of binary figures that were sent. In a quantum network, the "data" is a quantum state—the particular configuration of an atom's energy, spin, etc.—and the transfer of information is successful if the state is reproduced in a separate quantum system some distance away.

Extant quantum networks are capable of either receiving or sending signals, but not both simultaneously. A new experiment reported by Stephan Ritter et al. in Nature has achieved a simple two-node quantum network, in which a single photon successfully transferred the spin state of one rubidium atom to a second atom 21 meters away. Since the nodes are identical, both being rubidium atoms, signals are bi-directional. This type of quantum network should be scalable to encompass more than two nodes, leading to the possibility of larger networks with full communication between arbitrary nodes within them.

The network was constructed by Ritter et al. at the Max Planck Institute for Quantum Optics. Each node consisted of a single neutral rubidium atom in an optical trap, which is a standing wave of laser light. Since this kind of trapping is very delicate, the entire system was cooled to 5 mK (5 thousandths of a degree above absolute zero) to minimize the chance that thermal fluctuations would kick the atom out.

The researchers used a control laser to send a photon into the atom, which absorbed it and emitted a new photon. When this is done, the polarization (orientation of the electric field) of the second photon will be correlated with the spin state of the atom. To demonstrate that the information was actually carried by the photon, Ritter et al. used a polarized photon, and checked that the same information was present before and after interacting with the atom.

The key portion of the experiment, however, was to copy the quantum state of the first rubidium atom to the second. To accomplish this, Ritter et al. bounced the photon off their atom, then sent the emitted photon down a fiber optic cable 60 meters long to a second optical trap. (The extra cable length ensured that the coherence of the photon was maintained over greater distances than the space between the two optical traps.) The photon was then absorbed by the second atom, which set its spin state based on the photon's polarization—which in turn was dictated by the spin state of the first atom. In this way, the "data" of the first node was passed to the second node, with an accuracy between 83 and 85 percent.

This is mostly a proof of concept, but it proves a lot of concepts. Since absorption and emission are both repeatable processes, there is no reason the same two atoms couldn't be used to send and receive additional messages after the first transfer. Similarly, nothing changes if the atoms' roles are swapped: the second node can just as easily act as the transmitter. Finally, nothing prevents a larger network of atoms from working in exactly the same way—the system is entirely scalable, although we'd face the problem of controlling which path through the network a particular photon should go.

In addition, after the photon exchange, the two atoms' states may be correlated—entangled with each other. While this only occurs in 2 percent of the cases, it opens up the possibility of distributed quantum computing across the network, since the results of measurements performed on one atom will determine the outcome of measurements on the other, even across wide distances.

Nature, 2012. DOI: 10.1038/nature11023 (About DOIs).