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.

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A major triumph in physics involved using photons to produce a quantum state in a mechanical oscillator that was visible to the naked eye. Mechanical oscillations are vibrations of atoms within a solid, which act as particles called phonons (since they are quantized sound waves). The control of phonons is more difficult than the control of photons; if we could control them, it would open up a whole new area of nanotechnology research, including the entanglement of mechanical systems.

While the manipulation of phonons using light has been achieved, researchers in Japan have now found a way to control mechanical vibrations using electrically induced oscillations: controlling phonons with other phonons. In a Nature Physics paper, I. Mahboob, K. Nishiguchi, H. Okomoto, and H. Yamaguchi describe the construction of an acoustic resonant cavity, where one set of vibrations sets up a second oscillation, much as certain musical notes can cause other objects to vibrate. The difference is that both sets of vibrations are in the same object (the acoustic cavity), and the secondary oscillations are tunable, so their properties can be controlled by the external electrical signal.

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Quantum entanglement is the process by which two physical systems have correlated properties, even though they may be widely separated in space. As a practical matter, though, it is difficult to create a truly entangled system with parts that are widely separated, even though that's precisely what we need for a quantum network or other long-range communication systems. 

For ordinary electrical wires (such those used for ethernet), signals can travel long distances because they're boosted by means of repeaters. Researchers in Geneva, Switzerland have now built a possible model for a quantum repeater, using entangled photons to excite rare-earth atoms embedded in two crystals. The atoms themselves have correlated quantum states, and when they emit new photons, those are also entangled, guaranteeing that the original "message" is passed along. Devices built along these lines could act as solid-state nodes within quantum networks, allowing for larger quantum computing systems.

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