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.

As with many other successful entanglement experiments, the core of the Swiss apparatus is a type of crystal that absorbs one photon and then emits two that have opposite polarization states. The specific state of each of these photons is undetermined until measured. But because they are entangled—correlated—measuring the polarization of one photon instantly reveals the state of the second, no matter how far they are separated in space.

(No information can be transmitted this way since the people on either end would need to discuss how the measurement should be taken, and that discussion takes place at light-speed.)

However, single photons can't travel a arbitrary distances. Even fiber-optic cables exhibit small losses through absorption or scattering, and transmission through open air is even worse. However, atoms can reemit photons that are indistinguishable from a photon they absorbed, meaning they can act as repeaters. A particularly helpful atom, used in the Swiss experiment, comes from the rare-earth element neodymium. Many of these can be embedded in a silicon-based crystal, a process familiar in semiconductor devices called doping.

As the children's game of "telephone" shows, message passing is worthless if the final version doesn't match the original—it's important to make sure a photon emitted by a quantum repeater is the precise one needed for the experiment.

The absorption-remission process and a photon passing through without interaction both may trigger a detector, but the wrong photon means the message isn't actually received. To ensure that only the right photons are counted in the repeater, Imam Usmani and colleagues prepared the initial signal so that each "proper" photon is partnered with a second photon of a different wavelength known as a herald.

To do this, the team directed light from a laser onto a PPKTP (periodically poled potassium titanyl phosphate) crystal. In some cases, the crystal will respond by emitting a photon of the exact wavelength needed to excite the neodymium ions, along with a second photon with a much higher energy. Since this particular pair of photons isn't emitted every time the crystal is excited, the higher-energy partner acts as a herald to the photon that is needed. Detecting the herald photon means the rest of the system can work as desired, allowing entanglement to occur.

The experiment, which is cooled to 3 degrees above abolute zero, works as follows.

Laser light is used to excite the PPKTP crystal; if the right reaction occurs, a herald photon shows up in the detector, while the photon needed for the rest of the experiment travels on. That photon is split into two entangled photons, which are sent down separate paths until they reach the crystals doped with neodymium. The neodymium ions absorb the light and reemit it. The new photons are then recombined at a beamsplitter. Depending on the polarization states of the entangled photons, they will either pass through or reflect, landing in one of two detectors. This confirms that entanglement between the neodymium-doped crystals actually took place.

Thanks to the presence of herald photons, the problem of false negatives (due to non-entangled photons showing up in detectors) was mitigated.

Timing is of the essence in this experiment: the neodymium ions must receive the entangled photons at the same time for everything to work. Also, the reemission takes a relatively long 33 nanoseconds, so the larger the spatial separation, the more challenging getting all the components of the system to act in concert. Whether larger numbers of correlated photons and crystals can be used is a matter of some practical concern.

Nevertheless, this experiment stands as a step toward implementing a real quantum repeater, even though barriers still exist to scaling the apparatus to larger distances and more realistic physical parameters. Making actual quantum memory that lasts significantly longer than 33 nanoseconds is a potential happy side effect to studying the entanglement of these crystals.

Nature Photonics, 2012. DOI: 10.1038/nphoton.2012.34 (About DOIs).