Quantum applications, from cryptography to computation, all benefit from the use of entangled particles, often photons. Creating and manipulating these photons is generally pretty straightforward, but storing them is not, which makes the issue of providing memory for a quantum computer a significant hurdle. It has been possible to successfully store some photons, but the media involved—single atoms or cold atomic gasses—aren't necessarily the most practical things to work with. In today's issue of Nature, researchers demonstrate that it's possible to keep two photons entangled even as one of them is held in a crystal.

Sticking a photon in a crystal involves getting rid of the photon itself, at least in these examples. Both teams used crystals doped with rare earth elements to store the photons, since previous reports had indicated that these can be made to hold photons for a few seconds. The trick to making these crystals hold a photon involves manipulating the energy state of the electrons in the crystal. Normally, transitions between different energy levels in the doped atoms happen very quickly, but it's possible to manipulate them so that they take much longer; the papers use either a laser or a magnetic field to shift the crystals between these two states.

With the crystal properly prepared, it's all just a matter of preparation. By matching the photon and crystal, it's possible to arrange things so that the photon can only be absorbed when the crystal is in its fast transition state. Once it's absorbed, however, the crystal can be shifted to its slow transition state. Once that shift occurs, the photon is trapped. It'll either be released at the slow rate (which takes seconds), or will stick around until the next time the crystal is switched to the fast state.

In the intervening time, the photon remains in the crystal in the form of an excited state that is diffused throughout all the doped atoms present. In essence, it occupies the entire crystal, which can be up to a centimeter long.

Although the storage of photons in these crystals had been demonstrated, it wasn't clear whether they were able to preserve an entangled state. The new papers both go this extra step, and show that entangled temporal modes are maintained when the photon emerges from the crystal it's stored in.

Both also make a few extra steps toward making the technique practical. In one case, the authors's crystals could accept light at much more useful wavelengths than the previous demonstrations. In the other, the second entangled photon was at a wavelength that is commonly used in fiber optic communications, meaning that it could potentially be used in a quantum encryption system, which requires long-range transmission of photons. (That team actually detected the photon in another lab 50m down the hall.)

That said, these things still aren't exactly practical. As noted above, the crystals need to sit within a few Kelvin of absolute zero, so they're not quite ready for deployment in a typical computing environment. Although the entanglement could be demonstrated at several standard deviations, the efficiency of putting the photon into the actual crystal wasn't all that great; 21 percent in one case, a fraction of a percent in the other.

Both teams also kept the crystals oscillating between the receive/transmit and storage states, and oscillating very rapidly; in each case, the photon came back out in less than 200 nanoseconds. It should be possible to tune things so that this oscillation isn't used and storage times are extended out much further, but that certainly hasn't been done in either of these papers.

It would be nice if that last issue is cleared up soon. It would be really neat to see a demonstration that a photon can hit a crystal, vanish, and reemerge a second later with its quantum properties intact. As the authors themselves note, "The creation of entanglement between a single photon and a macroscopic object—in this case a single collective atomic excitation delocalized over a 1-cm-long crystal—is fascinating in itself."

Nature, 2011. DOI: 10.1038/nature09662, 10.1038/nature09719 (About DOIs).