Quantum applications, from cryptography to computation, all benefit from the use of entangled particles, (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. 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. 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. Still it’s a good step towards bringing quantum memory into reality.
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