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.
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