ETHEREAL quantum entanglement has been captured in solid crystals, showing that it is more robust than once assumed. These entanglement traps could make quantum computing and communication more practical.

In the quantum world, two or more objects can be entangled so that measuring one affects the outcome of measuring the others, no matter how far apart the objects are. This property is central to quantum cryptography, where it allows two people to be sure a secret key they shared was not intercepted, and to quantum computing, as entangled bits occupy a superposition of two or more states at once and so can be used to solve some problems much faster than conventional computers.

A missing element is memory, which is needed to do complex calculations and to transmit quantum states over large distances. “Photons travel at the speed of light, which is bad for storage,” says Wolfgang Tittel at the University of Calgary in Alberta, Canada. Chilled clouds of atoms can act as quantum memory, but this requires bulky equipment and trained physicists.

My professors used to say that entanglement is like a dream. Now we can show that it is pretty robust


Now Tittel’s group, and a separate team led by Nicolas Gisin at the University of Geneva in Switzerland, have created practical, solid-state quantum-memory devices.

Tittel’s team started by channelling one photon of an entangled pair into a crystal of lithium niobate doped with ions of thulium. This sends the crystal into a quantum superposition, in which many thulium ions absorb the photon at once and vibrate at different frequencies. The frequencies aren’t random, however. Beforehand, the team strategically removed some thulium ions, leaving only those that absorb a particular sequence of frequencies.

This tuning ensures that after about 7 nanoseconds the oscillations all come back into sync to recreate a copy of the original photon.

The researchers verified that such photons were still entangled by comparing measurements of the stored photon with measurements of its partner (Nature, DOI: 10.1038/nature09719). Gisin’s device is similar but used a different type of crystal (Nature, DOI: 10.1038/nature09662).

“My professors used to say that entanglement is like a dream: as soon as you think about it, it is gone,” says Gisin. “Now we can show that it is pretty robust.”

For some applications, 7 nanoseconds might be useful, and for now this storage time is fixed. In future it could be extended to seconds, perhaps by using electric fields to alter the tuning of the ions.