Quantum encryption systems are already up and running, as are simplified versions of quantum computers. But to fully take advantage of these developments, we need the equivalent of a quantum memory: something that will take a quantum state and hold it in place for extended periods of time. Quantum memory will be necessary for any form of advanced quantum computing, and it could extend the range of quantum encryption beyond the distance that we can expect to send individual photons.

Unfortunately, developing quantum memory is a very hard problem, since any interactions with the environment can erase any information held in an object's quantum state. The things that are easy to manipulate, like electrons, tend to undergo lots of interactions. Things that are a bit insulated against the environment, like the spins of an atom's nucleus, tend to be hard to address—you often have to go through the electrons to get to them.

Today's issue of Nature contains a paper that describes a nuclear spin that can be addressed directly with radiofrequency radiation. And the spin is so well insulated that it can store quantum states for as long as six hours. The downside? It needs to be held in an intense magnetic field at nearly absolute zero.

Most of the work on quantum storage in nuclear spins involves the use of doped materials. These have a regular crystal structure, but a small number of atoms within the structure are replaced by a foreign atom. In this case, the main crystal is a yttrium silicate; the atom it's doped with is an isotope of europium. The spin of the europium nucleus can be manipulated directly using photons, which simplifies the setup. And preliminary work had indicated that its nuclear spin is extremely stable—some experiments have observed lifetimes of up to 23 days.

The primary thing that causes the spin states to flip is the influence of nearby yttrium atoms, which may change orientations of their spin and thereby change the local environment. But the europium's own electrons also have spins, and these change much more rapidly than any of the nuclear ones. The authors found that by applying a strong magnetic field (on the order of a Tesla), they could create what they called a "frozen core" in which all the spins of the europium atom's electrons stayed put. This stabilizes the magnetic environment around the nucleus, stretching out the lifetime of the spin.

This process reduces the chance of that europium's spin will flip down to 9 x 10-5 each second. When the researchers stored a quantum state in the spin, it stayed there for about six hours (give or take an hour). That's remarkably stable for something like this, and the authors think they can do better. There are other isotopes of europium that are expected to be twice as stable as this one.

This development could potentially serve as the basis for a quantum relay—the europium atoms receive the state of a photon and then emit a new photon that's sent further down the communication chain, limiting the loss of photons that naturally occur over longer fiber-optic connections. But the authors are positively giddy, suggesting that it might make sense just to throw the material into the back of a truck and drive it to the destination.

At a very moderate speed of 100 km/hour, the authors calculate that by the time you got the crystals 600 km, 13 percent of them would still hold the intended information. Sending photons the same distance would result in the loss of all but one in 10-12 due to things like scattering.

What doesn't show up in their calculations is that everything operates in a very intense magnetic field at a temperature of two Kelvin. So, it's not quite as simple as throwing a crystal in the glove compartment and hitting the gas; chances are good that photons will still do the long-distance transmission. But it's certainly possible that this material, or an improved variant of it, will be on the receiving end of some of those photons when we develop the first quantum networks.

Nature, 2015. DOI: 10.1038/nature14025 (About DOIs).