One of the building blocks for the next generation of quantum computing and communications systems is a way of storing and regenerating photonic qubits. These are generally encoded in the polarisation of photons.

To date, physicists have done this by transferring the qubit from a photon to an ensemble of quantum particles such as a crystal lattice or a small cloud of atoms.

Today, Holger Specht and pals at the Max Planck Institute for Quantum Optics in Garching, Germany, have gone one better. They’ve found a way to store the qubit from a polarised photon in a single atom of rubidium and then release it again later.

The trick here is first to find an atom with the suitable two-level state that will absorb photons in the right way and second, to find a way to force the photon to give up its qubit to the atom.

It turns out that rubidium has the just right energy levels. Specht and co force the atom and photon to interact by trapping them in a high quality mirrored cavity in which the photon can enter but not easily escape. It then rebounds inside until it gives up its goods to the atom.

To accept the qubit, the atom first has to be placed in the right state by a weak laser beam. A second laser beam later forces the atom to spit out the qubit in the form of an identical polarised photon.

The result is a single atom memory that can read, store and write quantum information.

That’s a useful piece of kit. For example, such a device could form the basis of a quantum repeater, an enabling technology for a quantum internet that could be vastly more capable than the one we have today.

And although the device can store qubits for only 180 microseconds and has an overall efficiency of 9 per cent, Specht and co say they know how to make significant improvements, “with the prospect of storage times exceeding several seconds”.

This is a crowded field with many groups working on similar devices. It’s not clear whether the German team has the edge over its competitors but they’re certainly in the running.

Ref: arxiv.org/abs/1103.1528: A Single-Atom Quantum Memory

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