Just like a regular computer, a quantum computer based on photon entanglement requires memory as well as processing. However, storing the quantum state information necessary for computations has either suffered from inefficiency—losing most of the bits—or too short-lived to be useful as memory. Quantum memory that is both efficient and long-lived would enable more scalable computing, as well as long-range communication between computing nodes.

A new approach to quantum memory, described in Nature Physics by Xiao-Hui Bao et al., uses cold rubidium atoms in a special trap. The quantum state of a photon gets stored in the form of spin waves: transfer of electronic spin from one atom to another in a coherent way. A control laser beam can then stimulate the emission of a new photon—one that contains the quantum state information of the initial photon.

The researchers were able to get back between 71 and 75 percent of the quantum information they fed into the trapped atoms, and were able to hold the data for about 3.2 milliseconds. While this may sound pathetic by digital computing standards, it's a significant improvement over previous quantum memory benchmarks, opening up the possibility of further advances.

Most efforts in quantum computing involve entangling pairs of photons, and manipulation of their polarization states acts as the quantum bit, or qubit. Photons obviously don't stay in one place, so we need some other way to store the information about their polarization. Additionally, individual photons are easily lost through absorption or other processes, so storage is also essential for communication over large distances.

In the new experiment, the researchers confined rubidium atoms in a magneto-optical trap, in which the atoms are slowed by laser light until they can be held in a small space by magnetic fields. The atoms began in their lowest energy state, and the input ("write-out") photon induced a transition in one of those atoms that depends on the polarization of the light. This translates to the spin degree of freedom in the atom, which is key, because the spin doesn't end up limited to that one atom. The atoms propagated that spin through the entire ensemble in a coherent way, something known as a spin wave.

By sending a second beam of light ("read-out") into the atoms, the experimenters induced them to convert the spin wave back into a photon. The write-out and read-out beams had the same frequency but opposite polarization.

The whole process was controlled by building it into a ring cavity, a closed triangular path for the photons provided by a series of mirrors, with the rubidium atoms lying along one leg of the triangle. Additionally, the apparatus had a vertical orientation, with the path of the light traveling down through the ensemble of atoms, which ensures good overlap between the spin wave and the control laser beams.

Prior quantum memory methods have been efficient but short-lived (up to 84 percent retrieval of information, but only holding it for 240 nanoseconds), or more stable but inefficient (millisecond retrievals, but losing more than 75 percent of the bits). The use of the ring cavity and spin waves allowed 73 percent retrieval efficiency, but also had a lifetime of 3.2 milliseconds, which is a potent combination.

The authors suggest using optical lattices (traps that use crossed laser beams of slightly different frequencies) to hold the atoms and increasing the efficiency of the control beams to achieve even better results. By achieving both efficient and relatively long-lived storage, the researchers have shown a new possible route toward reliable quantum memory.

Nature Physics, 2012. DOI: 10.1038/NPHYS2324 (About DOIs).