Slow Dance Unites Atom, Photon

Hank Hogan

Researchers from the Max Planck Institute for Quantum Optics recently created a photonic molecule. They developed a setup to temporarily bind a single photon to a single atom, a technique that could lead to applications in quantum computing, as well as to research tools that produce a single photon on demand.

From left, Thomas Fischer, Pepijn Pinkse, Gerhard Rempe and Peter Maunz of the Max Planck Institute for Quantum Optics work at the apparatus they use to generate a bound state between an atom and a photon. Courtesy of Max Planck Society/Filser.

The work is similar to that done by the California Institute of Technology in Pasadena and the University of Auckland in New Zealand. However, the German research differs in some fundamental ways, such as how measurements are made.

"We count transmitted photons. Caltech uses a homodyne technique to measure electric field strengths," said Pepijn Pinkse, the lead author in the German group's March 23 Nature paper detailing its work. By counting photons, he said, researchers were able to see that atoms hopped from one energy-favorable position to another, rather than being largely confined to only one such antinode.

At the heart of both experiments are the carefully constructed optical cavities where light interacts with a single atom. The German technique used a larger mirror spacing, decreasing the atom-photon coupling and leading to a wider range of atom movement. The researchers spaced the mirrors -- each with a reflectivity better than 99.999 percent -- 0.12 mm apart and controlled cavity length to less than 0.1 pm. Thus, the cavity was highly optically efficient, able to confine a single wavelike photon for up to 100 ns.

For their research, the scientists placed this microcavity in a vacuum at the apex of a cooled fountain of rubidium atoms. When a single atom entered the microcavity, they detected it with a low-power beam from a 780-nm laser. They quickly increased the beam power until the cavity contained, on average, a single photon.

The atom and the photon formed a bound state, a molecule consisting of the two. The interaction provided information about the atom, which remained in the microcavity long after the photon escaped, whereupon the researchers injected another photon.