Light usually goes both ways. Shed a little light and it can be reflected back by a mirror. Now, researchers at TU Wien, Austria, have developed the equivalent of an electronic diode but for photons that acts as a one-way street for light travelers. The device could open the way to a future of optical circuitry, signal processing and computing that would not be possible without it; Phys Rev X.

According to Arno Rauschenbeutel elements that allow light to pass in only one direction are referred to as "optical isolators and are commonly based on the Faraday effect: A strong magnetic field applied to a transparent material held between two crossed polarizing filters controls the direction in which the photons can travel. This does not work at the nanoscale, which precludes micro-photonic devices based on this phenomenon, unless high intensity beams are used, but faint light signals are the reality of any system.

Rauschenbeutel's team has now taken a different tack. They coupled chilly - 30 thousandths of a degree Kelvin above absolute zero, 30 microkelvin - alkali metal atoms, cesium atoms, with a light field carried by an ultrathin glass fiber. The interaction of light and glass modulates the polarization and the sense of rotation depends on the direction of light propagation through the fiber. By establishing the alkali metal atoms in a specific quantum state it is possible to affect the atoms only if light is polarized one way and not the other, in the other direction the photons are then simply scattered out of the glass fiber. This makes the nanofiber a one-way street to light using 30 atoms. High transmission occurs in one direction, almost 80%, ten times less in the opposite direction.

The team also demonstrated the same effect with a single rubidium atom and photons trapped in an optical microresonator to allow a relatively long-lived interaction to occur. "When we only use one single atom, we have a much more subtle control over the process," explains Rauschenbeutel. "One can prepare the atom in a quantum superposition of the two possible states, so that it blocks the light and lets it pass at the same time." Of course, superchilled clouds of atoms are not amenable to construction of an everyday optical device, so the next step will be to find a way to recreate the phenomenon using a solid state system at ambient temperature.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".