Like in an electrical diode, which allows current to pass only in one direction, this glass fibre-based device transmits light only in one direction. The one-way-rule holds even if the pulse of light that passes through the fibre consists of a few photons. Such a one-way transfer of light can now be used for optical chips and may become used in optical signal processing.
“In principle, such components have been around for a long time”, said Arno Rauschenbeutel, from the Vienna Centre for Quantum Science and Technology at the Atominstitut at TU Wien. “Most optical isolators, however, are based on the Faraday Effect: A strong magnetic field is applied to a transparent material between two crossed polarisation filters. The direction of the magnetic field then determines the direction in which light is allowed to pass.”
However, devices using the Faraday Effect cannot be constructed at the nanoscale and other methods for breaking this symmetry only work at very high intensities. But in nanotechnology, an ultimate goal is to work with extremely faint light signals, which may even consist of individual photons.
Rauschenbeutel’s team chose a completely different approach: Alkali atoms were coupled to the light field of an ultrathin glass fibre. In a glass fibre, the light can propagate forwards or backwards. There is, however, another property of light which has to be taken into account: the direction of oscillation of the light wave, or polarisation.
“The polarisation rotates, like a helicopter’s rotor”, Rauschenbeutel explained. The sense of rotation depends on whether the light travels forwards or backwards. In one case, the light wave oscillates clockwise and in the other, anti-clockwise. The direction of propagation and the state of oscillation of the light wave are locked to each other.
If the alkali atoms are prepared in the right quantum state and coupled to the light in the ultrathin glass fibre, it is possible to make them react differently to the two senses of light rotation. “The light in the forward direction is not affected by the atoms. However, light which travels backwards and consequently rotates the other way around, couples to the alkali atoms and is scattered out of the glass fibre”, Rauschenbeutel continued.
This effect has been demonstrated in two different ways at TU Wien: In the first approach, about 30 atoms were placed along the glass fibre. Upon sending in light, a transmission of almost 80% was measured for one propagation direction while it was ten times less in the other direction. In the second approach, only a single rubidium atom was used. In this case, the light was temporarily stored in an optical microresonator, so that it could interact with the atom for a relatively long time. This way, similar control over the transmission could be achieved.
“When we only use one single atom, we have a much more subtle control over the process”, said 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.”
According to classical physics, this would be impossible, but quantum physics allows such combinations, opening the door for optical processing of quantum information.
