Scientists in the US have demonstrated a novel "light-switch" in an optical fibre that could become a new tool in the communications industry. The device created by Michal Bajcsy at Harvard University and colleagues could be developed to share both classical and quantum information.

Quantum information systems could bring a revolution to global data-sharing, by encrypting, processing, and transmitting information using the properties of quantum mechanics. However, as strings of ��1s�� and ��0s�� are represented by the quantum states of individual subatomic particles, such as the polarization of photons, they are very delicate and information can be easily lost. Prototype quantum devices have been developed but the move towards commercial applications requires more robust systems to compete with established ��classical�� technologies. A common approach is to transmit the quantum states of photons via their interaction with matter, which acts as a mediator. Here, photons of a particular state are absorbed by an atom before being re-emitted in the same, or a related state. A difficulty arises, however, in attempting to transmit information over significant distances as photon scattering causes very high signal losses.

In the past few years, several research groups have proposed a way around this problem by transmitting the photons through a hollow optical fibre, which is filled with a vapour of atoms. The state of atoms can be altered by interaction with photons to render the optical fibre either transparent or opaque to light �C an optical switch. However, given the tiny ratio of atoms to empty space, the vast majority of photons never actually come into contact with the atoms and significant losses still occur.
Bajcsy and his colleagues have offered a solution to this problem by replacing the vapour with ultracold rubidium atoms. By holding the atoms in a steady configuration using a magnetic field called a ��dipole trap��, they could then aim a pulse of photons at the atoms with an increased chance of hitting their target. Initially, the fibre was almost completely transparent with light passing through as if the atoms were not there. Then after a "switch" pulse, containing only 800 photons was injected into the fibre, the atoms absorb these photons and the system became completely opaque.