Glass fibre brings light to a standstill

Scientists have managed to slow down the speed of light and even bring it to a complete stop for a split second with a customised glass fibre.

A team of researchers at the Vienna University of Technology (TU Wien) coupled atoms to glass fibres and slowed light to a speed similar to that of an express train.

This proven ability to control its speed could lay the groundwork for a future glass-fibre-based quantum internet, in which quantum information could be teleported over great distances, but could also be harder to intercept.

Although a worldwide glass-fibre net already exists and we are using it to transmit data, there are different ways of quantum mechanically transferring information, the researchers said.

Light pulses slowed down

The usual speed of light in a vacuum is approximately 300 million metres per second, but when the light is sent though a medium such as glass or water it is slowed down a little due to its interaction with the material.

Professor Arno Rauschenbeutel, one of the scientists, said: “The speed in our glass fibre is only 180 kilometres per hour; any express train can top that.”

The method devised at the TU Wien involved cesium atoms coupled to an ultra-thin glass fibre and used an additional control laser in an experiment that coupled the high-energy state to a third atomic state.

When the atom absorbs laser light it can pass from a state of low energy to a state of higher energy, provided that the energy of the absorbed photon matches the energy difference between the two states.

“The interplay between these three quantum states prevents the photon from just being absorbed and randomly emitted. Instead, the photon’s quantum information is transferred to an ensemble of atoms in a controlled way, and it can be stored there for some time,” Rauschenbeutel explained.

After two microseconds, a period of time in which the light would normally have travelled about half a kilometre, the control laser was used to prompt the atoms to emit the light back into the glass fibre. This means the properties of the photon stay exactly the same.

Being able to store photons is an important technological step towards quantum communication over great distances, which could lead to an internet with secure connections.

“Quantum physics allows us to create a connection between sender and receiver, which makes eavesdropping impossible”, Rauschenbeutel said. “The fundamental laws of quantum physics make sure that no one can tap the connection without being noticed.”

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