Towards faster-than-light transmission by slowing down light

Bringing light pulses to a complete halt in optical fibre, then restarting them, may be an important step towards instantaeous data transmission

Quantum teleportation is one of the weirdest concepts in the very weird field of quantum mechanics. It refers to the instantaneous transmission of information over long distances, which should be impossible under classical physics, where no information can be transmitted instantaneously because of the insurmountable limit of the speed of light. But physicists at the Technical University of Vienna believe they may have taken an important step towards it — not by speeding up the transmission of information but by slowing a light pulse to a dead stop, then speeding it up again.

Coupled caesium atoms reversibly absorb light

The team, led by Professor Arno Rauschenbeutel, who is also attached to the Vienna Centre for Quantum Science and Technology, was looking at the transmission of data by light pulses along optical fibre: a standard method of moving information. When light enters transparent glass, it slows down a little because even though glass is transparent, its atoms interact with photons. The Vienna team slowed the pulse down to a much greater degree than usual by coupling the glass atoms in a tapered scetion of the fibre to caesium atoms.

This, Rauschenbeutel explained, creates a very strong interaction between light and matter. The caesium is sent into a higher energy state, by using the light from a laser whose energy, which is related directly to the wavelength of its light, matches the gap between the caesium’s high and low energy states exactly. This slowed the light pulse down to 180km/hr. “Any express train can top that,” Rauschenbeutel comented.

The team then introduced light from second laser, which coupled the high-energy caesium to a third atomic state within the fibre. This effectively turns the photon from the laser pulse into a collective excitation of atoms in the fibre. After two milliseconds — long enough for the light to travel a third of a kilometre under normal conditions — this excitation was still in exactly the same place. A further pulse from the second laser released the trapped light pulse back into the fibre without changing any of its properties.

This ability to trap photons then release them is a vital step in quantum communication over great distances, Rauschenbeutel said. “Quantum physics allows us to create a connection between sender and receiver, which makes eavesdropping impossible. The fundamental laws of quantum physics make sure that no one can tap the connection without being noticed.” The team explains the research in detail in a paper in the journal Optica.