The fastest thing in the universe has come to a complete stop for a record-breaking minute. At full pelt, light would travel about 18 million kilometres in that time – that’s more than 20 round trips to the moon.
“One minute is extremely, extremely long,” says Thomas Krauss at the University of St Andrews, UK. “This is indeed a major milestone.”
The feat could allow secure quantum communications to work over long distances.
While light normally travels at just under 300 million metres per second in a vacuum, physicists managed to slow it down to just 17 metres per second in 1999 and then halt it completely two years later, though only for a fraction of a second. Earlier this year, researchers kept it still for 16 seconds using cold atoms.
To break the minute barrier, George Heinze and colleagues at the University of Darmstadt, Germany, fired a control laser at an opaque crystal, sending its atoms into a quantum superposition of two states. This made it transparent to a narrow range of frequencies. Heinze’s team then halted a second beam that entered the crystal by switching off the first laser and hence the transparency.
The storage time depends on the crystal’s superposition. A magnetic field extends it but complicates the control laser configuration. Heinze’s team used an algorithm to “breed” combinations of magnet and laser, leading them to one that trapped light for a minute.
They also used the trap to store and then retrieve an image consisting of three stripes. “We showed you can imprint complex information on your light beam,” says Heinze.
Tens of seconds of light storage are needed for a device called a quantum repeater, which would stop and then re-emit photons used in secure communications, to preserve their quantum state over long distances.
It should even be possible to achieve longer light storage times with other crystals, says Heinze, as they have pushed their current material close to its physical limit.
If light is a time traveller, as it seems to be given double-slit experiments where photons fired one at a time form a wave-like interference pattern, holding light in one place may have consquences we do not yet understand.