快猫短视频

Read more: 鈥�Tricks of the light: Nine fabulous photon spin-offs鈥�

Imagine overtaking light on your bicycle. It鈥檚 a surreal image 鈥� but perfectly possible.

In a vacuum, light travels at a constant speed just shy of 3 脳 10⁸ metres per second. On entering any more substantial medium, however, it slows down. This causes the effect we call refraction. Light rays reflected from a stick partly inserted into water, for example, speed up as they leave the water and change direction, making the immersed portion of the stick look like it bends upwards.

The factor by which an everyday material slows light down from its vacuum speed 鈥� its refractive index 鈥� is generally small. For glass it is about 1.5, for water 1.33, and for air just 1.0003, resulting in speeds still far beyond human imagining.

That gave of Harvard University and her team a clue to how to slow light right down: create a material with a huge refractive index. In 1999 they produced a Bose-Einstein condensate (see 鈥淟ight: Thank Einstein for the new super-photons鈥�) of sodium ions cooled to within a whisker of absolute zero. By zapping this condensate with a cunningly tuned laser, it could briefly be made transparent 鈥� but only just. The medium had a refractive index of about 18 million, and slowed a second laser pulse sent through it to just 17 metres per second 鈥� the speed of a sporty cyclist ().

Such 鈥渟lowed light鈥� might benefit optical metrology by giving laser interferometers more time to measure very small distances. It could also increase the efficiency of optical-fibre communications by allowing light signals travelling at different speeds to be interwoven without colliding with each other. In 2010, and colleagues at the University of California, Santa Cruz, took a decisive step in that direction by making the technique work at less frigid temperatures. They built a centimetre-sized demonstration silicon chip containing rubidium atoms that slows light by a factor of 1200 at 80 掳C ().

If slowing light doesn鈥檛 impress you, how about bringing it to a screeching halt? In 2009, Hau and her group trapped a passing light pulse before releasing an identical pulse more than a second later by quickly switching a Bose-Einstein condensate transparent, opaque and then transparent again. Such 鈥渟topped light鈥� could be used to store information on light-based memory chips ().