Turning light into matter
How do you make a light wave vanish and then reappear elsewhere?
It sounds like a conjuring trick. You shine a light into a gas, and the light gets swallowed. Then you pump the gas into another container, say the magic words, and the light comes out again.
But this trick, demonstrated by physicist Lene Vestergaard Hau and her co-workers at Harvard University in Cambridge, Massachusetts, doesn't use magic. Instead, the researchers have harnessed the strangeness of quantum mechanics to conduct their vanishing act1.
First, they slow a light pulse — travelling, naturally, at the speed of light — to a crawl by beaming it into an ultracold cloud of about two million sodium atoms. Then they destroy the light beam entirely, but imprint a memory of it in the sodium.
They shunt some of these atoms into a second cloud, and tickle them with another laser beam. This triggers their 'memory' of the original pulse, which emerges, much weaker but otherwise unchanged, from the second cloud.
Hau says that the 'messenger' atoms that move between clouds are basically a 'matter copy' of the original light pulse: a piece of light cast in atoms, you could say.
This process could be used for manipulating information in quantum computers, which would be potentially much more powerful than conventional devices. It might also prove useful in conventional optical telecommunications, for example for storing information held in light beams.
The experiment relies on the way that, according to quantum mechanics, atoms may behave as waves as well as particles. This enables atoms to do some counterintuitive things, such as passing through two openings at once.
Usually, each atom displays its wavelike behaviour independently of all its neighbours, like a crowd of soccer fans each waving their arms about at random. But if a group of atoms is cooled to very low temperatures — a mere fraction of a degree above absolute zero — then they may all come into step, like the fans conducting a Mexican wave.
In this state, called a Bose-Einstein condensate, information encoded in a light pulse can be transferred to the atom waves. Because all the atoms move coherently, the information doesn't dissolve into random noise and get lost.
Hau and her team have previously shown that a Bose-Einstein condensate — which they make here by cooling sodium atoms to about 600 billionths of a degree (colder than deep space) — can slow down a light beam and even bring it to a standstill.
In this case, the Bose-Einstein condensate slows the light speed to a mere 24 kilometres an hour. This means that a pulse lasting less than a millionth of a second, which normally travels about a kilometre in that time, covers only about 20 micrometres (thousandths of a millimetre) in the sodium gas. So one of these pulses fits comfortably inside the cloud, Hau says.
Spreading the news
A second 'control' laser then writes the shape of the pulse into the atom waves. When this control beam is turned off and the light pulse disappears, the 'matter copy' remains.
At the same time, the light's momentum is transferred to the atoms, so they move out of the Bose-Einstein condensate cloud to a second, similar cloud that the researchers hold suspended in a magnetic trapping field less than a millimetre away.
By turning the control beam on this second cloud, the added atoms are encouraged to spread their 'message' throughout the whole cloud. All the atoms come into step with each other again, giving this second cloud a memory of the original laser pulse. They then re-radiate this pulse, albeit with only about one-fiftieth as much of the original light energy. The pulse crawls through the second cloud and speeds up once it leaves.
The end result is rather like telling a story to a crowd of people, some of who move off to another crowd and spread the story there.
Making 'matter copies' of light could be valuable in optical communications, Hau adds. "Matter is much easier to manipulate than light. For example, we could grab these copies and store them on the shelf." Alternatively, matter copies of light pulses arriving at an overworked hub could be put into a holding pattern, like aircraft at a busy airport.
- Ginsberg N. S., et al. Nature, 445. 623 - 626 (2007).