快猫短视频

A PULSE of light can be stopped dead, and then sent on its way again at the
flick of a switch, say two American research teams. Their achievement takes us a
step closer to quantum computers, because it provides a way to pluck quantum
information from a beam of light without having to keep individual atoms in a
fragile quantum state.

Light travels through empty space at 300,000 kilometres per second, or
somewhat slower in a dense medium such as glass or water. In 1999, Lene Hau of
Harvard University stunned physicists by slowing light to a few metres per second
(快猫短视频, 20 February 1999, p 10).

Now Hau has gone one step further and brought light to a complete standstill
in a specially prepared gas of cold sodium atoms. At the same time, a team at
the Harvard-Smithsonian Center for Astrophysics has reported achieving similar
results in a hot gas of rubidium atoms.

According to Ron Walsworth of the Harvard-Smithsonian team, similar
techniques could play a key role in future super-fast quantum computers. Such
machines will need to transfer quantum information from light beams to atoms for
processing. Previous attempts to do this have used light to push individual
atoms into an excited state. But these states are so delicate they are liable to
be destroyed by background noise.

In the latest experiments, when the light stops, the information in its
electromagnetic fields is stored in the arrangement of many gas atoms. 鈥淲e have
over 10¹² atoms, which makes the state very robust,鈥� says David Phillips of the
Harvard-Smithsonian team. This means the information can be retrieved with 100
per cent efficiency.

The key to stopping light is to nudge the gas atoms into a 鈥渄ark state鈥� in
which their electrons are unable to jump up to higher energy levels. This means
that the atoms cannot absorb light, so when the researchers shine a pulse of
light into the gas it interacts with the 鈥渟pin鈥� of the gas nuclei instead. This
is what slows the pulse down.

Both groups used a second carefully tuned laser beam, known as the coupling
beam, to create a gas in a dark state. The light pulse鈥檚 speed depends on the
intensity of the coupling beam. The dimmer the beam, the slower the pulse
travels, and switching off the coupling beam brings the light to a complete
stop. The researchers found that they could set the trapped light pulse moving
again by restoring the coupling beam.

The tricky part is switching off the coupling beam without destroying the
dark state, says Mikhail Lukin of Harvard-Smithsonian, who led the theoretical
work which inspired both experiments. But Hau says her team found that 鈥測ou can
slam it on and off.鈥�

Either way, 鈥渆verybody thought it was pretty wild,鈥� says Seth Lloyd, a
quantum computing engineer from the Massachusetts Institute of Technology who
attended the Physics of Quantum Electronics conference in Utah last week where
Lukin presented his experimental results.

Engineers like Lloyd would prefer to be able to make their quantum computers
out of a solid, rather than a gas. Phil Hemmer of the Air Force Research
Laboratory at Hanscom in Massachusetts may have the answer. He has slowed light
in a crystal of yttrium silicate, and is about to try stopping it completely
using the new technique. 鈥淣ow they鈥檝e shown it鈥檚 possible, the next step is to
show it鈥檚 practical,鈥� he says.

Their success isn鈥檛 guaranteed. In a solid, some atoms won鈥檛 settle into a
dark state and could absorb the pulse. Hemmer plans to use a third laser beam to
dump the uncooperative atoms out of the way into a different energy level.