Scientists have built an optical device smaller than a penny
that slows light down to 155 miles per second, the slowest ever
managed on a chip.
The tiny silicon chip works at room temperatures and can be
mass-produced, with 32 chips on a 4-inch silicon wafer. Previous
efforts slowed light to just 0.01 miles per second, but this
required a roomful of equipment and temperatures near absolute
Those experiments were "fantastic and very inspiring, but with
limited practical applications," said electrical engineer Holger
Schmidt of the University of California, Santa Cruz, who led the study
published in November's Nature Photonics.
Chips based on Schmidt's work could be used to create
all-optical systems that would potentially be "cheaper, faster and
use less power," said physicist John Howell from the University of
Rochester, who was not involved in the study. Slowing light on a
chip could eventually be used for optical memory, quantum
cryptography, and to create simple quantum computers, he said.
Schmidt's team's method involves shining a red laser through a tabletop maze of mirrors
into the optical chip. The laser, just a few times stronger than a
laser pointer, travels through a channel on the chip, and hits a
4mm-long capillary full of rubidium atoms. As the light smacks into
the atoms, they absorb it and don't let it through.
The scientists then shine another red laser into the rubidium
atoms, triggering a quirky quantum mechanical effect that causes
the rubidium's electrons to occupy a different physical state. This
turns the previously dense rubidium vapor transparent.
"That was really exciting," Schmidt said. "Without that second
beam, it would be opaque."
As the light passed through the rubidium atoms, they acted like
a speed bump, slowing it down by a factor of 1,200. The
6-metre-long incoming light pulse was squashed like a Slinky,
fitting into just 5mm on the chip. Reducing the second laser's
intensity could slow light even more, potentially even bringing it
to a stop, Schmidt said.
"The work is extremely impressive, and a big step forward," said
Stephen Harris, an electrical engineer and physicist at Stanford
University. Being able to slow down light on a chip at room
temperature "could have a lot of impact," he said.