Ultra-Simple Desktop Device Slows Light to a Crawl

Though Einstein put his foot down and demanded that nothing can move faster
than light, a new device developed at the University of Rochester may let
you outpace a beam by putting your foot down on the gas pedal. At 127 miles
per hour, the light in the new device travels more than 5 million times
slower than normal as it passes through a ruby just a few centimeters long.

Instead of the complex, room-filling mechanisms previously used to slow
light, the new apparatus is small and, in the words of its creator,
“ridiculously easy to implement.” Such a simple design will likely pave the
way for slow light, as it is called, to move from a physical curiosity to a
useful telecommunications tool. The research is published in a recent issue
of Physical Review Letters.

The new technique uses a laser to “punch a hole” in the absorption spectrum
of a common ruby at room temperature, and a second laser shines through that
hole at the greatly reduced speed. A recent successful attempt to slow light
to these speeds used a Bose-Einstein condensate (BEC), a state of matter
existing 459 degrees below zero Fahrenheit where all atoms act in unison
like a single, giant atom. The laser shining through the BEC was slowed to
38 miles per hour, but the system had enormous drawbacks, not the least of
which was that the equipment needed to create the BEC wouldn’t fit in the
average living room, and the created BEC itself was little bigger than the
head of a pin.

“If that was the world’s hardest way to slow down light, then what we’ve
found is the world’s easiest way to do it,” says Robert Boyd, the M. Parker
Givens Professor of Optics at the University. “We can slow light just as
much in a space the size of a desktop computer.”

Slowing light, at least a little, isn’t as difficult as it may seem. Light
passing through a window is 1.5 times slower while moving through the glass,
and is slowed slightly less so when passing through water. But to achieve
the 5.3-million fold slowdown, Boyd and his team, students Matthew Bigelow
and Nick Lepeshkin, used a quantum quirk called “coherent population
oscillations” to create a special gap in the frequencies of light that a
ruby absorbs. Rubies are red because they absorb most of the blue and green
light that strikes them. Shining an intense green laser at the ruby
partially saturates the chromium ions that give ruby its red color. They
then shine a second beam, called the probe laser, into the ruby. The probe
beam has a frequency slightly different than the first laser, and these
offset frequencies interact with each other, causing variations the same way
two ripples encountering each other on a pond might create waves higher and
lower than either one had alone. The chromium ions respond to this new
frequency of rhythmic highs and lows by oscillating in sympathy. One
consequence of this oscillation is that it allows the probe laser to pass
through the ruby, even though the laser is green, but it only allows it to
pass 5.3 million times more slowly than light would otherwise travel.

Boyd anticipates that the slow light device will find a role in the
telecommunications industry. When two signals from fiber optic lines merge,
the two signals may reach the merging router at the exact same moment and
need to be separated slightly in time so they can be laid down one after
another. Like two cars merging on a highway where one may need to slow down
to let another car into the lane, a light-slowing device could help ease
congestion on fiber optic lines and simplify the process of merging signals
on busy networks.

One drawback to the new technique is currently being scrutinized by Boyd and
his coworkers-the duration of the pulses of light that it delays are very
long. The BEC experiments were able to delay a short pulse, which meant that
a plain pulse of light and a slowed pulse would differ by several times the
pulses’ lengths. The Boyd technique slows light by roughly the same amount
as the BEC method, but since the pulses are much larger, the delay is only a
fraction of the pulses’ size. It would be the difference between slowing an
economy car a few feet to let another economy car merge, and a
double-tractor trailer slowing only a few feet and expecting another double
trailer to merge into the gap. Boyd suspects that different materials may
yield slowed light that can transmit shorter pulses that would be more
useful for telecommunications work.