You can't get much smaller than this: Physicists
have fashioned a mirror from a single atom. The advance might lead to an
atom-sized transistor for light, and experts say it bodes well for broader
efforts to shrink optical elements to the nanometer scale.
"In terms of the basic physics, it's
incredibly cute," says Christian Kurtsiefer, an experimental physicist at
the National University of Singapore, who was not involved in the work.
"It's a very striking effect because you wouldn't necessarily expect that
a single atom would exert a lot of influence on the flow of light."
In fact, the atom effectively reflects less than
1% of the light that hits it. So to detect the reflection, Gabriel Hétet,
Rainer Blatt, and colleagues at the University of Innsbruck in Austria relied
on a wave effect known as interference. They fashioned a device called a
Fabry-Pérot interferometer, which ordinarily consists of two mirrors facing
each other. Laser light of a fixed wavelength shines on the back of one mirror
and some leaks through the mirror, entering the "cavity" between the
mirrors. A small amount of light then leaks through the second mirror, while
most of it reflects back toward the first. The reflected light can make
multiple roundtrips between the mirrors. Each time, a little more light can
leak through the second, farther mirror. (A similar effect takes place at the
first mirror, too.)
Here's where the interference comes in. If the
roundtrip distance between the mirrors equals a multiple of the light's
wavelength, then all the light waves leaking through the second mirror will be
in sync and reinforce each other, greatly increasing the transmission. If this
roundtrip distance is slightly different, all those waves will be out of sync
and cancel each other out, reducing the transmission. So the amount of
transmitted light goes up and down as the distance between the mirrors
increases.
Hétet, Blatt, and colleagues replaced the second
mirror with a single atom—actually a barium ion. To focus the light on the atom
and collect the light bouncing off it, they put a 1.5-centimeter-wide lens
between it and the mirror. To hold the ion steady 14 millimeters away from the
mirror, they captured it in an electronic trap and used other laser beams to
cool it so that it jiggled no more than 20 nanometers from the trap's center.
Finally, they tuned the wavelength of the light entering the interferometer so
that it could "excite" the atom from a particular low-energy state to
a higher-energy one. Without such a light-atom interaction, the atom can't
affect the light.
The interferometer wasn't perfect. As the
researchers moved the ion away from the mirror, the amount of light coming
through the system varied by about 6%, they report in a paper to be published
in Physical Review Letters. In a standard Fabry-Pérot, the transmitted
light will fall to essentially zero if the roundtrip distance between the
mirrors is just a little off the required spacing. Still, the data show the atom
working as a mirror.
So what's this tiny mirror good for? In
principle, it helps extend a theoretical approach known as cavity quantum
electrodynamics. A cavity like a Fabry-Pérot can change the vacuum of empty
space to allow only certain quantum states of light to exist between its
mirrors—those with correct wavelengths. The new experiment shows that a mirror
and a single atom can exert the same sort of influence.
More practically, with a better lens to increase
the effective reflectivity of the atom, the device might make a building block
for an optical version of electronics. "One can think of moving the mirror
to make the atom transmit or reflect the light, which would make it a
transistor" for light, Hétet says. In principle, such an optical system
could be faster and more efficient than current electronics. A tack better
suited to all-optical systems would be to use a single photon from yet another
laser to control the atom's reflectivity by changing its internal state,
Kurtsiefer says: "That's the hard part."
The experiment comes as good news to scientists
striving to make ever smaller optical devices, says David Kielpinski, a
physicist at Griffith University in Brisbane, Australia. Physicists don't fully
understand whether the properties of optical devices will change as the devices
shrink to atomic scale, Kielpinski says: "This work tells you, 'Hey, it's
okay to build optical components out of a few atoms. There's nothing lurking
around the corner to kill that enterprise.' "
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