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.' "