Scientists at Princeton have built a tiny microwave laser (a "maser") the size of a rice grain. The laser is made of artificial atoms called quantum dots -- miniature bits of semiconductor material.

"It is basically as small as you can go with these single-electron devices," lead study author Jason Petta, an associate professor of physics at Princeton, said in a recent press release.

The primary goal of the engineering project was to see if they could coax two quantum dots into talking to each other. They succeeded, using light photons as their language.

The so-called quantum dots are minuscule bits of semiconductor material carved out of already infinitesimally thin nanowire. The dots are so small only a single electron can cross the dot at one time. To build their talking maser, researchers placed two quantum dots just six millimeters apart.

When the device is switched on, a single-file line of electrons is squeezed through the double quantum dot. The photons emitted in the microwave region of the spectrum cross from one dot to the other due to a difference in energy level between the two dots. After crossing the double quantum dot, they bounce off mirrors and are concentrated into beam of microwave light -- viola, a laser (or maser).

One difference between the double dot maser and traditional semiconductor lasers, Petta says, is that energy levels inside their mini laser can be adjusted to produce photons at different frequencies. The larger the discrepancy in energy levels between the two dots, the higher frequency light the double dot maser produces.

"In this paper the researchers dig down deep into the fundamental interaction between light and the moving electron," explained Claire Gmachl, Princeton's Eugene Higgins Professor of Electrical Engineering. Gmachl, who wasn't involved in the study, said the findings are important for the continued development of quantum computers.

"The double quantum dot allows them full control over the motion of even a single electron, and in return they show how the coherent microwave field is created and amplified," she added. "Learning to control these fundamental light-matter interaction processes will help in the future development of light sources."

The new study was published this week in the journal Science.