Researchers successfully built a laser that is no larger than a grain of rice and uses about one-billionth of the electrical current needed to power a hairdryer.
The tiny laser is powered by single electrons that tunnel through man-made atoms called "quantum dots," Princeton University reported. The findings could help researchers figure out how to use quantum dots as components of quantum computers.
"It is basically as small as you can go with these single-electron devices," said Jason Petta, an associate professor of physics at Princeton who led the study, which was published in the journal Science.
The findings represent major steps towards building a quantum-computing system out of semiconductor materials.
"The goal was to get the double quantum dots to communicate with each other," said Yinyu Liu, a physics graduate student in Petta's lab.
In order to accomplish this feat the researchers created quantum dots that emit photons when single electrons move from a higher energy to a lower one; each dot can only transfer one electron at a time.
"It is like a line of people crossing a wide stream by leaping onto a rock so small that it can only hold one person," Petta explained. "They are forced to cross the stream one at a time. These double quantum dots are zero-dimensional as far as the electrons are concerned - they are trapped in all three spatial dimensions."
The team created the quantum dots using nanowires made of the semiconductor material indium arsenide, which were patterned over even smaller metal wires that act as "gates" for the electrodes and control the energy of the dots. Two double dots were place about six millimeters apart in a cavity made of the superconducting material niobium, representing the first time a connection between two double quantum dots has ever been demonstrated.
When the device was switched on the electrons proved to flow single-file through each double quantum dot, triggering them to emit photons in the microwave spectrum. The photons bounced off mirrors at each end of the cavity, building a beam of microwave light. In this process, the energy levels within the quantum dots can be tweaked to produce light in other frequencies, which has never been possible in the past.
"In this paper the researchers dig down deep into the fundamental interaction between light and the moving electron," said Claire Gmachl, who was not involved in the research and is Princeton's Eugene Higgins Professor of Electrical Engineering and a pioneer in the field of semiconductor lasers. "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. Learning to control these fundamental light-matter interaction processes will help in the future development of light sources."
