A microwave laser — also called a "maser" — has been built by Princeton researchers and it's no larger than a single grain of rice, reports Princeton News. The minuscule device is powered by individual electrons that tunnel through artificial atoms known as "quantum dots," and the innovation could lead to new advancements in quantum computing.

"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.

The successful maser demonstration represents a breakthrough in efforts to build a quantum computer out of semiconductor materials. Basically, the device makes it possible to use double quantum dots — two quantum dots joined together — as quantum bits, or qubits, which are the basic units of information in quantum computers.

"I consider this to be a really important result for our long-term goal, which is entanglement between quantum bits in semiconductor-based devices," said collaborator Jacob Taylor, an adjunct assistant professor at the Joint Quantum Institute, University of Maryland-National Institute of Standards and Technology.

Essentially, the maser allows the double quantum dots to communicate with each other. 

To construct the tiny contraption, researchers placed two double dots about 6 millimeters apart in a cavity made of a superconducting material, niobium, which requires a temperature near absolute zero. When turned on, electrons flow single-file through each double quantum dot, which causes them to emit photons in the microwave region of the spectrum. The photons can then be channeled into a coherent beam of light using mirrors.

Aside from its importance in the development of quantum computers, the maser could also lead to advancements in a variety of fields such as communications, sensing and medicine, or any other discipline that utilizes technology which relies on coherent light sources.

"In this paper the researchers dig down deep into the fundamental interaction between light and the moving electron," said Claire Gmachl, professor of electrical engineering at Princeton. "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."

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