Researchers at Princeton University believe that they have taken a major step toward quantum computing by creating a microwave laser or maser. The maser is a rice grain-sized laser powered by single electrons from artificial atoms called quantum dots.

It was the quantum dots, and not lasers, that the researchers were originally interested in exploring. Quantum dots act like single atoms, as components for 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 in a statement.

The discovery represents a step forward in efforts to use semiconductor materials to build quantum-computing systems, according to the researchers.

“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 Jacob Taylor, an adjunct assistant professor at the Joint Quantum Institute at the University of Maryland-National Institute of Standards and Technology.

The original aim of the research was to explore the use of quantum dots, two quantum dots joined together as quantum bits or qubits. In quantum computing quits are the basic units of information.

According to a July, 2014 article on Phys.org “a single electron trapped in a semiconductor nanostructure can form the most basic of building blocks for a quantum computer. Before practical quantum computers can be realized, however, scientists need to develop a scalable architecture that allows full control over individual electrons in computational arrays.”

The researchers designed the dots to emit photons when single electrons jump from a higher to a lower energy level across the double dot. Quantum dots communicate through the entanglement of photons, or light particles. 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. 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,” said Petta.

The double quantum dots are made from extremely thin nanowire, about a billionth of a meter wide. That wire is made from indium arsenide, a semiconductor material. Those wires are laid over even smaller wires which control the energy levels of the dots.

The dots were placed in a space constructed of niobium, a superconducting material, about 6 mm apart. The cavity was cooled to a temperature of about minus 459 degrees Fahrenheit which is close to absolute zero.

When it was turned on, a single-file line of electrons flowed through the double quantum dots which caused them to emit photos as lasers in the microwave spectrum. The photons were then bounced off of mirrors at the ends of the cavity which created a solid beam of light.

“This is the first time that the team at Princeton has demonstrated that there is a connection between two double quantum dots separated by nearly a centimeter, a substantial distance,” said Taylor.

According to the team one advantage of the microwave lasers is that the energy levels can be fine tuned to produce light at other frequencies, which other semiconductor lasers cannot. The frequency is normally pre-set during manufacturing.