Chuck Bednar for redOrbit.com – Your Universe Online
While investigating the use of semiconductor material fragments as components for quantum computing, Princeton University researchers developed a laser the size of a grain of rice.
The researchers, who published their findings Friday in the journal Science, explain that the tiny microwave laser or “maser” is powered by single electrons that tunnel through those tiny bits of semiconductor material. These act like single atoms and are known as quantum dots.
They had been exploring how to use those quantum dots as quantum computing components when they built the device, which they claim is a demonstration of the fundamental interactions between light and moving electrons.
The laser uses approximately one-billionth the electric current required to power the average hair dryer, and lead author and associate professor of physics Jason Petta said that the new maser is “basically as small as you can go with these single-electron devices.”
Co-author Jacob Taylor, an adjunct assistant professor at the Joint Quantum Institute, University of Maryland-National Institute of Standards and Technology, added that the device demonstrates a huge advance in efforts to build quantum-computing systems from semiconductor materials.
“I consider this to be a really important result for our long-term goal, which is entanglement between quantum bits in semiconductor-based devices,” he explained.
However, building the microwave laser was not the original goal of the project. Rather, the authors were attempting to figure out how to use double quantum dots (two quantum dots joined together) as quantum bits (qubits), the basic units of information in quantum computers.
Yinyu Liu, a physics graduate student in Petta’s lab, explained that the goal “was to get the double quantum dots to communicate with each other.” Since quantum dots can communicate through the entanglement of light particles (photons), they designed dots that emitted photons when single electrons leap from a higher energy level to a lower one to cross the double dot.
Each double quantum dot can only transfer one electron at a time, Petta explained. He compared it to a line of people who are crossing a stream by leaping onto rocks that are only large enough to fit one person at a time. Petta added that the double quantum dots “are zero-dimensional as far as the electrons are concerned – they are trapped in all three spatial dimensions.”
The double quantum dots were built from extremely thin nanowires, approximately one-billionth of a meter in diameter, that were made out of a semiconductor material known as indium arsenide. The researchers patterned the indium arsenide wires over other even smaller metal wires that act as gate electrodes, controlling the energy levels in the dots.
In constructing the maser, they placed the two double dots about six millimeters apart in a cavity made of niobium, a superconducting material that requires a temperature near absolute zero (459 degrees below zero Fahrenheit). Taylor said that this was the first time that a connection between two double quantum dots separated by nearly a centimeter had been demonstrated.
Once the maser was powered up, electrons flowed single-file through each double quantum dot, which caused them to emit photons in the microwave region of the spectrum. These photons then bounced off mirrors at each end of the cavity, forming a coherent beam of microwave light.
One advantage of the new maser is that the internal energy levels of the dots can be fine-tuned to produce light at different frequencies – something that Petta said cannot be done with other semiconductor lasers because they have their frequencies fixed during manufacturing. The larger the energy difference, the higher the frequency of the emitted light, he added.
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