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Quantum Dots Set Stage for Next-Generation InfoTech

A new process has made it possible to control characteristics of the light coming from quantum dots with laser technologies, according to a report last week from the National Institute of Standards and Technology (NIST) and the University of Maryland’s Joint Quantum Institute (JQI).

Researchers at NIST and JQI have developed a technique that promises to significantly improve quantum dots as a source of pairs of “entangled” photons, a property with important applications in quantum information technologies. The accomplishment could accelerate development of powerful advanced cryptography applications, projected to be a key 21st-century technology. The study appeared recently in the journal Physical Review Letters.*

Cross-section scanning tunneling microscope shows indium arsenide quantum dot regions embedded in gallium arsenide. Each 'dot' is approximately 30 nanometers long–faint lines are individual rows of atoms. Credit: J.R. Tucker

Entangled photons are a peculiar consequence of quantum mechanics. Tricky to generate, they remain interconnected even when separated by large distances. Merely observing one instantaneously affects the properties of the other. The entanglement can be used in quantum communication to pass an encryption key that is by its nature completely secure, as any attempt to eavesdrop or intercept the key would be instantly detected.

One goal of the NIST-JQI team is to develop quantum dots as a convenient source of entangled photons.

Quantum dots are nanoscale regions of a semiconductor material similar to the material in computer processors but with special properties due to their tiny dimensions. Though they can be composed of tens of thousands of atoms, quantum dots in many ways behave almost as if they were single atoms. Unfortunately, almost is not good enough when it comes to the fragile world of quantum cryptography and next-generation information technologies. When energized, a quantum dot emits photons, or “particles” of light, just as a solitary atom does. But imperfections in the shape of a quantum dot cause what should be overlapping energy levels to separate. This ruins the delicate balance of the ideal state required to emit entangled photons.

Schematic of NIST-JQI experimental set up. Orienting the resonant laser at a right angle to the quantum dot light minimizes scattering. Credit: Solomon/NIST

The researchers used lasers to manipulate the energy levels of quantum dots, a process pioneered by physicists in the early 1970s using to control single atoms of the artificial quantum dot variety. With their customized set-up, which includes two lasers—one shining from above the quantum dot and the other illuminating it from the side—the researchers were able to manipulate energy states in a quantum dot and directly measure its emissions. By adjusting the intensity of the laser beams, they were able to correct for imperfection-caused variations and generate more ideal signals. The process allows laser-tuned quantum dots to efficiently generate photons one at a time, as required for quantum cryptography and other applications.

The device, which currently requires very cold temperatures and sits in a liquid helium bath, is compact enough to fit in the palm of your hand.

*A. Muller, W. Fang, J. Lawall and G.S. Solomon. Emission spectrum of a dressed exciton-biexciton complex in semiconductor quantum dot. Physical Review Letters, 101, 027401 (2008), posted online July 11, 2008.