August 19, 2008--Researchers from the National Institute of Standards and Technology (NIST; Gaithersburg, MD) and the Joint Quantum Institute (JQI), a collaborative center of the University of Maryland (College Park, MD) and NIST, have developed a new way to fine-tune the light coming from indium arsenide quantum dots by manipulating them with pairs of lasers.1 The technique could help quantum dots serve as a source of pairs of entangled photons--potentially useful in advanced cryptography applications.
Entangled photons, a consequence of quantum mechanics, are 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.
Though semiconductor quantum dots can be composed of tens of thousands of atoms, they behave in many ways almost as if they were single atoms. Unfortunately, almost is not good enough when it comes to quantum cryptography. When excited by light of the right wavelength, a quantum dot emits photons, 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.
To overcome this problem, the NIST-JQI team uses lasers to precisely control the energy levels of quantum dots, just as physicists have been doing with actual single atoms since the mid-1970s and, much more recently, with the artificial quantum dot variety. With their customized set-up, which includes two lasers--one illuminating the quantum dot from above and the other 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 signals that were closer to ideal. In so doing, the team was the first to demonstrate that laser-tuned quantum dots can efficiently generate photons one at a time, as required for quantum cryptography and other applications.
While the device currently still requires quite cold temperatures and sits in a liquid helium bath, it is compact enough to fit in the palm of one's hand.
1. A. Muller, et al., Physical Review Letters, 101, 027401 (2008), posted online July 11, 2008.