'Quantum Pointer' Improves Measurements
Daniel S. Burgess
An optical system called a "quantum laser pointer," developed by scientists at Australian National University in Canberra and at Université Pierre et Marie Curie in Paris, promises to improve measurements of the position of a laser beam. The system may have implications for microscopy and for the characterization of such phenomena as refractive index gradients.
Nicolas Treps of the university in France explained that the quantum laser pointer is a beam of light in which the photons are spatially ordered. In a demonstration of the technique, the researchers mixed 1064-nm laser radiation in an intensely coherent state with two weak beams in a squeezed vacuum state from a pair of optical parametric amplifiers.
A "quantum laser pointer" improves the accuracy of the measurement of a laser beam's position by canceling quantum noise in the beam. It employs double-flipped wave plates (left) to spatially order photons in two laser beams that are subsequently mixed without loss with a third beam in a mode-mixing cavity (right).
"Each of these weak beams has spatial quantum properties such that it increases the precision of positioning in one direction," he said, and the correlations between these nonclassical beams cancel the inherent randomness generated by the distribution of photons in the transverse plane of the beam, known as quantum noise. As a result, the standard quadrant detectors used in beam measurement offer higher precision: by a factor of two in the demonstrations, but perhaps by as much as a factor of five with improvements in the squeezing process.
Although many optical measuring devices actually are limited instead by classical noise, Treps noted that there are other important devices, such as atomic force microscopes, with detection limits that are close to the quantum noise in their beam measurement systems. These latter instruments thus could witness significant advantages if partnered with a quantum laser pointer.
Moreover, as other systems that rely on the measurement of a laser beam's position are refined, classical noise becomes less of an issue, making the approach more attractive. "It is when a detection system reaches the quantum noise limit that we can improve it," he said. "The conclusion is: The better the classical measurement system is, the more useful our technique can be.
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