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Quantum sensor sets new limits

Photonics Spectra
Jun 2010
Marie Freebody, marie.freebody@photonics.com

BATON ROUGE, La. – Researchers at Louisiana State University (LSU) are taking advantage of the quantum properties of light to design the world’s most sensitive optical interferometer. Optical interferometers are used in a vast range of applications, including metrology, surface profiling, microfluidics, mechanical stress/strain measurement and velocimetry.

These instruments operate by combining two or more light sources (typically laser light) so that interference fringe patterns are produced. Information derived from such fringe measurements is used to determine precise wavelength and to measure very small distances and thicknesses.


Researchers at Louisiana State University are exploiting the quantum nature of light to design the world’s most sensitive optical interferometer. Courtesy of Louisiana State University.


But rather than using laser sources, which adhere to the general laws of classical physics, the LSU team speculated that using quantum sources would introduce a different set of laws and lead to a breakthrough in sensitivity.

“It used to be thought that the ultimate limit on sensitivity was set by a scaling law inversely proportional to the square root of the laser power – the so-called shot noise limit,” said Dr. Jonathan P. Dowling, Hearne professor of theoretical physics at LSU and lead researcher on the project. “From the 1980s until now, it had become clear that, by exploiting the quantum nature of light, a new limit might be proposed where the sensitivity scales inversely as the laser power – the Heisenberg limit.”

Dowling and colleagues show that a nonclassical light source (two mode-squeezed light sources) combined with a parity detection scheme actually could beat this Heisenberg limit.

“We do not violate the Heisenberg uncertainty principle, but show the connection between the principle and the limit was a bit tenuous and the limit has a little more wiggle room in it to do better than previously thought,” Dowling said.

The quantum sensor design is a conventional Mach-Zehnder interferometer but with the classical laser light source replaced with a quantum squeezed light source. Details of the design are reported in a paper published in Physical Review Letters in March 2010. In the proposed setup, the classical intensity difference counting is replaced with a more elaborate photon counting scheme. So, although the optics remains the same, the photon source and detection scheme are quantum.


This setup is based on two mode-squeezed vacuum generated in an optical parametric amplifier with parity measurement achieved through coincidence homodyne measurement, thus avoiding photon counting. A Mach-Zehnder interferometer fed with specially prepared squeezed light, with a detection scheme based on a novel double homodyne – which performs parity detection by proxy of intensity-intensity correlations. This device has the ability to beat the Heisenberg limit on phase sensitivity and saturates the Cramer-Rao bound. Courtesy of Petr Anisimov.


The LSU group is already privileged to have access to one of the most sensitive measuring devices on Earth – the large LIGO optical interferometer. The LIGO, or Laser Interferometer Gravitational-Wave Observatory, was set up in 1992 to attempt to directly detect gravitational waves. When a gravitational wave passes through the device, the space time in the local area is altered, resulting in a very slight phase change.

Dowling hopes that the breakthrough quantum sensor could have an array of applications ranging from enhanced gravity wave observations to optical gyroscopes and commercial navigation systems.

“This work is a theoretical design for a quantum sensor,” Dowling said. “The first step will be to collaborate with our experimental colleagues at other universities to do an experimental proof of principle. This will be followed by investigations into commercial applications such as magnetometers or gyros.”


GLOSSARY
mach-zehnder interferometer
Derived from the Twyman-Green interferometer, the Mach Zehnder is an amplitude splitting interferometer that consists of two beamsplitters and two fully reflecting mirrors. Light from an extended source passes through the first beamsplitter resulting in two lightwaves traversing equal and separate optical paths. The two paths are later recombined with a set of mirrors at a second beamsplitter in which the resultant beam is then passed to an observation plane where interference fringes are...
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