Germans Measure Quantum States of Squeezed Light

Michael D. Wheeler

KONSTANZ, Germany -- More than a decade ago, scientists discovered a new state of light that, if harnessed, promised to increase the sensitivity of interferometers and other photonic instruments. There was only one problem: "Squeezed light" existed mostly in theory. It was difficult to generate and impossible to measure.
A group of scientists from the University of Konstanz has found a way to do both. In setting up their experiment, the researchers took into account squeezed light's unique nature. It exhibits fewer energy irregularities -- also known as quantum noise -- resulting from the effects of vacuum fluctuations, than traditional light does.
Because of its lower signal-to-noise ratio, squeezed light could feasibly be harnessed for more precise measurements. The problem facing the researchers: How do you measure a state of light that exists only in the strange world of quantum physics?
To solve that problem, the group turned to an unlikely source: ancient Greek philosophy. Plato's "cave theory" posed the question: If you lived in a cave and could not see the world, how would you know it exists? The only proof that the world exists, the philosopher surmised, lay in the shadows of living things cast upon the wall of the cave.
Gerd Breitenbach and his colleagues at the University of Konstanz thought Plato's theory provided a good model to analyze and measure squeezed states of light. So they devised an experiment in which they made predictions about the light's behavior. They did this by employing homodyne tomography, a technique similar to a popular medical imaging technique, computed tomography, in that the measurements performed are not of the objects themselves, but of their projections. The images of squeezed light are obtained via reconstructions using algorithms.
The group set up an experiment centered on a monolithic standing-wave lithium-niobate optical parametric oscillator. The oscillator was pumped by a frequency-doubled continuous-wave Nd:YAG laser at 532 nm. Using a homodyne detector, Breitenbach's team probed the field of light, viewing it from different angles. Gathered together, those images provided the group with valuable information on the average photon number, the energy distribution and the quantum mechanical purity.
Through their experiments, the researchers demonstrated that homodyne tomography is a powerful and practical tool, useful in the laboratory and able to provide real-time diagnostics about the quantum states.
Their work has also opened up the possibility of performing experiments in which accurately known quantum states are made to interact with optical systems. The resulting changes of the quantum states are determined in order to learn about the properties of the system.