A quantum trick based on interferometric measurements allows a team of researchers at LMU to detect even the smallest movements of a laser beam with extreme sensitivity.

Precisely measuring minute shifts or slight tilts of a laser beam is crucial in many scientific and technological applications, such as atomic force microscopy. So-called weak value amplification (WVA), a method that grew out of thinking about the foundations of quantum mechanics, has already shown that under certain conditions the output signal of an interferometer changes markedly when the beams inside it are altered only minimally. An interferometer is a measuring device that can detect such tiny differences by comparing overlapping light waves.

LMU physicist Carlotta Versmold and her colleagues, all members of the MCQST Cluster of Excellence, working together with researchers at Tel Aviv University, have now extended this type of measurement. The team developed a trick that also amplifies changes in the incoming beam. This makes it possible to carry out far more precise measurements that were previously difficult to achieve. A laser beam reflected from a distant window, for example, could pick up vibrations in the glass caused by conversations inside the building, allowing those conversations to be overheard.

In conventional interferometers, changes in the incoming light affect both arms of the instrument in the same way and therefore cancel out in the output signal. In the WVA approach for shifts inside an interferometer, a light beam is split and sent along two slightly different paths that later recombine and lead to two outputs. Versmold and her colleagues inserted a so-called Dove prism into the beam path of one arm of the interferometer. This type of prism creates an additional reflection, causing shifts in the two paths now in opposite directions. The result is an amplified shift.

Carlotta Versmold measured the tilt and displacement of a beam entering the interferometer with a precision of tenths of a microradian and tenths of a micrometre, much less compared with the diameter of the beam of about 2 millimetre, respectively. As a demonstration, she encoded music into the vibrations of a mirror and then sent the laser beam reflected from that mirror into her interferometer. The sound quality was clearly better than that of audio signals converted into light and back into sound without an interferometer. “This shows the potential of the method for particularly sensitive measurements,” says LMU physicist Harald Weinfurter, the study’s senior author.