Scientists measure distance with laser accurate to quantum noise

Scientists have created a method for measuring distances using laser beams that works even in an irregular and difficult environment. Writes about this Nature Physics.

Laser beams can be used to accurately measure the position or speed of an object. But this usually requires a clear and unobstructed view of the object. This condition is not always feasible. Biomedicine, for example, examines structures that are found in an irregular and complex environment. Under these conditions, the laser beam is simply deflected, scattered or refracted.

Scientists from the University of Utrecht and TU Wien were able to obtain measurements of a given accuracy even in such difficult conditions. They specifically modified the laser beam to deliver the desired information in a disordered environment.

“The highest possible measurement accuracy is central to all-natural sciences,” says Stefan Rotter of TU Wien. “For example, in the huge LIGO installation, which is used to detect gravitational waves, laser beams are sent to a mirror, and changes in the distance between the laser and the mirror are measured with extreme precision.”

It only works so well because the laser beam travels through an ultra-high vacuum.

“But let’s imagine a glass panel that is not perfectly transparent, but rough and unpolished, like a window in a bathroom,” continues Allard Mosk of Utrecht University. “Light, of course, passes, but refracts. Light waves change and scatter, so we cannot accurately see the object on the other side of the window with the naked eye. ” A similar situation occurs when it is necessary to examine tiny objects inside biological tissue: a disordered environment interferes with the light beam. Then a simple, regular, straight laser beam turns into a complex wave structure that deflects in all directions.

But knowing exactly what the interfering medium is doing with the light beam can make a difference: create a complex wave pattern instead of a simple direct laser beam that is converted into exactly the desired shape. because of the riots and shocks exactly where you want the best result. “To achieve this, you don’t even need to know exactly what these violations are,” explains Dorian Boucher, the study’s first author. “It is enough to first send a series of test waves through the system to study how they change by the system.”

The method was confirmed experimentally at the University of Utrecht: laser beams were directed through a disordered medium in the form of a turbid plate. The researchers then calculated the optimal waves to analyze the object outside the plate – this was done with nanometer precision.

Scientists were able to show that the method not only works, but is also optimal in the physical sense: “The accuracy of our method is limited only by the so-called quantum noise,” explains Allard Mosk. “This noise comes from the fact that light is made of photons – nothing can be done about it.”

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Alexandr Ivanov earned his Licentiate Engineer in Systems and Computer Engineering from the Free International University of Moldova. Since 2013, Alexandr has been working as a freelance web programmer.
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Alexandr Ivanov

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