News | January 25, 2021

Optimal Information About The Invisible

How do you measure objects that you cannot see under normal circumstances? Utrecht University and Vienna University of Technology open up new possibilities with special light waves.

With laser may radiate to measure precisely where an object is located, or if it changes its position. Normally, however, you need a clear, unclouded view of this object - and this requirement is not always met. In biomedicine, for example, one often wants to examine structures that are embedded in an irregular, complicated environment. There the laser beam is deflected, scattered and refracted, which means that a meaningful measurement result is often no longer possible.

The University of Utrecht (Netherlands) and the Vienna University of Technology have now been able to jointly show that this need can be turned into a virtue. The new approach is based on the possibility of changing the laser beam in a targeted manner in such a way that it still delivers the exact information required in the complex, disordered environment - and not just approximately, but in a physically optimal way: nature leaves more precision with coherent Laser does not light up at all. The new technology can be used in very different areas of application - also with different types of waves - and has now been presented in the journal " Nature Physics ".

The vacuum and the bathroom window
“You always want to achieve the best possible measurement accuracy - that is the essence of all natural sciences,” says Stefan Rotter from the Institute for Theoretical Physics at Vienna University of Technology. "Let us think, for example, of the huge LIGO system that can be used to detect gravitational waves: There you send laser beams onto a mirror in order to measure variations in the distance between the laser and the mirror with extreme precision." However, this is the only reason why it works so well because the laser beam spreads there through an ultra-high vacuum. Every disturbance, no matter how small, should be avoided.

But what can you do when you are faced with interference that cannot be removed? "Let's imagine a pane of glass that is not perfectly transparent, but rather rough and unpolished like a bathroom window," says Allard Mosk from the University of Utrecht. “It lets light through, but not in a straight line. The light waves are changed and scattered, so we cannot exactly see an object on the other side of the glass pane with the naked eye. ”The situation is very similar when you want to examine tiny objects inside biological tissue: the disordered environment disturbs you Beam of light. The simple, regular, straight laser beam then becomes a confusing wave pattern that is deflected in all directions.

The optimal wave
However, if you know exactly what the interfering environment is doing with the light beam, you can reverse the situation: Instead of the simple, straight laser beam , it is then possible to generate a complicated wave pattern that is given the exact shape you want due to the interference hits exactly where it can deliver the best result. “To achieve this, you don't even have to know exactly what the disturbances are,” explains Dorian Bouchet, the first author of the study. "It is sufficient to first send suitable waves through the system in order to examine how they are changed by the system."

The scientists involved in this work jointly developed a mathematical procedure with which one can then calculate the optimal wave from these test data: "One can show that certain waves exist for different questions, which bring a maximum of information: for example via the spatial coordinates, where a certain object is located. "

If you know, for example, that an object is hidden behind a cloudy sheet of frosted glass, there is an optimal light wave with which you can obtain the maximum amount of information about whether the object has moved a little to the right or a little to the left. This wave looks complicated and disordered, but is then changed by the frosted glass pane in such a way that it arrives at the object exactly in the desired manner and returns the greatest possible amount of information to the experimental measuring device.

Laser experiments in Utrecht
The fact that the method actually works has been confirmed experimentally at the University of Utrecht: Lasers are directed through a disordered medium - in the form of a cloudy plate. The scattering behavior of the medium was characterized, then the optimal waves were calculated to analyze an object on the other side of the plate - and it succeeded with a precision in the nanometer range.

The team then carried out further measurements to explore the limits of the method: The number of photons in the laser beam was significantly reduced in order to see whether one still gets a meaningful result. This made it possible to show that the method not only works, but is even optimal in the physical sense: "We see that the precision of our method is only limited by what is known as quantum noise," explains Allard Mosk. “This noise arises from the fact that light consists of photons - nothing can be done about that. But in the context of what quantum physics is for a coherent laserbeam allows, we can actually calculate the optimal waves to measure different things: Not only the position, but also the movement or direction of rotation of objects. "

These results were obtained within the framework of a program to measure semiconductor structures in the nanometer range, in which universities collaborate with industry. In fact, the team sees possible areas of application for this new technology in very different areas such as microbiology, but also in the manufacture of chips, where extremely precise measurements are also indispensable.

Source: Vienna University of Technology