Error-Effect Due to Spin-Orbit-Coupling

With modern optical imaging techniques, the position of objects can be measured with a precision that reaches a few nano­meters. These techniques are used in the labora­tory, for example, to determine the position of atoms in quantum experi­ments. “We want to know the position of our quantum bits very precisely so that we can manipulate and measure them with laser beams,” explains Gabriel Araneda from the Depart­ment of Experimental Physics at the University of Innsbruck.

The spiral wavefront of the elliptically polarized light hits the lens at a slight angle, leading to the impression that the source of the light is somewhat off its actual position. (Source: IQOQI Innsbruck, H. Ritsch)

A colla­borative work between physicists at TU Wien, Vienna, led by Arno Rauschen­beutel, and researchers at the Univer­sity of Innsbruck and the Institute of Quantum Optics and Quantum Infor­mation, led by Rainer Blatt, has now demon­strated that a systematic error can occur when deter­mining the position of particles that emit ellip­tically polarized light. “The ellip­tical polari­zation causes the wave­fronts of the light to have a spiral shape and to hit the imaging optics at a slight angle. This leads to the impression that the source of the light is somewhat off its actual position,” explains Yves Colombe from Rainer Blatt’s team. This could be relevant, for example, in biomedical research, where luminous proteins or nano­particles are used as markers to determine biolo­gical structures. The effect that has now been proven would possibly lead to a distorted image of the actual structures.

More than 80 years ago, the physicist Charles G. Darwin, grandson of the British natural scientist Charles Darwin, predicted this effect. Since that time, several theo­retical studies have substan­tiated his predic­tion. Now, it has been possible for the first time to clearly prove the wave effect in experi­ments, and this twice: At the University of Innsbruck, physicists determined, through single photon emission, the position of a single barium atom trapped in an ion trap. Physicists at Atom­institut of TU Vienna determined the position of a small gold sphere, about 100 nanometers in size, by analyzing its scattered light.

In both cases, there was a dif­ference between the observed and the actual position of the particle. “The devia­tion is on the order of the wave­length of the light and it can add up to a consi­derable measure­ment error in many appli­cations,” says Stefan Walser from Arno Rauschenbeutel’s team. “Super-reso­lution light micro­scopy, for example, has already pene­trated far into the nano­meter range, whereas this effect can lead to errors of several 100 nano­meters.” The scientists believe it is very likely that this funda­mental sys­tematic error will also play a role in these applications, but this has yet to be proven in separate studies.

The researchers also assume that this effect will not only be observed with light sources, but that radar or sonar measure­ments, for example, could also be affected. The effect could even play a role in future appli­cations for the position esti­mation of astro­nomical objects using their gravi­tational waves emission. (Source: Univ. Innsbruck)

Reference: G. Araneda et al.: Wavelength-scale errors in optical localization due to spin–orbit coupling of light, Nat. Phys., online 15 October 2018; DOI: 10.1038/s41567-018-0301-y

Link: Quantum Optics and Spectroscopy, Dept. of Experimental Physics, University Innsbruck, Innsbruck, Austria

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