Adaptive optics (AO) was developed to solve a stubborn problem facing astronomers: The same atmospheric aberrations that cause stars to twinkle lead to blurry images as the path of light from those stars to a telescope is distorted. To correct for aberrations from both the atmosphere and the telescope's optics, observers monitor a guide star, either a real one or an artificial one created in the upper atmosphere by a laser, to measure how the final image is being distorted. Then they adjust their optical equipment in a way that counteracts the distortion, ideally resulting in a point-like star and in-focus astronomical images.
AO systems are also used in optical microscopy, where the lenses and the complex structure of specimens can induce significant aberrations. Without a sky full of stars as candidate guide stars for homing in on, scientists must identify a point-like part of their specimen to use. That is difficult to do, especially in label-free microscopy, in which the sample needs to be illuminated because it lacks fluorescent probes. Additionally, the correction blueprint for the optics needs to be adjusted any time the specimen or the arrangement of lenses changes.
Now researchers at the University of Glasgow and the Paris Institute of Nanosciences at Sorbonne University have developed a novel AO method to correct label-free microscopy images. Rather than adding to the optics, they take advantage of the photons that illuminate the sample. They use entangled photons, created by shining a laser through a nonlinear crystal. That allows photons reaching the camera to be used first to derive the aberration corrections and then to obtain a corrected image of the specimen. In each case, the photons captured by the camera are subject to the same aberrations.
The new technique relies on each of the photons in a correlated pair mirroring the other’s movement to conserve angular momentum. Any deviation from that mirrored position serves as a signature of the aberrations. The researchers calculate the deviations from many photon pairs to create a map of the distortions. A spatial light modulator then implements an iterative program to undo the effect of the distortions. The researchers create an accurate correction mask by transforming a blurry image mapping the deviations of the paired photons into a point-like correlation map so that the correlation map closely matches how it would’ve looked in the absence of any aberrations. By applying the same optical correction to the sample image, the specimen is refocused as well.
Currently, the quantum AO system is limited in applicability by the type of microscopy system used and the hours needed to calculate the correction, especially if there are multiple sources of aberrations. But according to the Glasgow and Paris teams, the advantages outweigh the limits. Future advancements in technology are expected to reduce the correction times to minutes. (P. Cameron et al., Science 383, 1142, 2024.)
