To make the finest structures visible, researchers use special microscopes that use entangled photon pairs as a light source. However, these systems are very sensitive to optical distortions that blur the image. Previous methods for eliminating these imaging errors were not applicable, especially for many biological samples. A study now shows a new method in which the relationship between the entangled photons themselves is used to determine and calculate the distortion. This could mean an important improvement for future quantum microscopes.
When particles are entangled with each other at the quantum level, it means that they are brought into a common state that connects them over arbitrarily large distances. Entangled pairs of light particles, i.e. photons, have numerous applications in imaging. Among other things, they are used in special forms of microscopy. The entangled photons are generated by guiding a laser beam through a nonlinear crystal. When they hit a sample, they are reflected in a specific way, which in turn is recorded by a camera that reacts to individual photons.
Calculate imaging errors
The problem: “All of these methods are very sensitive to optical imaging errors caused by the samples to be imaged or by the imaging system itself,” explains a team led by Patrick Cameron from the University of Glasgow in Great Britain. “If these effects remain uncorrected, the benefits of these techniques will be negated and their practical use will be compromised.”
One way to correct the distortions is to insert a specific marker into each sample – a so-called “guide star”. Since the shape and size of this marking is known, it is possible to calculate exactly how large the aberration is from its distortion in the image. This way the error can be eliminated and a sharp image is created. “Without such a guide star, information about the aberration is not directly accessible,” write Cameron and his colleagues.
Accurate images without a guide star
However, especially in biological samples, it is often not possible to apply appropriate markings without changing the sample. Additionally, the technique requires its own adjustments for each type of microscope and each type of sample. This limits their effectiveness. Cameron and his team therefore set out to find an alternative that does not require a guide star and can be applied to as many applications as possible.
And they actually found a way: “We have developed a quantum-assisted adaptive optics (QAO) method that uses the entanglements between pairs of photons to directly infer the aberration without the need for a guide star,” reports the team. “All information about the distortions at each individual point is encoded in the spatial relationship of the photon pairs. The approach is also independent of the imaging modality and the sample examined.”
To test their method, they examined, among other things, the mouthparts of a bee under the microscope, sometimes using conventional adaptive optics methods and sometimes adding their quantum-based calculations. This showed that QAO delivered significantly sharper images and also avoided errors that other approaches to image correction built into the results. “QAO thus has the potential to optimize the operation of any photon pair-based imaging system,” write Cameron and his colleagues. “It could therefore play an important role in the development of future quantum optical microscopes.”
Source: Patrick Cameron (University of Glasgow, UK) et al., Science, doi: 10.1126/science.adk7825