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When it comes to quantum technologies, computing has dominated headlines around the world. Computers that exploit the laws of quantum mechanics are significantly faster for several classes of problem than even the most powerful supercomputers.
But behind the scenes other quantum technologies are emerging with the potential to revolutionize other areas of science. One of these technologies is quantum imaging, using the quantum properties of photons to enhance images.
Now Lihong Wang and colleagues at the California Institute of Technology in Pasadena have unveiled a quantum microscope that produces images with twice the resolution of conventional microscopes.
Conventional imaging systems are limited in various ways by the properties of light. For example, the resolution of any image is limited by the wavelength of light and the size of the lens aperture collecting the light. At the edge of a lens, the aperture diffracts or bends the light causing it to interfere and produce light and dark rings around any detail in the image. This ultimately limits what can and cannot be resolved.
Over the years, physicists have discovered various ways to achieve “super-resolution”, such as by making lenses out of exotic substances called metamaterials, although this works only at short distances from the subject.
Quantum mechanics offers another solution with entangled photons. These are particles of light that are separated in space but share the same existence. Wang and co say that these photons travel along symmetric paths of the same length and then recombine. When this happens “they behave like a single photon with half the wavelength, leading to a 2-fold improvement in resolution.”
This allows any stray photons at the original wavelength to be easily ignored, which also improves the image.
The trick, of course, is to construct an optical set up with exactly these properties, which is what Wang and co have achieved. They call their technique “quantum microscopy by coincidence”.
Their quantum microscope generates pairs of entangled photons and sends them down separate paths. It turns out that only one of the photons need have contact with the object to be imaged. So one path passes through the object while the other passes through a reference plane.
The photons then recombine at the detection plane where they are picked up by a sensitive photo detector. The process is repeated thousands of times to build up an image, which currently takes about 15 minutes per image.
Carbon fibres imaged with classical microscopy and quantum microscopy by coincidence (Source: arxiv.org/abs/2303.04948)
Nevertheless, the results are impressive. Wang and co have used the technique to image of test targets, carbon fibers and even cancer cells. “With the low-intensity illumination, we have demonstrated that quantum microscopy by coincidence is suitable for nondestructive bioimaging at a cellular level, revealing details that cannot be resolved by its classical counterpart,” they say
Further improvements are theoretically possible. Quantum physicists predict that by entangling N photons, it should be possible to improve the resolution by a factor of N. That will take a little longer to achieve.
Ref: Quantum Microscopy of Cancer Cells at the Heisenberg Limit: arxiv.org/abs/2303.04948