Hong-Ou-Mandel effect harnessed for quantum microscopy
A quantum imaging breakthrough could lead to advanced forms of microscopy for use in medical research and diagnostics, claim physicists in Scotland.
A team from Glasgow University and Heriot-Watt University said they have found a new way to create detailed microscopic images under conditions that cause conventional optical microscopes to fail.
In a new paper published in Nature Photonics, the team describe how they have generated images by finding a new way to harness a quantum phenomenon called the Hong-Ou-Mandel (HOM) interference.
According to Glasgow University, HOM interference occurs when quantum-entangled photons are passed through a beam splitter – a glass prism that can turn a single beam of light into two separate beams as it passes through. Inside the prism, the photons can be reflected internally or transmitted outwards.
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When the photons are identical, they will always exit the splitter in the same direction, a process known as ‘bunching’. When the entangled photons are measured using photodetectors at the end of the path of the split beam of light, a characteristic ‘dip’ in the output probability graph of the light shows that the bunched photons are reaching only one detector and not the other.
That dip is the Hong-Ou-Mandel effect, which demonstrates the perfect entanglement of two photons. It has been put to use in applications like logic gates in quantum computers, which require perfect entanglement in order to work.
It has also been used in quantum sensing by putting a transparent surface between one output of the beam splitter and the photodetector, introducing a very slight delay into the time it takes for photons to be detected. Analysis of the delay can help reconstruct details like the thickness of surfaces.
The Glasgow-led team has now applied it to microscopy, using single-photon sensitive cameras to measure the bunched and anti-bunched photons and resolve microscopic images of surfaces.
In their paper, they show how they have used their setup to create high-resolution images of some clear acrylic sprayed onto a microscope slide with an average depth of 13 microns and a set of letters spelling ‘UofG’ etched onto a piece of glass at around eight microns deep.
Their results demonstrate that it is possible to create detailed, low-noise images of surfaces with a resolution of between one and 10 microns, producing results close to that of a conventional microscope.
In a statement, lead author Professor Daniele Faccio, from Glasgow University’s School of Physics and Astronomy, said: “Conventional microscopy using visible light has taught us a vast amount about the natural world and helped us make an incredible array of technological advances.
“However, it does have some limitations which can be overcome by using quantum light to probe the microscopic realm. In bioimaging, where cells can be almost entirely transparent, being able to examine their fine details without using conventional light could be a major advantage – we chose to image transparent surfaces in this research precisely to demonstrate that potential.
“Similarly, samples in conventional microscopes need to be kept perfectly still – introducing even a small vibration could introduce a level of blur which would ruin an image. However, HOM interference requires only measuring photon correlations and there is much less need for stability.
“Now that we’ve established that it’s possible to build this kind of quantum microscopy by harnessing the Hong-Ou-Mandel effect, we’re keen to improve the technique to make it possible to resolve nanoscale images. It will require some clever engineering to achieve, but the prospect of being able to clearly see extremely small features like cell membranes or even strands of DNA is an exciting one. We’re looking forward to continuing to refine our design.”
The research was supported with funding from EPSRC, the European Union’s Horizon 2020 programme, RAEng, and the Marie Sklodowska-Curie grant programme.
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