Hong-Ou-Mandel effect harnessed for quantum microscopy

A breakthrough in quantum imaging could lead to advanced forms of microscopy for use in medical research and diagnostics, physicists in Scotland say.

Detailed microscopic view of UofG ticks (Image: Professor Daniele Faccio)

A team from Glasgow University and Heriot-Watt University said they have found a new way to capture detailed microscopic images under conditions that cause conventional optical microscopes to fail.

In a new article published in Nature photonicsThe team describes how they generated images by finding a new way to take advantage of a quantum phenomenon called the Hong-Ou-Mandel (HOM) interference.

According to the University of Glasgow, 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 it. Inside the prism, the photons can be reflected inward or emitted outward.


If the photons are identical, they will always exit the splitter in the same direction, a process known as ‘bundling’. When the entangled photons are measured using photodetectors at the end of the split light beam’s path, a characteristic ‘dip’ in the light output probability plot shows that the bundled photons reach 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 used in applications such as logic gates in quantum computers, which require perfect entanglement in order to work.

It has also been used in quantum detection by placing a transparent surface between an output of the beam splitter and the photo detector, introducing a very small delay in the time it takes to detect photons. Delay analysis can help reconstruct details such as surface thickness.

The Glasgow-led team has now applied it to microscopy, using single-photon-sensitive cameras to measure the bundled and anti-bundled photons and resolve microscopic images of surfaces.

in their paper, show how they used their setup to create high-resolution images of clear acrylic sprayed onto a microscope slide with an average depth of 13 microns and a series of letters spelling “UofG” etched onto a piece of glass around eight o’clock micrometers deep.

Their results show that it is possible to produce detailed, low-noise images of surfaces with a resolution between one and 10 microns, with results close to those of a conventional microscope.

In a statement, lead author Professor Daniele Faccio of Glasgow University’s School of Physics and Astronomy said: “Conventional visible light microscopy has taught us an enormous amount about the natural world and has helped us make an incredible array of technological advances.

“However, it has some limitations that can be overcome by using quantum light to investigate the microscopic realm. In bioimaging, where cells can be almost completely transparent, it could be a great advantage to examine their fine details without conventional light.” – we chose to depict transparent surfaces in this study precisely to demonstrate that potential.

“In the same way, samples in conventional microscopes must be kept perfectly still – introducing even a small vibration can cause a degree of blur that would ruin an image. However, HOM interference only requires measuring photon correlations and there is much less need for it.” stability.

“Now that we have established that it is possible to build this kind of quantum microscopy using the Hong-Ou-Mandel effect, we want to improve the technique to make it possible to solve nanoscale images. It requires some smart engineering to achieve this, but the prospect of being able to clearly see extremely small features such as cell membranes or even DNA strands is exciting, and we look forward to further fine-tuning our design.”

The research was supported with funding from EPSRC, the European Union’s Horizon 2020 program, RAEng and the Marie Sklodowska-Curie grant program.

Abhishek Maheswari
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