Conventional optical microscopes, which use light as their source of illumination, have hit a barrier, known as the Rayleigh limit. Set by the laws of physics, this is the point at which the diffraction of light blurs the resolution of the image. Equal to around 250 nanometres – set by half the wavelength of a photon – the Rayleigh limit means that anything smaller than this cannot be seen directly.
The EU-funded SUPERTWIN project’s goal was to create a new generation of microscopes capable of resolving imaging below this limit by making use of quantum physics. The technology resulting from this FET Open research project could one day be used to view the tiniest of samples – including many viruses – directly and in detail.
Although direct outcomes will not be measurable for some time, the SUPERTWIN team expect that refinement of their platform will result in novel tools for imaging and microscopy, delivering new scientific findings with a huge societal impact in fields such as biology and medicine.
‘The SUPERTWIN project achieved a first proof of imaging beyond classical limits, thanks to three key innovations,’ says project coordinator Matteo Perenzoni of the Bruno Kessler Foundation in Italy.
‘First, there is the deep understanding of the underlying quantum optics through novel theory and experiments; secondly, advanced laser fabrication technology is mixed with a clever design; and thirdly, there is the specifically tailored architecture of the single-photon detectors.’
Under specific conditions, it is possible to generate particles of light – photons – that become one and the same thing, even if they are in different places. This strange, quantum effect is known as entanglement.
Entangled photons carry more information than single photons, and SUPERTWIN researchers capitalised on that ‘extra’ information-carrying capacity to go beyond the classical limits of optical microscopes.
In the new prototype, the sample to be viewed is illuminated by a stream of entangled photons. The information these photons carry about the sample is extracted mathematically and automatically pieced back together, like a jigsaw puzzle. The final image resolution can be as low as 41 nanometres – five times beyond the Rayleigh limit.
To achieve their ultimate aim, the project team had to make several breakthroughs, including the creation of a solid-state emitter of entangled photons which is able to generate intense and ultrashort pulses of light.
The researchers also developed a high-resolution quantum image sensor capable of detecting entangled photons. The third key breakthrough was a data-processing algorithm that took information about the location of entangled photons to generate the image.
One of the project’s greatest challenges – yet to be completely solved – was in determining the type and degree of entanglement. By carrying out additional experiments, the team created a new theoretical framework to explain the atom-scale dynamics of generating entangled photons.
Looking to the future
‘Several follow-ups to the SUPERTWIN project are under way,’ says Perenzoni. ‘The solid-state source of non-classical light and super-resolution microscope demonstrators will be used in the ongoing PHOG project, and they are also expected to pave the way to a future project proposal.
‘The potential of our quantum image sensor is currently being explored in the GAMMACAM project, which aims to develop a camera exploiting its capability to film individual photons.’
The FET Open programme supports early-stage science and technology researchers in fostering novel ides and exploring radically new future technologies.