Quantum optics: a discovery by the University of Insubria opens new frontiers for biomedical technologies

A discovery in the field of quantum optics, led by the University of Insubria, paves the way for new, more accurate and less invasive biomedical investigation techniques. The study, published in the prestigious journal Science Advances, demonstrates that two-photon processes can be enhanced by quantum light even at much higher light intensities than previously thought possible.

The result comes from an international collaboration involving, in addition to Insubria, the Universities of Strathclyde and Glasgow and the Institute of Photonics and Nanotechnology of the National Research Council (CNR) in Milan.

Two-photon processes such as two-photon absorption or second harmonic generation are fundamental in advanced biomedical applications: from non-linear microscopy to deep tissue imaging and the diagnosis of neurodegenerative diseases such as Alzheimer's. However, these techniques normally require very intense light beams, with the risk of damaging the cells and tissues being observed.

To overcome this limitation, the scientific community has hypothesised over the years the use of pairs of “entangled” photons, i.e. photons that are quantum-correlated. These particular quantum states, produced through parametric conversion processes, can increase the efficiency of two-photon interactions, reducing the intensity required. Until now, however, it was believed that this advantage was confined to conditions of very weak light, which are insufficient for practical applications.

The new study led by Lucia Caspani, professor of physics in the Department of Science and High Technology, has overturned this belief. Thanks to an innovative experimental approach, the researchers have demonstrated that the quantum advantage persists even in light conditions up to ten times more intense than those previously considered possible.

Using a particular type of quantum light called squeezed vacuum, the researchers observed that second harmonic generation, a typical two-photon process, maintains higher efficiency than classical light, even when the average number of photons per mode exceeds unity. In other words, the quantum advantage does not disappear as soon as one moves from the low-intensity regime, but persists well beyond the threshold accepted until now.

This result was verified through a direct comparison between quantum beams and equivalent classical laser beams, under the same intensity and experimental conditions: in all cases, entangled light demonstrated greater efficiency in two-photon processes.

‘We have demonstrated that quantum effects remain advantageous at light intensities that do not damage samples but are high enough to allow reliable measurements,’ explains Professor Lucia Caspani. ‘This could greatly expand the use of quantum light in applied technologies, particularly in the biomedical field.’

The implications of this discovery are significant: a new generation of quantum light-based diagnostic and medical instruments could be developed, capable of exploring tissues in depth, with sharper and more detailed images, minimising the risks of photodamage. Possible applications range from high-resolution microscopy to spectroscopy and photodynamic therapy.

The paper, entitled “Quantum-enhanced second harmonic generation beyond the photon pairs regime”, is authored by an international team comprising Thomas Dickinson, Ivi Afxenti, Giedre Astrauskaite, Lennart Hirsch, Samuel Nerenberg, Ottavia Jedrkiewicz, Daniele Faccio, Caroline Müllenbroich, Alessandra Gatti, Matteo Clerici and Lucia Caspani.

The study was made possible thanks to the support of various research bodies and international agencies, including the European Research Council, ERC, EPSRC, Innovate UK and the PRIN programme of the Ministry of University and Research.