In Vivo Quantum Time-Domain Diffuse Optical Spectroscopy

Light is a remarkable tool for looking inside the human body without making a single cut. When doctors shine a beam of near-infrared light through a finger or a breast, that light does not travel in a straight line. Instead, it bounces off cells and molecules, scattering in a chaotic, random walk before it emerges on the other side. By measuring exactly how long it takes for those scattered photons to arrive, scientists can calculate the thickness of the tissue and how much it absorbs, revealing details about blood flow and oxygen levels. This technique, known as time-domain diffuse optical spectroscopy, has long relied on firing incredibly fast, ultra-short pulses of laser light to act as a stopwatch. The timing of the pulse start and the photon arrival tells the story. However, these lasers are complex, expensive, and can be too bright for delicate biological samples, potentially causing damage if used for too long.

A team of researchers at Politecnico di Milano and the University of Glasgow has now demonstrated a fundamentally different way to run this experiment. They replaced the powerful, pulsed laser with a continuous, steady beam of light and swapped the traditional stopwatch for a trick of quantum physics. Instead of timing a single pulse, they used pairs of light particles that are born together and remain mysteriously linked, no matter how far apart they travel. One particle of the pair is sent through the tissue, while its twin is caught by a detector to signal the exact moment the journey began. This method, which the researchers call quantum diffuse optical spectroscopy, allows them to map the path of light through tissue with extreme precision, even when using very dim light and simple, noisy detectors.

The researchers began by testing their new system on blocks of material designed to mimic the way human tissue scatters light. These blocks, made of a silicone-like substance mixed with white powder to scatter light and black ink to absorb it, were cut into slabs of varying thicknesses. They measured how the light traveled through these blocks using their quantum setup and compared the results to a standard, high-end pulsed laser system. The quantum method produced the same detailed maps of light travel times as the conventional laser, accurately calculating how much the material absorbed and scattered the light. This proved that the timing correlations between the linked photon pairs could serve as a perfect substitute for a pulsed laser, providing the necessary reference to reconstruct the journey of light through a scattering medium.

To see if this approach could handle the messy reality of the real world, the team introduced a significant amount of background noise. They swapped their high-end detector for a much cheaper, noisier silicon photomultiplier and shone a bright, uncorrelated light into the room to simulate a noisy environment. In a traditional setup, this background light would drown out the faint signal from the tissue. Yet, because the quantum system only counted photons that arrived in perfect sync with their twin partners, it successfully filtered out the noise. The system could still distinguish the true signal from the chaos, retrieving accurate measurements of the tissue's properties even when the background noise was thousands of times stronger than the signal. This ability to find a needle in a haystack without needing a perfect detector suggests the technology could eventually be made affordable and portable.

The researchers then moved from plastic blocks to biological fluids. They created mixtures of animal blood and a milky scattering liquid to simulate the complex environment inside a human body. By changing the amount of blood in the mixture, they showed the system could detect changes in how much light was absorbed. More importantly, they demonstrated sensitivity to oxygen. As they removed oxygen from the blood, the system detected a shift in absorption, tracking the change as the blood became deoxygenated. This is crucial because the ability to distinguish between oxygen-rich and oxygen-poor blood is the foundation of functional brain imaging and tumor detection. The quantum system tracked these changes clearly, even when the measurements were taken in just one second, proving it could capture rapid biological shifts.

Finally, the team took the experiment to living humans. They placed a blood-pressure cuff on a volunteer's arm and measured the light passing through the little finger. As the cuff was inflated to stop blood flow, the researchers watched the absorption of light change in real time. The data showed a steady increase in absorption as the blood in the finger became deoxygenated during the blockage, followed by a rapid decrease once the cuff was released and fresh blood flowed back in. This dynamic tracking of blood flow and oxygenation in a living person confirmed that the quantum method works on real, moving tissue. It is the first time such measurements have been performed on humans using light generated by a continuous source and linked photon pairs.

This work does not claim to be a finished medical tool ready for hospitals tomorrow. The measurements were proof-of-concept, showing that the physics works and that the data is reliable. The researchers note that their system is not necessarily more sensitive than the best existing lasers, but it offers a completely new path forward. By using continuous light and quantum correlations, they have shown that time-domain spectroscopy can be done without the need for expensive, complex pulsed lasers. This opens the door to using simpler, cheaper, and safer light sources for medical imaging, potentially allowing for long-term monitoring of patients or delicate tissues that cannot withstand the intensity of traditional lasers. The study establishes that quantum light is a viable resource for seeing inside the human body, turning a theoretical curiosity into a practical new way to measure life.