Quantum light unlocks nature’s little secrets

University of Michigan researchers have found a way to probe tiny structures like bacteria and genes with less damage compared to traditional light sources.

The new technique involves spectroscopy, which studies how matter absorbs and emits light and other radiation, and exploits quantum mechanics to study the structure and dynamics of molecules in ways not possible using conventional light sources.

„This research examined a quantum light spectroscopy technique called engineered two-photon absorption. The study’s senior author Theodore Goodson, UM professor of chemistry and macromolecular science and engineering, said.

Entangled two-photon absorption allows researchers to study molecules using two photons linked together through a quantum phenomenon called entanglement.

Photons are tiny particles of electromagnetic energy, so tiny particles of light allow us to see details of molecular structure—things ordinary light can’t show. Quantum light spectroscopy is extremely fast and can reveal properties that are normally hidden.

This discovery opens up opportunities for non-invasive, low-intensity imaging and sensing applications with minimal photolysis for delicate biological samples such as proteins, DNA and cells.

„Measurements with entangled photons can provide biological signatures with high selectivity and protection against photosynthesis at very low light levels.” said lead author Oleg Varnovsky, a research laboratory specialist in the UM Department of Chemistry.

The research, published in the Proceedings of the National Academy of Sciences, used the organic molecule zinc tetraphenyl porphyrin to study the two-photon absorption phenomenon—where one molecule absorbs two particles of light simultaneously.

Using quantum entangled pairs of photons, the researchers found that the ZnTPP molecule exhibited absorption in the red spectrum. With two unentangled photons, ZnTPP molecules showed absorption in the blue spectrum.

A laser creates pairs of entangled photons through a process called arbitrary parametric down-conversion. These photons were focused onto a cuvette containing ZnTPP solution. Transmittance was measured using a highly sensitive single-photon detector.

This work paves the way for advances in quantum light-based spectroscopy and microscopy, leading to higher performance ETPA sensors and low-intensity detection schemes. The ability to access discrete molecular positions with complex photons may improve the sensing of biological signatures with significant selectivity and sensitivity even at minimal light levels to inhibit photosynthesis.

„This offers the opportunity to study the states of molecules with non-classical light, which has fundamentally different properties than what is accessible with classical light.” Varnavski said.

Contributing authors are Sajal Kumar Giri, Tse-Min Chiang, Charles Zeman IV and George Schatz of Northwestern University. This research was supported by grants from the US Air Force Office of Scientific Research, the National Science Foundation, and the US Department of Energy.