Spectroscopy – that is, the study of materials using light – plays a major role in biology, medicine, materials testing and research, and is a widely used measurement technique. Quantum spectroscopy – that is, the characterisation of materials using quantum light – promises significantly reduced measurement times and greater accuracy than previously used techniques by exploiting specific quantum properties. Nevertheless, many theoretical and practical questions remain unresolved in its application. A new study by René Pollmann and colleagues has now, for the first time, investigated the limits of a specific approach to quantum spectroscopy that utilises so-called entangled light particles, known as photons.
The use of entangled photon pairs to excite two-photon absorption processes in organic molecules promises a massive gain in efficiency; however, this has not yet been conclusively demonstrated. A research team at Paderborn University is now investigating which measurement methods are best suited to detecting entangled two-photon absorption.
For a molecule to be excited by two photons, both photons must strike the molecule simultaneously and with the correct amount of energy. As energy and time are subject to the uncertainty principle, this is only possible to a limited extent with classical light. In other words: either the photons strike the molecule simultaneously, or they possess the correct amount of energy.
However, this uncertainty can be circumvented using entangled photon pairs, which is why a massive gain in efficiency was anticipated. In practice, however, despite the improved efficiency, the two-photon absorption signal is still very, very small and can only be distinguished from background signals using highly specialised measurement methods.
By mathematically modelling the entire measurement system, including all background effects, the Paderborn researchers are now able to make quantitative statements about the detectability of entangled two-photon absorption, which will help the Paderborn team and others to further develop this methodology. The paper is freely available to everyone at: https://doi.org/10.1103/qpb1-hk5l
The research was funded by the Federal Ministry of Research, Technology and Space (BmFTR) as part of the “E²-TPA” project (number: 13N16352).
