Description
Molecular absorption spectroscopy is one of the most widely used tools for identifying and characterising matter. In a typical experiment, light is sent through a sample and the transmitted signal reveals information about the molecules present. Usually this light is well described by a classical laser field, or coherent state, which provides the standard benchmark for performance.
In this work, I will discuss how tools from quantum information can be used to ask a simple question: how can nonclassical light improve our ability to distinguish different molecular samples? We formulate molecular absorption spectroscopy as a problem of distinguishing between different optical absorptions, which we model as quantum channels, produced by different samples. This allows us to quantify how well different probing strategies perform, from standard coherent-state illumination to more general quantum states of light.
A central tool in this work is measured relative entropy, which measures how distinguishable two physical processes are after an optimal measurement. While many optimisation tasks in quantum information are exponential, we show how this quantity can be computed using semidefinite programming, making it possible to turn an abstract distinguishability measure into a practical numerical method. We then compare this approach with other channel distinguishability measures, which correspond to different levels of experimental capability, and apply the framework to bosonic dephasing models.
Finally, I will describe our recent extension to molecular spectroscopy, including multimode optical probes and more realistic situations where the goal is to distinguish between families of possible samples rather than two perfectly specified alternatives. By benchmarking against coherent states, we identify when quantum probes can offer an advantage over the classical-laser baseline. More broadly, this work connects quantum hypothesis testing, numerical optimisation, and near-term quantum sensing in a setting directly motivated by spectroscopy.
[1] https://arxiv.org/abs/2406.19060
[2] https://arxiv.org/abs/2603.19911
[3] Manuscript in preparation
| I am the presenting author | Yes |
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