Speaker
Description
Polarimetry is a fundamental optical characterisation technique that reveals material-specific properties such as internal structure, molecular composition, and anisotropy. Whereas classical approaches rely on intensity averages, quantum polarimetry enables phase-sensitive measurements at ultra-low light levels, an advantage for fragile or photosensitive samples. Additionally, quantum measurements deliver enhanced sensitivity and increased capacity to detect subtle responses in weakly depolarising samples, including microorganisms and biological monolayers [1]. Most quantum polarimetry schemes employ polarisation-entangled photon pairs, where one photon interacts with the sample, while its partner acts as a reference. The possibility of further enhancing the sensitivity or accessing additional polarimetry information remains an open question with significant implications for quantum imaging and metrology.
We address this problem by introducing two-photon probing quantum polarimetry, an approach in which both photons of an entangled pair interact with the sample simultaneously [2]. Whereas conventional quantum one-photon polarimetry measures the linear transformation of first-order polarisation moments, the two-photon configuration probes second-order polarisation correlations encoded in the joint quantum state. This enables fundamentally enhanced sensitivity in polarimetry due to a quadratic scaling of depolarisation effects with respect to linear scaling in single-photon probing.
We experimentally implement the proposed two-photon quantum polarimetry for tissue-mimicking phantoms with controlled scattering properties [2]. The photon pairs generated via spontaneous parametric down-conversion and prepared in a polarisation entangled state interact with the scattering samples and are characterised using full quantum state tomography. Our measurements confirm that when both photons traverse the turbid medium, the entangled state undergoes significantly stronger changes in concurrence, purity, and coherence, revealing polarisation transformations with sensitivity higher than that of traditional one-photon probing quantum polarimetry. These results establish two-photon probing as a distinct sensing modality, with prospective broader applications in quantum sensing, imaging, and the study of complex optical environments.
$\small{\mathrm{[1]~Y.D.~Zhang~\mathit{et~al},~Sci.~Adv.~\mathbf{10},~eadk1495~(2024).}}$
$\small{\mathrm{[2]~J.~Ren~\mathit{et~al},~arXiv~2604.09257~(2026).}}$
| I am the presenting author | Yes |
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