The measurement of correlations between different degrees of freedom is an important, but in general extremely difficult task in many applications of quantum mechanics. Here, we report an all-optical experimental detection and quantification of quantum correlations between the polarization and the frequency degrees of freedom of single photons by means of local operations acting only on the polarization degree of freedom. These operations only require experimental control over an easily accessible two-dimensional subsystem, despite handling strongly mixed quantum states comprised of a continuum of orthogonal frequency states. Our experiment thus represents a photonic realization of a scheme for the local detection of quantum correlations in a truly infinite-dimensional continuous-variable system, which excludes an efficient finite-dimensional truncation.
@article{arxiv.1311.5034,
title = {Experimental detection of polarization-frequency quantum correlations in a photonic quantum channel by local operations},
author = {Jian-Shun Tang and Yi-Tao Wang and Geng Chen and Yang Zou and Chuan-Feng Li and Guang-Can Guo and Ying Yu and Mi-Feng Li and Guo-Wei Zha and Hai-Qiao Ni and Zhi-Chuan Niu and Manuel Gessner and Heinz-Peter Breuer},
journal= {arXiv preprint arXiv:1311.5034},
year = {2018}
}