We propose and demonstrate a method for measuring the spectral density matrix of a single photon pulse. The method is based on registering Hong-Ou-Mandel interference between photon to be measured and a pair of attenuated and suitably delayed laser pulses described by a known spectral amplitude. The density matrix is retrieved from a two-dimensional interferogram of coincidence counts. The method has been implemented for a type-I downconversion source, pumped by ultrashort laser pulses. The experimental results agree well with a theoretical model which takes into account the temporal as well as spatial effects in the source.
@article{arxiv.quant-ph/0702200,
title = {Spectral density matrix of a single photon measured},
author = {Wojciech Wasilewski and Piotr Kolenderski and Robert Frankowski},
journal= {arXiv preprint arXiv:quant-ph/0702200},
year = {2009}
}