Abstract
In an experiment reported recently (Mosley et al 2008 Phys. Rev. Lett. 100 133601), we demonstrated that, through group velocity matched parametric downconversion, heralded single photons can be generated in pure quantum states without spectral filtering. The technique relies on factorable photon pair production, initially developed theoretically in the strict collinear regime; focusing—required in any experimental implementation—can ruin this factorability. Here, we present the numerical model used to design our single photon sources and minimize spectral correlations in the light of such experimental considerations. Furthermore, we show that the results of our model are in good agreement with measurements made on the photon pairs and give a detailed description of the exact requirements for constructing this type of source.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Photon-pair sources have been used extensively to perform experiments thought by the pioneers of quantum mechanics to be possible only in the realm of the imagination. This has allowed not only tests of the fundamental tenets of quantum physics but also the exploration of the practical implications of these laws in fields such as precision measurement and information processing. At the forefront of this research are spontaneous parametric downconversion (PDC) photon sources in which photon pairs are created when a laser pulse propagates through a nonlinear crystal.
Main results. In this paper we describe our method of spectral engineering of a PDC photon-pair source to generate heralded single photons directly in pure quantum states, capable of high-visibility interference. We show that by choosing a nonlinear material with the correct chromatic dispersion and pumping it with ultrashort laser pulses one can control the spectral structure of the daughter photons. This paper provides a pedagogical introduction to this technique with details of how to model and build such a single photon source, along with details of our experimental results.
Wider implications. The extremely high purity of the heralded single photons demonstrated in this paper was achieved without any of the narrowband spectral filters that typically limit the performance of this type of experiment. Our technique is therefore ideally suited to building state-of-the-art single photon sources as well as extending the reach of experiments into the higher-photon-number regime.