Design of a Narrowband Single Photon Source

Abstract

Photons are the ideal carriers for quantum information: they travel at the speed of light, enabling the fastest communication possible, their interaction with the environment is weak, resulting in low decoherence, and they allow simple encoding of quantum information in multiple degrees of freedom, e.g. in polarisation or frequency. The development of optical fibres with losses ${<}0.15~$dB/km at telecom wavelengths made communication distances ${\sim }100~$km and most recently up to 300 km possible, fundamentally limited by chromatic and modal dispersion, scattering and absorption. This is insufficient for the implementation of large global networks and therefore the original information needs to be restored on a regular basis via so-called quantum repeaters. The repeater can either be a series of high fidelity gates (${>}99\%$) in an all optical approach, or the information is stored locally in a quantum memory, generally in some type of atomic transitions, purified, and then a new photon is emitted, carrying the initial information. To build such a network of quantum nodes, we have to achieve efficient interaction between atoms and single photons. This is not a trivial task because photons usually have bandwidths 5–6 orders of magnitude larger than the transitions they are aiming at. Cavity-enhanced SPDC can solve this problem while maintaining high rates of photon pair creation as discussed in Section. The upcoming sections will present the design considerations of the optical and the electronic control system in order to build a narrowband single photon source suitable for quantum memories based on the rubidium (Rb) D$_1$ transition at 795 nm.