Narrow-Band Single Photons as Carriers of Quantum Information

The use of quantum mechanical properties for information processing, the socalled quantum information processing (QIP) has become an increasingly popular research field in the last two decades. It turned out that single photons are the most reliable long distance carriers of quantum information, e.g., tools to connect different processing nodes in QIP. While several methods exist to produce single photons, only little research has been performed so far on narrow-band single photons with spectral bandwidths in the MHz regime. Such photons are, however, of particular importance when coupling of single photons to atomic systems, which are often used in QIP as processing nodes, shall be realized. This thesis covers several research aspects on narrow-band single photons, all of which are important if such photons should be used as quantum information carriers. At first, a source for narrow-band single photons is introduced. This source is based on the concept of parametric down-conversion inside an optical resonator. It emits a constant stream of photon pairs. One of the two photons from the pair can be detected heralding the presence of the other photon. A statistical description of these photon pairs is introduced and for the first time also directly measured. A strong non-classical correlation of the two photons in each pair is shown and single-photon character is proven. In order to reach single-mode single-photon emission, the stream of photons was filtered with a specifically developed multi-pass Fabry-Pérot etalon. This filter has a passband FWHM of only 165 MHz and 65% transmission and a suppression of offresonant light by 46 dB. It can be widely used, e.g., to extract quantum information carrying photons out of a background of noise photons. A potential use in the field of space-based long range quantum cryptography experiments is motivated. Photon-atom interactions are shown in the second part of the thesis. The effect of electromagnetically induced transparency (EIT) is introduced and experimentally demonstrated. The first EIT experiments in cesium gas cells at room temperature with a probe pulse containing only a single photon are described. Also, singlephoton pulses are amplitude-modulated. Finally, a comprehensive outlook shows how the developed experimental building blocks can be extended in order to show single photon storage and demonstrate quantum repeater technology.