Quantum Networks Based on Single Photons

Quantum networks rely on the transfer of quantum information between stationary quantum nodes. Physically the nodes are connected by single photons. In a first part of this chapter we address different components of a quantum network. We start with a discussion of semiconductor photon sources with an emission wavelength near 900 nm. In order to make them suitable for fiber-networks a conversion to the telecom band is required. We describe how such converters can be realized with the help of nonlinear optics. Next we address photon storage devices as crucial components of quantum repeaters, which are necessary to establish quantum key distribution (QKD) over long distances. We concentrate on the approach of room-temperature gas cells filled with alkali atoms and outline first promising result. In a second part we address a special QKD protocol, the so-called time-frequency (TF) protocol. It can mostly be realized with off-the-shelf components and its encoding of quantum bits in frequency and time suggests a straightforward way to utilize multiplexing. We analyze the TF-protocol numerically before we report on an actual free-space link over 100 m as a testbed for a quantum network in a realistic environment.