Entanglement between atomic systems and light is a promising approach for a quantum network of atomic quantum memories and photonic communication channels. In our project we work with single, optically trapped Rubidium atoms whose spin is entangled with the polarization of single photons. Two photons independently emitted from distant atoms are combined to create heralded entanglement between the atoms. These entangled atom pairs, together with fast and efficient state read-out are well suited for experiments on the foundations of quantum mechanics, e.g., for a test of Bell's inequality.
Quantum key distribution (QKD) allows two parties to exchange a secure key for cryptography using the quantum mechanical properties of light. We have achieved QKD over the record distance of 144 km, which is representative for a link to a low orbit satellite. In this context we also demonstrated that a key exchange with a fast moving device is possible by establishing a QKD link to an aircraft at a distance of 20 km to the ground station. Our current research focuses on the implementation of a compact transmitter suited for handheld user devices, where we plan to combine light from a laser diode array with laser written waveguides on a glass chip.
The development of reliable devices to generate single photons is crucial for future applications in applied physical and quantum information science, as well as for fundamental quantum optics experiments. In this context our group focuses on the efficient evanescent coupling of single photons radiated by a single defect center in diamond to photonic waveguide structures, e.g., tapered optical fibers and dielectric slot waveguides on a chip.