Quantum Optics with Hot Atoms

This project aims at a deterministic single photon source that is reproducible and scalable. Our atomic source directly offers intrinsic center frequency and bandwidth stability and can be scaled up to large arrays at minimal technical overhead. The emission of single photons is enabled by strong Rydberg-Rydberg interactions that realize strong correlations in the atomic medium and block multiple excitations at a time.

So far, we could demonstrate that it is possible to excited thermal atoms in a micrometer-sized vapor cell (µ-cell) to Rydberg states with long coherence times and sufficiently long population storage. We were also able to observe coherent dynamics on nanosecond timescales and could then switch on strong Rydberg-Rydberg interactions. Within a four-wave mixing scheme we could retrieve single photons with anti-bunched light statistics from a thermal atomic volume of the size of one cubic micrometer.

The current goal of this project on the one hand is to turn this emitter into a high-performance device based on leading-edge laser technologies at high repetition rates as well as to integrate the high-NA optics into the µ-cell by means of solid-immersion lenses. On the other hand, relevant physical processes such as Rydberg-Rydberg and Rydberg-wall interaction have to be investigated more in depth. Finally, we aim with this device at applications in the field of quantum information science.

This goal of this project is to develop a highly efficient source of heralded single photons with a high spectral brightness, based on four-wave mixing in an alkali-vapor contained in a hollow core optical fiber. Hollow-core optical fibers filled with atomic media are known to boost a variety of non-linear optical phenomena including four-wave mixing. This should allow lower pump laser powers to be used which in turn means simple diode lasers could be used to generate the photon pairs. Also, using a thermal vapor with fiber optics should allow for an easily integrated device for many applications requiring heralded single photons.

We use hollow-core fibers encased by alkali-metal-filled glass vapor cells. This setup allows the fibers to be filled simply by diffusion. In this way we can achieve desired optical depths without the use of extra desorption lasers.

A major challenge in this project is to split the photon pairs from each other. Since the photon pairs have the same polarization and spatial mode, and also have very similar frequencies, their separation is difficult using conventional techniques. However, we have developed an atomic Faraday beam splitter effective for this purpose. By using computational optimisation to efficiency guide experimental efforts, we have created an atomic Faraday dichroic beam splitter of unprecedented performance in spectral selectivity.