Rubidium Rydberg Nonlinear Quantum Optics

The Rubidium Quantum Optics-project (RQO) explores how the strong interactions between atomic Rydberg states in an ultracold gas of rubidium can be used for quantum optics. The simple level structure of the alkalis and the established techniques for cooling and trapping Rb make this element a natural choice for exploring Rydberg EIT and nonlinear quantum optics.

For our experiments we usually cool the a dilute gas of Rubidium to a few micro-Kelvin in first a magneto-optical trap and further in one or more optical dipole traps.

We probe the cold gas with a very weak probe-beam at 780 nm and employ a classical control field at 480 to couple to Rydberg states. The Rydberg-Rydberg-interactions between the atoms and the resulting excitation blockades are mapped onto the probe field. This allows us to create strong optical nonlinearities at single photon level for the probe light.

The individual probe photons are detected on in a Hanbury-Brown-Twiss setup of single photon counter modules. This allows us to investigate photon-photon-correlations. In addition we have the option to field-ionize the Rydberg-atoms and detect the created ions. The detection-time of the ions gives us information of the spatial distribution of Rydberg excitations in the rubidium gas. We also utilize the ion-statistics to determine the number of Rydberg excitations in the system.

The first demonstration of quantum optics with our rubidium experiment was done in 2013, and was followed by the demonstration of a single-photon transistor in 2014. The transistor was implemented by using the Rydberg excitation blockade between states of different principal quantum numbers.

The superatom-project is a collective project with awesome theory collaborators at University of Stuttgart.

Check out their webpage ➡ https://www.itp3.uni-stuttgart.de/

Since 2016 the RQO experiment has mainly been focusing on so-called Rydberg superatoms: Atomic ensembles collectively excited to Rydberg states. The collective nature of the Rydberg excitation causes a very strong coupling to the excitation field, and a high probability of reemission into the probe mode of the system. This has has allowed us to explore the coupling between single photons and quantum emitters.

Our Rydberg atoms host only a single excitation at a time, and they can be described as effective two-level systems where the ground state is a state without any Rydberg excitations, and the excited state is a superposition state with a single Rydberg excitation. In addition, a manifold of dark states which do not couple to the light but which contains a single Rydberg excitation have to be taken into account. These states can conveniently be exploited for technical applications of the Rydberg superatoms.

With the superatoms we have investigated two- and three-photon correlations, investigated the excitation decay which reveals the internal dipole-dipole dynamics, and demonstrated deterministic subtraction of single or few photons from an input pulse.