Bachelor- and Master-projects

In the Ytterbium Experiment, we aim to study the nonlinear quantum optical effects via Rydberg EIT at the single photon level. Ytterbium is a particularly promising element for this work thanks to its narrow-linewidth (6s2)1S0 to (6s6p)3P1 transition that leads to lower Doppler temperatures.

After loading the laser-cooled Ytterbium atoms into a dipole trap, we realize the Rydberg EIT scheme by two-photon excitation with a weak probe (wavelength 399nm) and a strong control beam (wavelength 395nm). With Ytterbium, the similarity of the probe/control wavelengths has the advantage of a very low momentum transfer to the polariton that is created, hence longer coherence times.

In this project you will work on generating Rydberg polaritons in a gas of ultracold Ytterbium atoms and on characterizing the effect of the Rydberg blockade depending on experimental parameters.

In our new Ytterbium lab, we want to create a scalable array of so-called Rydberg superatoms - ultracold atomic clouds of the size similar to the Rydberg blockade radius acting as an effective two-level system with large coupling strengths for single photons.

Placing the superatoms around an optical nanofiber with sub-wavelength diameter exhibiting an extended evanescent field will allow strong, homogeneous coupling and scaling up the number as the system will not be limited by diffraction. We will position the atoms next to the nanofiber in a few clouds of ~10000 atoms.

Now the goal is to develop and characterize an imaging system for the atoms in the optical tweezers. The constraint is, that the imaging system is under 45 degrees due to constraints of the other optics around the chamber. By playing tricks with the numerical aperture of the imaging system it is possible to image on the one hand the atoms in a single cloud with high resolution and on the other hand also possible to image the clouds of atoms relative to each other with lower resolution.

To connect ultracold Rydberg atoms to integrated photonic circuits, laser light at precisely defined optical frequencies is required to cool the atoms and excite them to Rydberg states. A key property of our hybrid quantum systems is the coherence time of the different components. For Rydberg atoms, this is typically determined by the lifetime of the atomic state (µs to ms) and the frequency bandwidth of the laser light used for manipulation. To maximize coherence times, it is therefore very important to reduce the linewidth of the laser to values comparable to or below the natural linewidth of the Rydberg states. In addition, it is important that the lasers are stable over long periods of time with low noise to ensure long, stable measurements.

In this project, you will build a laser setup that will allow you to test how stable the frequency of the lasers is in the experiment, and then you will be able to further optimize their stability and reduce the noise of the lasers. You will be working with state-of-the-art lasers that will allow long term stability of the experiment.

In the RQO experiment we create artificial two-level systems by trapping ultracold rubidium atoms in small optical traps. We couple the atoms to Rydberg states, but thanks to the Rydberg blockade effect, the atom cloud in the small trap can only contain a single excitation. These so-called Rydberg superatoms can be used to manipulate even few-photon states of light. So far, we used up to three supeatoms in a chain.

The next experimental step is to create multiple superatoms in a 2D pattern, and this will be the topic of your thesis. In this project you will create a pattern of laser beams with a radio-frequency-controlled 2D acousto-optical deflector. You will characterize the pattern and the radiofrequency signal generation, and if possible, you will implement your pattern in the main experiment where the laser beams will act as optical dipole traps to the atoms, and you will help trap ultracold rubidium atoms in these traps!

In the Fiber Cavity Optomechanics (FCO) lab we investigate the interaction between mechanical motion of 3D laser-written trampoline-like oscillators with photons of optical fiber cavity fields. This interaction allows to manipulate the mechanical properties of the oscillator via the photons (and vice-versa) on a fundamental level.  

The flexibility of the laser-written fabrication allows to print complex mechanical geometries and multi-oscillator systems, but these printed oscillators typically entail somewhat poor mechanical and optical properties. In this project you will continue ongoing work to improve the system by e.g. optimizing the fabrication process, developing new oscillator geometries or employing more exotic printing materials (“printing with glass”).

The goal of the Hybrid Quantum Optics project is to realize hybrid quantum systems of ultracold Rydberg atoms coupled to microwave electromechanical oscillators. In particular, we want to study the coupling between electromechanical oscillators near their quantum ground state. This requires the cooling of macroscopic objects to a few K using liquid He cryostats. Closed-cycle cryostats typically suffer from vibrations in the µm range induced during He compression and from limited optical access due to radiation shields protecting the samples from blackbody radiation. Both are typically incompatible with ultracold atom experiments that rely on good access for control, manipulation, and detection.

We are building a novel experimental platform that combines an ultracold atom setup with a commercial cryostat, providing a suitable environment for hybrid quantum systems of both ultracold Rydberg atoms and electromechanical microwave oscillators. For example, vibrations will be damped by a special vibration isolation system and atom trapping will be achieved with magnetic rather than optical traps. In this project, you will characterize crucial components and properties of the cryostat, such as vibration isolation, electrical performance of the superconducting chips, or the cooling performance of the system, and you will join us in producing Rb Rydberg atoms in a 4 K environment and in coupling them to microwave resonators.

Experiments with ultracold atoms rely on frequency-stabilized lasers. This is particularly important when working with Rydberg atoms that have narrow transitions and where stability becomes key to produce high quality scientific measurements. Phase and frequency noise is for instance thought to be one limiting factor for the achievable coherence time achievable with Rydberg superatoms.

In this project, you will work on characterizing frequency and phase noise in laser locks of different types. You will get familiar with different electronics and optimize laser lock systems. You will learn key skills in laser physics, fast frequency analysis, noise characterization and atomic physics.

In our new Ytterbium lab, we want to create a scalable array of so-called Rydberg superatoms - ultracold atomic clouds of the size similar to the Rydberg blockade radius acting as an effective two-level system with large coupling strengths for single photons. This will allow us to study collective light-matter interaction in regimes not yet explored.

Placing the superatoms around an optical nanofiber with sub-wavelength diameter exhibiting an extended evanescent field will allow strong, homogeneous coupling and scaling up the number as the system will not be limited by diffraction. For positioning the Ytterbium atomic clouds next to the nanofiber, and array of movable microscopic optical tweezer traps needs to be controlled with high precision with an acousto-optical deflector (AOD).

In this project, you will develop and characterize the setup for the generation of optical tweezer arrays with a two-axis AOD, driven by an FPGA-controlled frequency source. This involves planning, building and characterizing the AOD optical setup, as well as programming the control software.

Strong coupling of an optically active material to an optical resonator can create new hybrid light-matter particles, called polaritons. Two-dimensional semiconductors are especially suited to realize such systems due to their strong light-matter interaction. In this master thesis, a fiber-cavity system should be designed and constructed and an atomically thin layer of semiconductor should be integrated within this cavity to generate hybrid light-matter particles.

Experiments combining ultracold atoms and single photons to study fundamental aspects of quantum optics and to explore quantum technology applications require a large number of commercial and self-built individual devices - lasers, cameras, single-photon-detectors, optical elements, signal sources, data acquisition devices,... - to be precisely synchronized and ideally controlled through a user-friendly graphical interface (from the beach).

In the NQO group, a standardized solution used by our different labs as well as by others has been developed and maintained for years. Rapid progress in fast digital and analog control hardware now offer new opportunities that make it more sensible to develop a fully new version for the future. In particular, various open-source options now exist that can be co-developed with other research groups, but also adapted to our specific experiments.

For this project, we are looking for a programming enthusiast interested in joining such a development effort, which still is closely involved with the science and technology of a cold-atom experiment.