Bachelor- and Master-theses

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 cooling, trapping and probing the atoms, precise control of laser light-fields is required.

In this project, you will build a laser system for laser-cooling Ytterbium atoms, Rydberg excitation and probing of the atomic ensembles. You will, of course, work closely with the PhD students and postdocs. In particular, you will learn how to tune and characterize top-end lasers, stabilize their frequency to an ultrastable reference cavity and stabilize their intensity.

The Bonn Fiberlab develops advanced fiber Fabry-Perot Cavities (FFPC) with high stability and extended functionalities and attempts proof-of-principle demonstrations of possible FFPC applications.

Photo-thermal spectroscopy (PTS) is a highly sensitive technique to measure fingerprints of various gas samples via thermal absorption of a pump light source. We are collaborating with the Wroclaw University of Science and Technology to build a miniaturized fiber-based device that utilizes PTS for a compact, fiber-coupled gas sensing device.

In this project, you will build a monolithic 3-port fiber Fabry-Perot-Cavity-Device (2 ports for the FFPC and 1 additional for the PTS pump). You will first learn how to build regular FFPCs and will utilize a 3D direct-laser writing system to design and print the 3-port fiber housing to build the complete PTS device.

Laser cooling and trapping has opened the door to investigation of many aspects of fundamental atomic physics since ultracold atoms allows stable measurement conditions by limiting atomic motion.

In the RQO experiment rubidium is trapped and cooled in an optical dipole trap. One completely essential cooling technique is Raman sideband cooling (RSC). In the experiment, this step relies on a 780 nm laser driving Raman transitions between different hyperfine states in rubidium. At the moment, this laser is not frequency stabilized, and this presumably limits the cooling efficiency.

In this project you will be in charge of locking the 780 nm laser to a high-finesse ultra-low-expansion cavity. To do this, you will need to modulate frequency sidebands onto the laser, which you will do with an electro-optical modulator. You will build the resonance circuit for the EOM yourself, and use the EOM to create a Pound Drewer Hall-error signal that will allow you to stabilize the laser frequency to the cavity. You will use this error signal for a PID-loop that provides the frequency lock.

Finally, you will test the performance of the RSC with and without the laser lock active on the running experiment together with the PhD students.

The goal of the hybrid quantum optics project is to realize hybrid quantum systems of ultracold Rydberg atoms and superconducting quantum circuits operating in the microwave regime. In particular, we want to study the coupling between electromechanical oscillators near their quantum ground state. This requires cooling of macroscopic objects to just a few K using liquid He cryostats. However, closed-cycle cryostats typically suffer from vibrations in the µm range that are induced during He compression and limited optical access due to the radiation shields which protect samples from the thermal blackbody environment. Both of these are typically incompatible with ultracold atom experiments which rely on good access to control, manipulate, and detect them.

Towards the end of the year, we will receive a commercial cryostat that provides a suitable environment for both. 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 test and characterize crucial components and properties of the system such as vibration insulation, the electrical performance of superconducting chips or the system’s cooling performance and join us in producing first ensembles of ultracold Rb atoms in a 4 K environment.

 In the Fiber-Cavity-Optomechanics (FCO) lab we work with polymer-based drum-like mechanical resonators that are fabricated in a 3D direct laser-writing process. They are printed on a reflective substrate and interfaced through their interaction with light when enclosed in a miniaturized fiber Fabry-Perot-Cavities. To optimize this light-matter interaction platform, the mechanical and optical quality of the two constituents have to be maximized.

In this project you will be introduced to direct laser-writing using a Nanoscribe system. You will work on optimizing the optical quality of the polymer-based oscillators by finding the best printing parameters and apply post-processing techniques to the polymer-based drum.

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.

We have now loaded the laser cooled atoms into the dipole trap in which the next major step will take place: the realization of electromagnetically induced transparency (EIT) in Rydberg atoms of Ytterbium.

Excitation of atoms into the Rydberg states needs precise control of the laser frequency, power and timing. The detection of Rydberg atoms in the system are done by ionizing the Rydberg atoms and guiding the electrons into a MCP (multichannel plate) by applying large voltages to a set of electrodes. In this project, you will implement and characterize the high voltage electronics and the fast switching box detect the Rydberg atoms.  Moreover, you will also work with the team towards realizing the Rydberg EIT.

In the hybrid quantum optics project, we want to interface ultracold Rydberg atoms and superconducting quantum circuits including electromechanical oscillators that have resonance frequencies in the GHz regime. To characterize and control the latter, advanced test equipment such as vector-network analyzers (VNA) are required, which provide signals with well-defined frequency, amplitude, and phase and can detect all those properties at the output of a circuit as well as any back reflections.

In this project, you will design and build a measurement setup to study microwave circuits, which is based on a high-end 20 GHz VNA. You will test and conceive suitable measurement procedures, initially on classical circuits at room temperature before integrating the VNA and other test equipment such microwave signal generators into our existing experiment control and data acquisition systems for ultracold atoms experiments.

The long-range Rydberg-Rydberg interaction between highly excited atomic states allows the creation of complicated collectively excited states. The interactions allow atoms – few or many – in tailored geometries created with optical traps to behave collectively when the atoms are within the distance defined by the Rydberg blockade effects.

Such tailored geometries are normally made with spatial light modulators (SLMs), acousto-optical deflectors (AODs), or digital micromirror devices (DMDs). Independent of the choice of device, precise device control is key to a good implementation and creation of tailored light potentials.

In this project you will use a SLM to generate tailored intensity patterns which can be used for optical trapping of single or few atoms. The SLM works by changing the phase front of a reflected beam. As the beam is focused down, the phase differences turn into an intensity pattern. Therefore, you will work on finding optimal phase patterns to generate the intensity patterns of interest. You will build a test setup to benchmark this performance, and you will work on implementing active feedback where the applied phase pattern is modified based on the observed intensity pattern.

In the Ytterbium Experiment, we aim to study the nonlinear quantum optical effects via Rydberg EIT at the single photon level. To realize these effects, a cold and dense (very large optical depth) ensemble of atoms is prepared with standard cold atom techniques such as magneto optical trapping (MOT) and optical dipole trap.

The science chamber in our current setup limits the fine control of the magnetic fields and the optical access. We are working on designing a new custom UHV cell to replace the current science chamber. The planned UHV cell has the following requirements, maintaining ultra-high vacuum of the order of 10^-11 mbar or higher to minimize the background collisions and minimal thermal lensing.

The project is planned in two steps. First, you will design and optimize the UHV cell with Inventor – a CAD software and simulate the ion trajectories in the cell with COMSOL. Then you will learn and refine a specialized UHV gluing technique patented by IAP. Afterwards, you will test the UHV cell by integrating it into a test vacuum setup including baking up to 120 ^oC. Depending on your interests the project can be further extended. Once completed, we will integrate the new vacuum cell into the experimental setup and will transfer the technology into the future experiments.

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.