Bachelor- and Master-projects

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 constituents. For Rydberg atoms, this is usually determined by the lifetime of the atomic state (µs to ms) and the frequency bandwidth of the laser light used for manipulation. To maximize the 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 this project, you will build the laser systems required for both laser cooling and Rydberg excitation of the atoms. You will stabilize the frequency of the lasers to the resonances of an optical cavity with ultra-high finesse (> 15000). Using the Pound-Drever-Hall technique, we aim for linewidths in the order of 100 Hz, which corresponds to an accuracy of 10^-12 compared to the light frequency. You will be working with both state-of-the-art lasers and a commercially available optical resonator designed to experience almost no thermal drift when isolated from the environment in a vacuum chamber to provide a stable frequency reference.

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. Finally, if possible, you will implement the AOD setup in the main experiment.

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, employing more exotic printing materials (“printing with glass”) or designing a cryogenic fiber microscope to cool down the mechanical oscillators.

Exciting rubidium atoms to Rydberg states can be done with different two-laser combinations. These different schemes have different benefits. We have recently upgraded our laser collection with a 420 nm laser. This wavelength can drive transitions in rubidium between the ground state, 5S, and the second excited state, the 6P state. Combining this laser with a 1012 nm laser, it is possible to excite the rubidium atoms to Rydberg states in the so-called inverted scheme. In the future we want to implement this inverted scheme in combination with our magic wavelength lattice laser, which is exactly at 1012 nm. We want to use this combination to create Rydberg excitations and perform high-resolution spectroscopy of the Rydberg states.

To use the laser for actual experiments, it must be frequency stabilized. In this project you will work towards locking this laser to a rubidium spectroscopy cell using a home-built electro-optical modulator and modulation transfer spectroscopy. In this project you will learn advanced experimental skills, including working with lasers, and building electronics circuits.

If time allows, you will also work on introducing the laser in the main experimental setup and use it for Rydberg excitation. We hope that this laser will allow the Rydberg atoms to mediate nontrivial correlations between single photons.

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 interface these two systems an ultra-cold cloud of atoms is trapped closely above the surface of an atom chip that hosts the circuit. The first-generation chip that is going to be employed in the experiment features a classical microwave strip line resonator to study the interaction of Rydberg atoms with a classical resonator closely above the chip surface.

The goal of your project will be to design and integrate a second-generation atom chip into the experiment. The atom chip will have the capability to trap ultra-cold rubidium atoms at a controllable position above the chip surface and it will host an electromechanical oscillator close to its motional ground state enabling the study of large mechanical quantum system interacting with optically controlled ultra-cold Rydberg atoms.

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 polariton that is created. However, the proximity of the control light to resonance also generates a repulsive potential that causes loss of atoms from the dipole trap.

In this project you will work on techniques for shaping the potential of the control beam to reduce its detrimental effect. One option is to overlap another beam with same but attractive potential to compensate. Another option is to shape, with appropriate optics, the control light into a flat-top beam instead of a gaussian beam, such that the repulsive force would be homogeneous across the dipole trap.

3D-printing of optical elements is a growing research field. Printing miniaturized optical elements that can be attached directly to optical fibers would allow us to bring detection and addressing optics much closer to our ultracold atoms, that what we can do with conventional optics.

In this project you will work with a 3D laser-writing-system, printing miniaturized objectives in transparent polymer and attaching them to optical fibers. One core goal is to optimize the printing parameters for best performance, and to test the vacuum properties of your objectives.

Our first objectives printed with the nanoscribe. The objectives are only few mm in length.

You will also have the exclusive option to print a tiny statue of yourself that you can gift to someone you love <3

The goal of the hybrid quantum optics project is to realize hybrid quantum systems of ultracold Rydberg atoms which are interfaced with microwave electromechanical oscillators. 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. 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 blackbody radiation. Both of these are typically incompatible with ultracold atom experiments which rely on good access to control, manipulate, and detect them.

We are building a novel experimental platform that combines an ultracold atom setup with a commercial cryostat that provides 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 insulation, the electrical performance of superconducting chips or the system’s cooling performance and join us in producing Rb Rydberg atoms in a 4 K environment and interfacing them with microwave resonators.

Coupling 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. In this master thesis, a fiber-cavity system, based on a flat substrate and a concave fiber mirror should be designed and constructed.

The concave fiber mirror will be fabricated on our Fiber shooting apparatus with the challenge of realizing a small-volume cavity. After that an atomically thin layer of semiconductor should be integrated with the goal 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.