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.

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.

Quantum optics experiments, including those in the NQO group, rely on the efficient detection of single photons. Photon detectors based on superconducting nanowires can achieve efficiencies up to 90%, which is a significant improvement compared to room temperature detectors.

Superconducting photon detectors are based on the heating of a nanowire when it is hit by a photon. Heating leads to a local breakdown of superconductivity. The increased resistance due to the breakdown leads to a change in current flow that can be detected using amplification electronics.

The NQO group has acquired a system containing eight nanowire detectors in a cryostat, into which the photons are delivered via optical fibers. In this project, you will characterize the detectors and implement a data acquisition system to detect photons and analyze photon correlations with sub-ns time resolution.

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.

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.

The strong interaction between Rydberg atoms modify the propagtion of photons transmitted through the medium. This leads to nontrivial photon correlations, but also to modification of the spatial mode of the light. Measuring these spatial modifications require a mode-sensitive detection setup.

In this project you will build such a setup and image different spatial modes of light, in particular modes with orbital angular momentum. You will investigate how to create the different modes with a spatial light modulator (SLM) and image them with a single-photon sensitive camera. Eventually, a second SLM will be used to distinguish between different modes of light, sending some mode contributions onto the camera and others onto a single photon detector.

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.