Finished projects

Interferometer setup for detecting photon-number dependent phase shifts

Anthea  Nitsch, bachelor project 2023

For my Bachelor project I joined the Rubidium Quantum Optics experiment, where I worked with Nina Stiesdal as my advisor. The goal of my Bachelor thesis was to characterize an interferometer, which is capable of measuring conditional phase shifts.

In the RQO group we explore the use of strong interactions between atomic Rydberg states in an ultracold gas of rubidium for quantum optics. An important part of this exploration is measuring the intensity of the phase of the light passing through the ultracold rubidium gas, which acts as a nonlinear medium. This atom cloud can mediate a strong interaction between photons, resulting in a phase shift between two photons, which pass the nonlinear medium in a small time interval.

This is where my thesis research starts. Building on the thesis of my predecessor, Anna Speier, I worked on characterizing an interferometer test setup, which is capable of measuring exactly such conditional phase shifts at low light intensities. During the duration of my lab work, I rebuilt the previously existing interferometer setup to closer resemble the experiment dimensions and implemented a method to recreate the beat signal based on Anna’s work.

Furthermore, I implemented a method to automate the data taking process. To find out which ratio of signal beam and local oscillator beam power yields the most stable phase, I implemented a program simulating the interference of two laser beams at low intensities without taking external influences and the detector efficiency into account. With an Allan deviation plot, I could determine the phase stability of the interferometer and even see correlated noise of one of the used lasers. Lastly, I implemented the sorting of the measured photons according to their detection times to measure a conditional phase shift.

Building on this thesis, a final version of the interferometer can be implemented into the RQO main experiment.

Frequency stabilization of a laser and a high resolution optical setup for excitation of ultracold Rydberg atoms

Samuel Germer, bachelor project 2023

For my Bachelor thesis, I joined the Hybrid Quantum Optics experiment (HQO), where Rydberg atoms will be brought into interaction with a mechanical oscillator.

I had two main projects. The first thing I did was to frequency stabilize a 480 nm laser which will be used for the excitation of the atoms. This is necessary to ensure a long life time of the excited atoms and to achieve a blockade effect around them. The second project was to build an optical test setup for the excitation. It focuses two lasers (480 nm and 780 nm) to diameters of a few micrometers. Both lasers are aligned on a common optical axis. This way the atoms can be excited via a two photon transition. The small beam diameters and the blockade effect will allow us to achieve an effectively one-dimensional geometry of the excited atoms.

During my project, I had to think about how to set up the optical setup as precisely as possible. I learned a lot about the properties of optical fibers, laser beams and precise optic alignment techniques. With a construction that allows for micrometer precision of lens placements I was able to achieve a very high beam quality (see image) close to an ideal Gaussian beam profile. Furthermore, I wrote a software for a camera and calibrated the sensor. I used this camera to align the optics of the setup and measure the beam profiles around the focus. As a result, I was able to achieve a much higher level of precision in the set-up than before. The camera software and the alignment techniques can now also be used in other labs.

In the future, this setup will be implemented in the experiment to be able to excite Rydberg atoms.

Realization of a ⁸⁷Rb magneto-optical trap

Valerie Mauth, bachelor project 2023

During my bachelor thesis I joined the Hybrid Quantum Optics project where I worked together with Hannes Busche, Cedric Wind and Julia Gamper.

The aim of my thesis was the realization of the rubidium magneto-optical trap (MOT) as the first cooling step in the HQO experiment. After the rubidium atoms are initially cooled down and trapped in the MOT, they will be transported via a magnetic transport to the cryogenic science chamber. Therefore, the MOT is an important part for realizing the two-chamber system in the experiment.

The first step for realizing a MOT was to set up the laser system for cooler and repumper laser. For this I optimized the frequency lock of a third laser, the so-called master laser, and built the laser locking setup for stabilizing the cooler and repumper laser in frequency and phase relative to the master laser. Furthermore, I implemented fiber beam splitters and acousto-optical modulators into the system to guide the frequency stabilized light to the vacuum chamber, split it into six MOT beams and control the power of the light.

For optimizing the cooling process of the MOT an imaging system is needed. So, during my thesis I also built an absorption imaging setup to characterize the MOT. At the end of this thesis, it was possible to realize the 87Rb magneto-optical trap and first absorption images could be taken.

Dye solutions in mode-matched Fiber Fabry-Perot-cavities

Yasar Turgi, bachelor project 2022

The title of my project was 'Dye solutions in mode-matched Fiber Fabry-Perot-cavities’, which means that I worked with a Fabry-Perot-cavity consisting of a mode-matched fiber and a flat mirror in order to distinguish the medium inside said cavity. This mode-matched fiber is composed of a standard single-mode fiber and piece of Gradient-Index-fiber (as seen in the picture below) which acts as a lens to focus the incoming light onto the cavity and therefore improve the coupling of the light into the cavity.

First of all I analyzed the mode-matching capabilities of the fiber I used by measuring the Finesse and the coupling-depth of their reflectionsignal for different cavity-lengths. I derived from that, that the coupling of my mode-matched fiber is in fact better than for a single-mode fiber whereas the Finesse of the assembled fiber hardly differs.

After that I used this fiber-cavity to measure two dye solutions and distilled water. For this I placed a droplet of the medium on top of a horizontally placed flat mirror and poked the fiber mirror into the droplet (see picture). In order to distinguish them I tried to use thermal nonlinearities. These nonlinearities occur due to the heating of the medium inside the cavity and the associated change of the refractive index, which ultimately leads to a displacement of the cavity resonance. The extent of the pushing of the resonance, quantized through the pushing factor, I tried to extract from the recorded reflectionsignal and use as a parameter for distinction. My main result was, that through this pushing factor the medium inside the cavity (solution oder pure water) can be distinguished. The effect of the concentration of this solutions is yet unknown and may be an interesting investigation in itself.

Below: Yasar used a fiber consisting of a gradient index (GRIN) fiber spliced onto a single mode fiber.

Research project: Simulation of magnetic trapping

Florian Pausewang, research project 2022

The HQO-Experiment is capable of transporting a cloud of cold atoms (T = 50 µK) via a magnetic transport through a valve in a science chamber with excellent vacuum conditions - and in future with a blackbody radiation shielding via a cryostat.

In the science chamber the interaction between Rydbergatoms and an electromechanical oscillator will be explored. After the magnetic transport the atoms are captured in a quadrupole field generated by the last magnetic transport coils. As a quadrupole field has a zero potential, it is not suitable for a long trapping due to the occuring majorana spin flips.

The atoms need to be loaded from the magnetic transport quadrupole trap into a trap which has a minimum, but no zero field. And the trapping region should be close to the electromechanical oscillator. One approach is the miniaturized ZWire Trap that can be printed on an atom chip, which can be inserted into the chamber.

To understand the process of loading the atoms from one trap into the other one, a Monte Carlo like simulation programm using python has been written. The simulation initilizes a random atom sample in an initial potential and as the potential is changed the propagation of the atoms is simulated. Trajectories of the atom can be evaluated and quantities like the atom loss during the transfer will be estimated.

Experiment control for new experiment

Tore Homeyer, bachelor project 2022

The aim of my bachelor’s thesis was to set up the experiment control system for the Hybrid Quantum Optics (HQO) experiment. I worked in the hybrid quantum optics team together with Hannes Busche, Cedric Wind, Julia Gamper, and Florian Pausewang.

During my project I adapted the existing systems from other experiments in the group to the new HQO infrastructure. This included a good amount of Python programming for communication with various devices like pulse generators or function generators used to run the experiment once it is built, as well as some electronics work.

I learned a lot about the thoughts that go into designing and building a new experiment and the requirements of the supporting systems. Additionally, I gained insights into how to „talk“ to various instruments using Python scripts, which is of course very useful for any future labwork.

Measurement of dye concentrations in liquids using FFPCs

Jasper Schwering, bachelor project 2022

During my Bachlor project I worked in the Fiber Cavity Optomechanics group together with Lukas Tenbrake, Hannes Pfeifer and Florian Giefer.

In our team we work with so called „Fiber Fabry-Perot Cavities“ (FFPC). These FFPCs consist of two fiber mirrors opposing each other inside a glass ferrule as you can see in the picture on the right. The fiber mirrors are fabricated from bare glass fiber, where a high reflective coating is placed onto the fiber ends. The cavity length of the FFPCs can be controlled via the piezoelectric element, which is glued to the glass ferrule. This way the finesse of the cavity can be measured by scanning over the cavity length.

FFPCs have a broad area of aplications. One of them is the research on optomechanics, which is the main topic of the FCO group. Another one is using the FFPC as a spectrometer, as it was previously performed in our group for oxygen spectroscopy.

The aim of my project was to transfer the spectroscopy application to liquids, starting with the invastigation of the finesse change inside of different dye concentrations. The used solutions were made form a near-infrared dye mixed with destilled water. An FFPC was placed inside of such solutions and the finesse change due to the additional absorption losses was analyzed (compare the picture on the right).

During my research I had to do a lot of practical work, such as building several FFPCs, arranging optical setups or preparing the dye concentrations, but also finding solutions for various problems arising in the process of my project. Also some coding was done for results analyzation. Overall I made the first steps towards realizing this specific FFPC application, and I charaterized the main problems of measuring liquid concentration.

Laser frequency stabilization for Rydberg excitation

Florian Pausewang, bachelor project 2022

During my Bachelor project I joined the hybrid quantum optics team and worked with Hannes Busche, Cedric Wind, Tore Homeyer, and Julia Gamper.

I continued the work on the laser system of the hybrid quantum optics project, following in the footsteps of Julia. One of the steps of this HQO experiment is the excitation of rubidium atoms to Rydberg states (states with a high principal quantum number), which have a long lifetime and therefore a narrow linewidth. The prerequisite for a long coherence time of the cooled and excited atoms is that the lasers used in the process have a linewidth comparable to or smaller than the linewidth of the transition.

Two lasers at 780 nm and at 960 nm are used for the Rydberg excitation. My task was to narrow the linewidth of the 960 nm laser by locking it to a high finesse cavity. The ultra low expansion cavity which we use provides a very narrow and stable reference frequency. I performed many optimization-iterations of the feedback-loop that corrects the laser freqency to achieve a small linewidth.

By using a beatnote between two independent lasers, locked at the same frequency to different cavities, the linewidth of the locked locked lasers could be quantified below 1 kHz and a longtermdrift under 5 kHz in 12 hours.

These stabilized lasers can be used to perform the next steps towards producing Rydberg atoms.

Tailored light potentials with a SLM

Jan de Haan, bachelor project 2022

My bachelor thesis was about setting up a liquid crystal on silicon spatial light modulator (SLM). This device can imprint a freely choosable spatially varying phase shift onto light. The goal was to use it to create arbitrary intensity patterns and to characterize the results. In the future, the intensity patterns created with the SLM will be used for optical dipole trapping of atoms in the Rubidium Quantum Optics (RQO) experiment.

I really enjoyed working with Nina Stiesdal, Lukas Ahlheit and Simon Schroers in the RQO-lab, and got to learn a lot about the RQO experiment.
 

Over the course of my thesis, I built an optical setup for testing the SLM, including setting up the laser that will provide the light for the dipole traps that will be made with the SLM in the future, wrote various pieces of software to display phase patterns on the SLM, compute those phase patterns in the first place, and optimize the results iteratively.
 

The project was a nice mix of learning about how to build optical setups, applying Fourier optics and understanding and implementing algorithms and methods for finding and improving phase patterns to display on the SLM.
 
The results are both the tangible spot intensity patterns of high quality that can be made with the SLM, as well as knowledge of what factors of an optical setup including an SLM influence the image quality.

A Vacuum Fiber Microscope

Florian Giefer, bachelor project 2022

I did my bachelor thesis in the “Fiber Cavity Optomechanics” project. In the FCO group, we are examining the behavior of mechanical oscillators, placed inside fiber Fabry-Perot cavities. These fiber Fabry-Perot cavities consist of a fiber mirror (optical glass fiber, coated with a Bragg reflector) and a mirror substrate.
It is advantageous to observe these oscillators in vacuum, as the absence of air dampening improves their mechanical quality.

Additionally, we are interested in the behavior of interconnected resonator assemblies, as this could enable advances in optomechanical circuits and quantum computing. This led to the idea of my project, the “Double Tip Vacuum Fiber Microscope”, an apparatus capable of precisely placing two fiber mirrors on
top of a mirror substrate to form fiber Fabry-Perot cavities in vacuum. You can see what this setup looked like in the image on the right. Central components are the five electrically controlled translation stages (seen in black), as well as the fiber holders (aluminum block on the leftmost stage). The Fiber holders allow for a scan of the cavity length via an integrated piezo element.

I was very thankful for the possibility to work on such an interesting experiment together with Lukas Tenbrake and Hannes Pfeifer, who both are great teachers and advisors. My project involved lots of practical skills for assembling the experiment and vacuum setup, like screwing and gluing components together, planning the workflow, prototyping experiment parts, and testing the setup, as well as some
coding for a control software of the experiment.

Raman Sideband Cooling of Rubidium

Simon Schroers, bachelor project 2022

During my bachelor thesis I was working on the Rubidium Quantum Optics project together with Lukas Ahlheit, Nina Stiesdal, and Jan de Haan.

Even though the main topic of my thesis was Raman sideband cooling, we worked together on all the different parts of the experiment. This means we had to tackle various problems arising during the build-up and improvement on different parts of the setup.

This way I got an insight into all the different parts of experiment and an understanding of modern atomic physics reasearch. This is not just measurements and working at a computer, but also building optical setups using a screwdriver and fine alignment of the optics. Therefore, there was a broad range of tasks that needed to be done, and I never got bored.

For the Raman sideband cooling, which in the end was the main part of my project, I had to set up optics in order to create an optical lattice and prepare different laser beams in order to be used on our atomic cloud of rubidium atoms. To implement the cooling I had to do several adjustments  e.g. a power stabilization for the optical lattice and compensation magnetic stray fields using microwaves.

In the very end of my project, I was able to implement and optimize the Raman sideband cooling of Rubidium atoms. With the cooling scheme we can lower the temparture by a factor of 16. Using the cooled atoms it will be possible to do experiments using Rydberg excitation and a lot of cool nonlinear optics-experiments.

A series of absorption images of rubidium atoms after releasing them from the optical dipole trap. The atomic cloud expands after being released, and depending on how fast the cloud expand, we can extract the temperature of the atoms.

The two different animations are for different temperatures.

Laser frequency stabilization for laser cooling of Rb atoms

Julia Gamper, bachelor project 2021

For my Bachelor thesis I was able to work as a part of the Hybrid Quantum Optics project together with Hannes Busche and Cedric Wind.

The aim of my project was to build the laser locking setup for the lasers at 780nm which includes a reference laser and two lasers which are going to be used in a magneto optical trap (MOT). Therefore I built the needed optics and electronics to make this possible.

First of all, one of the three lasers has been frequency stabilized onto an ultra-stable external reference cavity. This stabilized laser can then be used as reference for additional lasers at 780nm e.g. for the two lasers which are going to be used in the MOT.

These two lasers need to be stabilized with a certain frequency offset to make sure they are at the transition needed, one for exciting the Rubidium atoms and the other one is going to be used as a repumper. This is why the lasers are phase-locked relative to the reference laser.
It is now possible to use the lasers in the MOT as soon as it is built and the reference laser can also be used as a reference for additional lasers at 780nm e.g for the first step in Rydberg excitation.