Finished projects

Balanced Homodyne Interferometry for Quantum State Reconstruction

Timo Bracht, bachelor project 2024

The goal of this bachelor thesis was to build an interferometer that is usable in an optical experiment using ultracold atomic gases. The idea is to reconstruct the wavefunction of light states that passed an ultra cold atomic cloud with the interferometer.

In this thesis a Mach-Zehnder Interferometer was built based on a previous design, where some challenges were identified. The thesis included a variety of interesting challenges, first designing and constructing the detector, than fine-tuning and chararcterizing it´s components and finally testing the detector with the vaccuum state, which included programming an alogrithm, that can reconstruct the Wigner-Function from the raw data.

Building the interferometer is a versitile practical task, including the design, the adjustment of Acousto-Optical-Modulators and beamsplitters. The pointing of the laserbeam must be extremely precise and the set-up needs to be shielded from the environment to suppress movement at the nanometer-scale! To characterize the stability of the set-up, a measurement of the Allan-Deviation was used. To make measurements more automated, a data-acquisition-card was installed. This allowed to replace an oscilloscope with a card, that can directly communicate with the other components of the whole experiment. Whereas the measurement of many physical quantities like voltage, temperature or the speed of an macroscopic object are often trivial, measuring a quantum state is more complex. Especially the probabilistic nautre of Quantum Mechanics makes measurements difficult to perform, as the same experiment has to be repeated enough times, to infer the probabilities of the quantum state.

Quantum State Tomography is a method to fully measure a quantum state's characteristics. Infering the probability distribution from the measured data is no trivial task. The algorithm used in this thesis for this reconstruction is based on the Maximum Likelihood Principle. The measurement consists measuring a voltage timetrace for different phases of the interferometer. The voltage timetrace taked the photodiode signal and records it over time. With a Maximum-Likelihood-Reconstruction the density-matrix and Wigner-Function can be reconstructed from the data.

Atom Preparation and Rydberg Excitation of Rubidium Atoms

Julia Gamper, Master project 2024

The goal of this thesis was to implement a Rubidium Rydberg excitation scheme in the Hybrid Quantum Optics (HQO) experiment, taking one step towards the ultimate aim of interfacing Rydberg atoms with electromechanical oscillators on a chip.

Due to the two-chamber design of the HQO experimental setup, which is needed to fulfil certain requirements (differential pressures and temperatures for atom loading and conducting the experiment) multiple steps of atom preparation have to be performed. The steps were characterized and optimized as part of this thesis. This includes trapping the atoms, preparing them for magnetic trapping and transporting them to the Science chamber, where the Rydberg excitation can be performed.

With 10⁸ rubidium atoms being prepared in the chamber where the experiment will be conducted, the excitation as well as detection scheme for Rydberg excitation was implemented, including the optics used. To be able to confirm the creation of Rydberg excitation, two different detection schemes, transmission spectroscopy of the probe light and ionization detection for ionized Rydberg atoms, were implemented and characterized.

Transmission spectroscopy was set up, characterized and used to map the cloud position, confirming the overlap of the cloud and the laser light for Rydberg excitation. Additionally, the theoretical transmission spectrum was fit to the probe transmission through the atom cloud in the magnetic trap. It was used to determine a temperature estimation of (∼ 400 μK ) and the number of atoms of 2 · 10⁸, contributing to the transmission spectrum. It was also possible to detect a first transmission signal under EIT (Electromagnetically Induced Transparency) conditions, verifying a coupling from the intermediate state to the Rydberg state. To verify the presence of Rydberg excitation, ionization detection was used. The detection efficiency was estimated to be 14%.

It was a great pleasure working in the HQO where I did not only learn a lot of great physics but also had an amazing time working together with Cedric, Leon, Valerie and Sam - thank you for the great time.

Characterization of a 1D magic wavelength lattice for increasing the coherence time of Rydberg superatoms

Jan de Haan, Master project 2023/2024

During my Master's thesis, I worked on the Rubidium Quantum Optics experiment. There, I took part in detailed characterization measurements of a 1D standing wave optical dipole trap. It was designed such that it traps not just ground state atoms, but also Rydberg atoms, and to do that with the exact same trap potential for both. To make the trap potentials for two states equal, one has to choose the right wavelength for the laser whose light forms the trap. This wavelength is called the "magic wavelength".
In the RQO experiment, clouds of atoms smaller than the Rydberg blockade radius that only contain a single, collective Rydberg excitation can be made. They are called Rydberg superatoms. This collective excitation is subject to different dephasing mechanisms, such as inhomogeneous differential light shifts, density-dependent effects, and thermal motion. The 1D magic wavelength lattice trap offers the possibility of reducing the thermal motion of the atoms, by tightly confining them in one direction, and of reducing inhomogeneous differential light shifts.

While the size of a ground state atom is a few Angstrom (0.0001 µm), Rydberg atoms can be about 1 µm in size. This is comparable to the wavelength of the light used to trap them. The intensity of the standing wave of light varies on this scale, and these variations cause a trap-geometry dependent change of the shape of the trap potential for Rydberg atoms that does not occur for ground state atoms. That makes it more difficult to find the magic wavelength.

During my thesis, I conducted parts of an extensive series of measurements of coherence times of the collective excited state for different principal quantum numbers, two different trap geometries, and various wavelengths of the trapping light. These measurements clearly showed the geometry dependence of the trap potential, and also demonstrated an overall increase of coherence times due to the confinement of the atoms in the lattice. During these measurements, we found that reducing thermal dephasing by tightly confining the atoms led to increased dephasing due to larger local density of the cloud. Measurements and calculations for gaining a quantitative understanding of this trade-off were also done.

I had an amazing time, both getting to know the technical side of the day-to-day operation of the RQO experiment, and learning tons of super-cool physics. I would like to thank the RQO team: Lukas, Daniil, and Nina, for teaching me so much and for having me as a Master student on the experiment.

Laser Frequency Stabilization to Optical and Atomic References

Matthias Metternich, bachelor project 2024

In my Bachelor thesis, I built two feedback loops to stabilize the frequency of two self-built interference filter lasers. These lasers were previously refurbished to 780nm by another bachelor student (for more information see Hannah Buss's bachelor thesis) to use them for Rubidium experiments.

The idea is to stabilize the first laser to a hyperfine transition in Rubidium and the second laser relative to the first. To realize stabilization, I built a feedback loop that detects frequency changes and provides feedback to the laser. I had three parameters that I was able to use for tuning the frequency of the lasers which were the laser diode current and temperature and the piezo element which controls the cavity length. I used the piezo element and the laser diode current in my feedback loop since the temperature response was too slow.

For both stabilizations, I had to find a way to create an error signal. That means that a frequency change results in a change in an electrical signal. I used Doppler-free saturated spectroscopy to detect frequency changes in the first laser and overlapped both laser beams to detect the beatnote frequency. The spectroscopy detects changes in the absorption rate for frequency drifts and fluctuations which is used as the error signal. To create an error signal with the beatnote frequency, I used a delay line box to generate an interference signal which depends on the beatnote frequency. To provide feedback to the laser in the most effective way, a PID controller was implemented as a control algorithm. In the end, I was able to stabilize the lasers for multiple hours. The bandwidth where the feedback loop can compensate for frequency fluctuations reached roughly 1 MHz.

Characterization of a superconducting nanowire single photon detector

Max Reicherd, bachelor project 2024

In this thesis, I did the precise characterization of the detector for one channel and the analysis of the dependencies in order to find the optimal operating parameters of the SNSPD (superconducting nanowire single photon detector).

Before going into the interesting details of the thesis, the cool thing about working in this group in general was the development (with the help of the group, of course) of an experimental setup and the investigation of physical properties from zero.

Now to the content of my thesis: The basis for the characterization of the detector was a faint laser source setup. This served as a source for measuring single photons and made measurements of SNSPD characteristics possible. For realizing a faint laser source set up a laser with a wavelength of 780nm was used and attenuated by ND filters. For the precise measurement of the OD values of the ND filters, a calibration of the filters was done in two ways to consider statistical and systematic errors. Moreover, the faint laser source set up was operated in a self-build optical enclosure (black box) to avoid environmental light coupling into the fibre. After the setup was built, first dark count measurements were performed.
Different settings were investigated to find the optimal settings for the lowest dark count rate. The lowest dark count rate was achieved by operating the faint laser source in the black box and wrapping aluminum
foil around the optical fibre connecting it to the SNSPD. Afterwards the recovery, dead and reset time was determined with a time tagger unit. With an autocorrelation evaluation of the time distances between detected photons, the three times were determined. Based on the evaluated dead time, this would make the detection of single photons, with count rate up to 72MHz possible.

Finally, the system detection efficiency was measured. It was found out that the efficiency is stable (when the trigger voltage is larger than 600mV) and independent of the trigger voltage for an input count rate up to 2.446MHz. Furthermore, the system efficiency was evaluated. As final result a system detection efficiency of 90% was calculated. However, the significance of this result is restricted by the large uncertainty due to the systematic error in the calibration of the ND filters. Next steps would be the investigations and characterizations of the other seven available channels.

Finally, the fully characterized SNSPD can be used in the HQO main experiment for measurements of strong nonlinearities in Rydberg physics.

Generation and Detection of Optical Beams with Orbital Angular Momentum using a Spatial Light Modulator

Kimberly Kurzbach, bachelor project 2024

In my Bachelor Thesis I worked with a Spatial Light Modulator (SLM), which is capable to change the phase of the initial laser light. The aim of my thesis was to generate various Laguerre-Gaussian modes from an initial Gaussian beam.

To achieve this, I first built an optical setup. The laser beam was fiber-coupled to clean its shape, reflected by the SLM, and finally focused onto a camera, which imaged the different modes. To create a Laguerre Gaussian mode I needed to implement a lot of holograms, which I could upload to the SLM. I began by examining the mathematical derivation of such beams, enabling me to implement various phase profiles of different modes. Because the created mode still overlapped with the zeroth-order beam, I needed to add a blazed grating term to shift the beam into the first diffraction order for imaging. I decided to also take the Hermite Gaussian modes into account. Due to their rectangular symmetry, it was easier to image them first before moving on to Laguerre-Gaussian modes, as the offset was easier to adjust.

Afterwards I imaged the Laguerre Gaussian modes and a lot of adjusting of the setup was performed in the progress of doing so. Because the waist size of the beams was smaller than expected I decided to implement an optimization method for the phase masks that I uploaded to the device. Here the idea was to only diffract the desired part of the beam into the first order of diffraction and this was done by using a grating pattern, whose diffraction efficiency is modulated by a function of the amplitude of the beam. This lead to much nicer results which an increased waist size and a more homogeneous distributed intensity for the modes

In the future, there will be attempts to create an inverse phase mask to transform an initial Laguerre-Gaussian beam back into a Gaussian beam. This could be useful for the RQO experiment, as it could enable the collection of light from more different modes.
I really enjoyed that my Bachelor's thesis offered a good mix of both laboratory work and theoretical programming, allowing me to build a setup and understand the theoretical background.

Refurbishing Interference Filter External Cavity Diode Lasers for Cold Atom Experiments

Hannah Marie Buss, bachelor project 2024

The title of my project was “Refurbishing Interference Filter External Cavity Diode Lasers for Cold Atom Experiments”. This means that I worked with  Interference Filter External Cavity Diode Lasers (IFLs) that were built in the lab. I did a lot of hands-on lab work, refurbishing two IFLs from 852nm to  780nm to use in the RQO-lab. For this I exchanged the laser diode and  interference filter of the two lasers and then realigned all optical elements  for optimal functionality. I then also worked with the newly refurbished   lasers, characterizing them in great detail. I documented things such as  the output power and frequency, the beam profile and the lasers’  tunability as well as their stability – here I looked at both the short and the  longterm stability. For the former I measured a beat note signal of the two  lasers, which is a first measurement for the future goal of locking the two lasers.

For the second part of my project, I did a first application of the refurbished lasers: Using a simple setup I did saturated/doppler-free absorption spectroscopy of the D2 line in the rubidium spectrum. To maximize the scan range for this, I implemented a current feedforward mechanism which ultimately allowed for scan ranges around 6GHz, making it possible to scan three of the four absorption dips in the line at
once, a result comparable to some commercially available external cavity diode lasers.

As a next step the characterized lasers will be stabilized by locking them,
so that they can be used in various applications in the RQO lab.

Creation of Error Signals for Frequency-stabilizing a Grating Laser to Rubidium Transitions Using Modulation Transfer Spectroscopy

Bennet Sohn, bachelor project 2024

For my bachelor thesis I joined the Rubidium Quantum Optics (RQO) Lab, where I worked with Lukas Ahlheit and Nina Stiesdal as my supervisors. The goal of my thesis was to use a laser to create electrical signals that can be used to stabilize the frequency of said laser.

In the RQO Rubidium atoms are excited to high energy levels, called Rydberg states. These states are achieved with two lasers, each with a very specific frequency. With my thesis I want to stabilize a laser to enable the excitation of Rydberg states using different frequencies from the ones in the current excitation scheme.

My thesis included work from different fields. Apart from quantum optics, atomic physics, lasers and optical experimental setups, I also had to deal with a lot of electronics and soldering.

The method I worked with is called modulation transfer spectroscopy (MTS).

It utilizes two counter propagating laser beams that are overlapped in Rubidium vapor. The non-linearity of the medium causes the phase modulation of one beam to be transferred to the other one. Demodulating the second beam gives a signal suited for frequency stabilization.

For this method to work I build a heating system for a Rubidium spectroscopy cell. With this heating system the Rubidium vapor inside reaches a temperature of about 100°C, which increases the vapor pressure inside. This way I can measure the transition, including ist hyper-fine splitting, which ultimately gives the frequency reference I want to stabilize the laser to.

Another important part of my experimental setup is the electro-optical modulator (EOM) which I build myself for the most part. It consists of a crystal in between two metal plates. Using an electric resonance circuit, I can apply high voltages to this crystal, therefore influencing its referactive index. If a laser beam passes through this crystal, I can modulate its phase, which is an essential part of MTS.

In the end i was able to create error signals suited for frequency stabilization! But a lot can be optimized. Once this is done, the setup can be implemented in the RQO main experiment.

Constructing a Homodyne Detector for 399nm Laser light

Wayne Ströbel, bachelor project 2024

For my bachelors thesis I joined the YQO team and started the homodyne interferometer with assistance from Eduardo.

With initial information from Alexei Ourjoumtsev and a lab tour of their setup for NIR light in Collége de France, we set out to implement this method for 399 nm laser light.

Homodyne detection is a method of measuring relative phase shifts between two input beams, one being a classical beam (local oscillator) and the other a few photon beam (probe beam). The measurements will allow the reconstruction of the density matrix and measurement of wigner functions of the probe beam.

I calibrated the parts used for the initial setup of the interferometer, and setup the first iteration of the detector, using it to test how low power the probe beam can be made whilst still seeing intereference when scaning the phase of the beams and started testing the phase stability. The continuation of these can be found in Theresa's beachelor thesis.

Once further improvments on the setup by future students have been made this setup is set to be implemented for measuring the single photons coming from the YQO experimental setup.

Stability and Sensitivity Limit of a Few-Photon Homodyne Interferometer for Yb Quantum Optics Experiments

Theresa Dewey, bachelor project 2024

In the course of my Bachelor thesis I worked on an interferometer setup using the method of balanced homodyne detection. This means that the two interferometer arms are laser beams of the same wavelength and the interference is measured on a detector. Using a balanced detector which measures the two output beams each on one photodiode and also puts out the difference of these two signals leads to being able to measure small relative phases between the two arms.

I characterized the interferometer in phase stability and detection sensitivity at very low laser powers using new evaluation methods like the application of the Hilbert transform. This means that I worked in the lab, aligning the optical elements on the table and taking measurement data as well as in the office where I looked into mathematics and theory and wrote the evaluation code.

For me, a very interesting part of the work in the lab was to find the source of a disturbing oscillation in the phase data (see picture) where I found one mirror mount to be especially resonant on acoustic waves of a specific frequency. In the future the setup is going to be implemented in the Ytterbium Quantum Optics experiment to measure the phase shifts inherited by photons that are sent through a non-linear medium to observe photon-photon interactions.

A Glued Ultra-High Vacuum Cell Using Titanium and Fused Silica and a High Resolution Optical Setup for Ultracold Atom Experiments

Valerie Leu, bachelor project 2023

In collaboration with the dynamic and dedicated Hybrid Quantum Qptics group (HQO), my bachelor's thesis delved into building a self-build titanium fused silica vacuum test cell and a probe and control setup for the HQO experiment with realistic dimensions, taking the space constrains around the experiment into account.
The HQO team plans to combine nonlinear quantum optics, which is based on the strong dipolar interaction of Rydberg rubidium atoms, with other quantum systems, such as electro-mechanical oscillators, which operate in the microwave regime. For this to work, the mechanical mode of the oscillator is cooled to near its quantum ground state. The HQO group plans to get close to this regime using a cryogenic ultracold atom apparatus, which cools the chip with the oscillator down to 4K. Additionally the atoms have to get trapped over the oscillator. Any atom trapping requires ultra-high vacuum (UHV) to prevent atom loss due to background collisions.

This is why I build a fused silica titanium vacuum cell for such UHV and tested its optical and vacuum characteristics. The problem with fused silica glass and titanium as a frame is that the coefficient of thermal expansion (CTE) of the glass is very different to the CTE of titanium. The challenge was to glue the windows to the frame of the cell without any leaks forming. On this test cell we tested different techniques to get the best glueing results. After that I tested if the gluing process did work by looking at the mass spectra of the vacuum. Also I located leaks in the the chamber with a helium leak check and fixed them with a silicone-based thin film sealant.

But not only the glass of the vacuum chamber has to be carefully constructed, the optics surrounding the vacuum chamber have to be of high quality as well and the setup has to be well aligned. The main components, which required focussing of the 780nm laser to 8µm, for the Rydberg excitation laser setup, like the achromatic lens, were already chosen and characterized in Samuel Germer’s bachelor thesis. This way I could start to build the setup with the realistic dimensions, taking the space constrains around the experiment into account. I tested the quality of the optics and set the polarization of the used laser to circular light inside of the vacuum cell. This could be done in a procedure that I visualized on a poincare sphere in Mathematica.

In the future this setup can then be build around the real vacuum chamber to excite Rydberg atoms in the experiment.

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.