If you are a talented student looking for doing a master in a thriving field of research, please contact me. Below you can find a number of possible master projects – both theoretical and experimental – that can be done in my group. Projects can be discussed and fine-tuned to the preference and capabilities of the candidate.
Make ultracold atoms: Experiment
To reach the temperatures we aim for, we will first have to cool further both the ions and atoms. This project focuses on the atoms. To cool these, we will first trap them in a laser beam, after which we will lower the intensity of the beam. This causes the atoms with the most energy to escape the trap, after which the remaining atoms will re-thermalise to lower temperatures. This is pretty much the same way that you cup of coffee cools down. For this to work, the atoms need to collide. Unfortunately, Li atoms have the peculiar property that they don’t – they completely ignore each other. However, we can tune a magnetic field to about 800 G – which causes a molecular resonance to occur. At these fields, the atoms are colliding a lot.
Fig: An ion crystal over lapped with a cloud of atoms.
Currently, the atoms are still about 500 microKelvin, and we will need to down to about 1 microKelvin. In the project, you will set up the trapping laser and characterize the evaporative cooling as well as the resulting ultra-cold cloud. We will characterize the effect of the trapping laser on the ions as well. Under the guidance of the PhD students working in the lab, you will tune the magnetic field even higher than 800 G to turn the fermionic atoms into molecular bosons and see how these interact with the ions. You will be the first to see ions colliding with weakly bound Bose-condensed molecules.
Detect the motion of ions: Experiment
We expect the ions to cool down under the influence of collisions with the atoms. The big question will be: how cold are they? To answer this question, we can try to detect the oscillation amplitude they have in their trap. Unfortunately, this amplitude is on the order of several 10s of nm, which cannot be resolved by any optical microscope. To solve this problem, we will employ a trick that was developed in the context of quantum information science. In particular, we will map the motion of the ion onto the electronic state of the ions. Since the amount of fluorescence detected when we shine in a laser depends on this internal state, we can detect the internal state with almost unit efficiency. Therefore, we should be able to actually ‘read out’ the nm amplitude motion of the ions.
Fig: Detecting the temperature of an ion-atom system. An ion immersed in a cloud of atoms undergoes collisions. A second ion detects its motion via the Coulomb interaction. We use a laser to map its motion onto the internal electron state, which we detect by fluorescence detection. How cold will it be?
In the project, you will build up a laser system that can perform this mapping of motion onto the internal state. You will characterize its performance and – under guidance of the PhD student working on the experiment – be the first to see a mixture of atoms and ions in the quantum regime.
Tuning the interaction of atoms and ions: Experiment and theory
One of the crucial aspects of a quantum simulator is that it has knobs you can turn to study its properties systematically. At first sight, atoms and ions do not really seem all that tunable. The charge of the ion polarizes the atom which causes an attraction. The polarizability of the atom can be calculated or looked up in a table, but to tune it seems to be a challenge. To solve this problem, we will make use of a laser field that mixes some Rydberg character into the orbit of the electron in the atom. Since the polarizability of Rydberg atoms scales with the principal quantum number to the power 7, it can be many orders of magnitude larger than that of ground state atoms. Tuning the frequency and intensity of the laser allows us to tune the polarizability of the atom and even to make it repulsive as can be seen in our recent paper.
We have already worked out the quantum theory of a single “dressed” Rydberg atom interacting with a single ion. In your project, you will work out the theory of many dressed Rydberg atoms interacting with trapped ions. We have the following questions in mind: (1) Can we use the soundwaves in ion crystals to mediate spin-spin interactions between many atoms to simulate emergent fermion-fermion interactions. (2) What is the lifetime of such a material build up of atoms with Rydberg character interacting with ions? What are the main collisional processes? Can the system ionize or decay otherwise?
Fig: A trapped atom that is dressed to a Rydberg state interacting with a trapped ion is easily understood. But what happens when there are many atoms? In the figure, the particles are trapped in harmonic traps and each have two internal states. The Rydberg laser couples one of the atomic states to a Rydberg state, significantly increasing the polarizability. This causes the ion to move depending on the internal state of the atom (a minimal instance of spin-motion coupling). Techniques from quantum optics allow us to map this onto an atom-ion spin-spin interaction. In the project we will study how this picture caries over to the many particles scenario.
The project will be complemented with the first experiments on producing and detecting Rydberg atoms. Depending on the interest of the candidate, the project can be experiment or theory oriented or both. Experimental students will work on cavity stabilization of the Rydberg lasers before proceeding with the production and detection of Rydberg atoms. An important goal on the experimental side it to gain a clear understanding of the effect of the electric fields in the experiment on the Rydberg levels. Under guidance of the PhD student working on the experiment you may be the first to see interactions between Rydberg atoms and ions in the quantum regime.
Fig: Optical cavity system to stabilize the Rydberg lasers.