Dipolar Quantum Gases

In 2005, we produced the first Bose-Einstein condensate (BEC) of chromium atoms. This was the first example of a BEC with a non-negligible dipolar interaction. While in the initial experiments the dipole-dipole interaction was still only a small perturbation compared to the usual contact interactions, only a few years later a purely dipolar BEC was achieved by lowering the contribution of the contact interaction using a so-called Feshbach resonance. In this setting, the anisotropic nature of the dipole-dipole interaction led to a trap geometry dependent stability diagram, and a dipolar collapse with an anisotropic d-wave symmetry.

Since 2014, we are working with a new atomic species and are able to create quantum gases of dysprosium atoms. Dysprosium belongs to the group of rare-earth lanthanides and has the highest magnetic moment of all atoms in the periodic table. Compared to the usual alkali-metal atoms used in other BEC experiments, the dipole-dipole interaction between dysprosium atoms is enhanced by a factor of ~100. Therefore, dysprosium atoms are ideally suited to study the dipolar phenomena.

In order to create degenerate gases of dysprosium atoms, the atoms first have to be evaporated at about 1200°C in a high temperature effusion cell. The atoms in the resulting hot atomic beam are cooled with a Zeeman slower and subsequently further cooled and trapped in a magneto-optical trap (MOT). The cold temperature of a few micro Kelvin in our narrow-line MOT directly allows us to transfer the atoms into an optical dipole trap (ODT) in which quantum degeneracy can be achieved by evaporative cooling of the atoms. Using this experimental sequence, we can generate quantum gases with up to 50.000 atoms. These degenerate quantum gases then form the starting point for our experiments.

• Griesmaier et al., "Bose-Einstein condensation of chromium",
  Phys. Rev. Lett. 94, 160401 (2005).
• Lahaye et al., "Strong dipolar effects in a quantum ferrofluid",
  Nature 448, 672 (2007).
• Lahaye et al., "d-Wave Collapse and Explosion of a Dipolar Bose-Einstein Condensate",
  Phys. Rev. Lett. 101, 080401 (2008).

In 2015 we discovered the self-organization of our dipolar BEC into an array of smaller droplets. This behavior was very similar to the Rosensweig instability observed in classical ferrofluids. However, our observation came unexpected, as all theories available at the time predicted a collapse of the BEC. Nevertheless, these droplets remained stable and could be observed over several hundreds of milliseconds in our apparatus. Based on our observation, it was shown theoretically and experimentally shortly afterwards that the observed droplets are stabilized by quantum fluctuations. Such fluctuations are a result of the Heisenberg uncertainty principle. They are a very small effect that can typically be neglected. In dipolar quantum gases, they can nevertheless play a decisive role because of the complex interplay of contact and dipolar interactions.

• Kadau et al., "Observing the Rosensweig instability of a quantum ferrofluid",
  Nature 530, 194 (2016).
• Böttcher et al., "New states of matter with fine-tuned interactions: quantum droplets and dipolar supersolids",
  Reports on Progress in Physics 84, 012403 (2021).

Similar to classical liquids, in which the liquid properties arise from the interplay between attractive inter-particle interactions and a strong short-range repulsion due to the Pauli principle, quantum droplets exhibit liquid-like properties due to the interplay between attractive mean-field interactions and repulsive quantum fluctuations. The most striking consequence is the self-bound nature of these quantum droplets. As a gaseous phase, a typical BEC needs a container to prevent it from expanding indefinitely. In contrast to this, quantum droplets constitute a self-bound state that remains bound even in the absence of an external confinement. They thus behave like a liquid, allthough they are more than a million times more dilute.  In the experiment, we were able to prove their self-bound nature by observing levitated quantum droplets for up to 100 milliseconds even without the presence of a trap.

• Schmitt et al., "Self-bound droplets of a dilute magnetic quantum liquid",
  Nature 539, 259 (2016).
• Ferrier-Barbut et al., "Observation of quantum droplets in a dipolar Bose gas",
  Phys. Rev. Lett. 116, 215301 (2016).

In 2019, based on our previous discovery of stable quantum droplets, we succeeded in proving experimentally that the long-sought supersolid state of matter exists. A supersolid is a counterintuitive phase of matter, which simultaneously features both the frictionless flow of a superfluid, as well as the crystalline structure of a solid.

The definitive proof that the regular patterns observed in the experiment form indeed a supersolid relies on our observation of two kinds of sound waves. Such sound waves propagate differently in different materials – in air, for example, sound waves travel much slower than in water. This "normal" sound wave is also present in the supersolid. However, because the supersolid is both solid and fluid, a characteristic second form of sound wave can be observed, in which the crystal and the superfluid move against each other. This results in characteristic sound waves (so called low-energy Goldstone modes) that travel at very low speeds, which we were able to observe for the first time in our experiment.

Furthermore, we were able to investigate the nature of the superfluid-supersolid phase transition by looking at the dynamical emergence of density fluctuations in the system. These density fluctuations are dominated by the roton excitations, which are periodic density modulations on top of the BEC. The softening of these roton excitations eventually trigger the crystallization process that forms the supersolid crystal. In the future, we plan to investigate the possibility of creating a supersolid where the droplets self-assemble in two dimensions. The increased complexity of this higher dimensionality offers the possibility of discovering many more new states of matter.

• Böttcher et al., "Transient Supersolid Properties in an Array of Dipolar Quantum Droplets",
  Phys. Rev. X 9, 011051 (2019).
• Guo et al., "The low-energy Goldstone mode in a trapped dipolar supersolid",
  Nature 574, 386--389 (2019).
• Hertkorn et al., "Density Fluctuations across the Superfluid-Supersolid Phase Transition in a Dipolar Quantum Gas",
  Phys. Rev. X, in print (2021).

We are currently also setting up a next-generation experiment for the production of quantum degenerate dysprosium gases. With this new experimental setup, the production of significantly larger BECs will become possible, thus allowing for larger droplet crystals and the study of more complex states of matter. In addition, we plan to implement an optical lattice in the near UV, which will lead to very strong dipole-dipole interactions between atoms on adjacent lattice sites. In combination with a novel quantum gas microscopy scheme based on a long-lived intermediate state in dysprosium, we will be able to resolve single atoms in the lattice with a single site resolution. This single site resolved imaging combined with long coherence times will allow us to study Fermi- and Bose-Hubbard model physics enriched by the long-range and anisotropic dipolar interactions.

The 5th Institute of Physics is looking for students who would like to experience everyday work in a modern quantum optics laboratory.

A variety of techniques and skills will be used:

• CAD Modelling / 3D printing,
• electronics,
• usage of optic devices/laser,
• measuring techniques.

Previous experience with these techniques is an advantage but not a requirement.

Salary: regular Hiwi salary per hour, hours per week and schedule upon individual arrangement.

Interested candidates should contact Tilman Pfau.

Are you interested in studying novel and unexpected quantum phenomena in dipolar quantum gases? Our group is currently looking for students to join our team studying novel phases of matter, such as quantum droplets and dipolar supersolids.

We are currently setting up a new experimental setup that will allow us to create quantum degenerate systems with a significantly larger atom number. This will facilitate the realization of new exotic phases of matter, which are predicted to emerge at higher atom numbers than currently achievable in our setup. In addition, we are planning to study the effect of a large nearest neighbor interaction in lattice systems by implementing a UV optical lattice. By implementing a novel quantum gas microscopy scheme based on a long-lived excited state in dysprosium, we expect to attain single-site resolution in a system with a small lattice spacing of only ~180nm. With this we plan to study Fermi- and Bose-Hubbard model physics enriched by long-range and anisotropic dipolar interactions.

If you are interested in learning more, please contact Tilman Pfau.

Many-body physics with dipolar quantum gases

In this project you will study superfluidity and many-body phases in dipolar quantum gases. The project will involve experimental research using ultracold dysprosium atoms imaged under a high-resolution optical microscope. Part of the experimental work will be conducted at the University of Stuttgart, Germany, and part at Swinburne University of Technology, Melbourne, Australia. This unique scholarship opportunity is supported by an established Swinburne/Stuttgart joint PhD programme and will provide experiences working in two leading research laboratories.

Candidates will enrole at both participating universities and receive a stipend from Swinburne University for up to 3,5 years.

The thesis will be supervised by Prof. Tilman Pfau (Stuttgart), Prof. Christopher Vale (Swinburne), Dr. Sascha Hoinka (Swinburne).

Please contact Tilman Pfau (University of Stuttgart) and Christopher Vale (Swinburne University of Technology) if you are interested in this opportunity.

Please contact Tilman Pfau if you are interested in joining our team as a Postdoc!