Dipolar quantum gases in optical lattices

Working on this experiment: Niels PetersenMarcel Trümper, Christina Meyer

Introduction

Bild2Ultra-cold dipolar quantum gases enable the study of many-body physics with long-range, inhomogeneous interaction effects due to the anisotropic character of the dipole-dipole interaction. These systems are expected to show novel exotic quantum phases and phase transitions which can be investigated e.g. with dysprosium atoms. In this project we are setting up a new experiment to generate and study strongly dipolar quantum gases of dysprosium. The experimental apparatus will be designed for trapping and cooling both bosonic and fermionic atoms.

 

Dipole-Dipole Interaction

DDI Compared to “standard” ultra-cold gases of alkali atoms, which are dominated by short-range, isotropic s-wave scattering, atomic systems that additionally interact via dipole-dipole interaction (DDI) show long-range, anisotropic interaction effects. For two polarized dipoles the interaction potential Udip depends on the distance r and the angle ϑ between their relative positions resulting either in a repulsive or an attractive interaction. The dipole potential decays like ~1/r3 which gives rise to its long-range nature. This anisotropy and the long-range character of the DDI lead to a strong dependence of the quantum gases’ properties on the trap geometry which significantly influences the stability of the trapped atomic cloud. Moreover, the dipole potential scales quadratically with the magnetic moment µ of the atoms. Thus, the choice of atomic species, and hence µ, is crucial to investigate interesting new physics with dipolar gases of neutral atoms. Considering these aspects, dysprosium is an ideal choice for these types of experiments as it is the element with the highest magnetic moment in the periodic table.

 

Dysprosium

Dysprosium
As an atomic species we will use the element dysprosium (Dy) which, with an atomic number of Z = 66, belongs to the group of rare-earth lanthanides. It is a silvery grey metal with large atomic mass (162.5 u) and a high melting point of ~1400 °C.

Dy level scheme
Dysprosium features several stable isotopes, the most abundant being the two bosonic isotopes 162Dy and 164Dy and the two fermionic ones 161Dy and 163Dy with nearly equally high natural abundances. Both bosonic isotopes possess a nuclear spin equal to zero (I=0) whereas the fermions have a nuclear spin of I = 5/2. The almost equally high relative abundances of these four isotopes allow for the generation of BECs, ultra-cold Fermi Gases and Bose-Fermi-mixtures. The electronic ground-state configuration of Dy is [Xe]4f106s2, with spin S = 2, orbital angular momentum L = 6 and total angular momentum J = 8. This open f-shell configuration leads to a very rich and complicated energy spectrum with a variety of atomic transitions that are suitable for laser cooling and performing quantum gas experiments. More importantly, it is the origin of Dy’s large ground-state magnetic moment which, with a value of about 10 Bohr-magnetons (µ~10μB), is the largest magnetic moment of all elements in the periodic table. Compared to alkali atoms (µ~1μB) the DDI in ultra-cold gases of Dy is about 100 times stronger.

 

Cooling and trapping of dysprosium

Cooling and trapping of Dy can be realized in several sequential steps: Initially the dysprosium atoms are evaporated in a high temperature effusion cell (EC). Due to its high melting point, temperatures of about 1200°C are needed to evaporate a sufficient number of dysprosium atoms for performing quantum gas experiments. As a result of these high temperatures, Dy atoms are leaving the effusion cell with a very high velocity of about 500 m/s. The atomic beam is then collimated by a set of apertures and transversally cooled (TC) to reduce the beam divergence. The next cooling step is done inside a Zeeman slower in which the atoms are slowed down to mean velocities of about 15 m/s. In a Zeeman slower a spatially varying magnetic field is combined with a laser beam to realize one-dimensional laser cooling. After being precooled in the Zeeman slower the atoms will be trapped inside a magneto-optical trap (MOT), where they are further cooled to temperatures in the micro Kelvin range. This allows for the transfer into an optical dipole trap (ODT) where the final evaporation to quantum degeneracy is achieved.
Setup
To realize these steps several different technologies have to be developed and combined into the experimental apparatus. The laser system has to deliver laser radiation at the right frequencies with the right intensities to address all relevant atomic transitions of Dy. Our experiment utilizes the blue transition at 421 nm for transverse cooling, the Zeeman slower and for absorption imaging while the MOT is operated on the orange 626 nm transition. The ODT is generated by a high power 1064 nm laser system. Additionally an ultra-high vacuum chamber is required to cool, trap and investigate ensembles of Dy atoms.

 

News from the lab

Dysprosium MOT
Our first Dysprosium-162 MOT

spectrum Dysprosium
Recently, we measured the spectrum of Dysprosium on the 421 nm cooling transition with our homebuilt laser system. After amplifying the master laser seed in a homebuilt tapered amplifier stage, it is frequency doubled in a homebuilt bow tie cavity which delivers up to 460 mW of blue laser power. The five most abundant isotopes of Dysprosium can clearly be identified in the spectrum shown below. The inset shows a picture of our main vacuum chamber where a beam of Dysprosium atoms crosses the spectroscopy beam.

 

Bachelor / Master Theses

We always offer projects both for bachelor and master theses. If you are interested in joining our team as a bachelor or master student, please contact Prof. Patrick Windpassinger or either of the PhD students.

 

Finished Theses

Aufbau und Stabilisierung eines ECD-Lasers, bachelor thesis by Ansgar Schaefer (2015)

Summenfrequenzerzeugung von 626 nm Laserlicht, bachelor thesis by Lena Maske (2016)

Laserkühlen von Dysprosium, master thesis by Carina Baumgärtner (2017)

Spektroskopie des 1001nm Übergangs von Dysprosium, master thesis by Lena Maske (2018)

Aufbau und Charakterisierung einer optischen Dipolfalle für Dysprosium, bachelor thesis by Gunther Türk (2018)

Laserintensitätsstabilisierung mit dem Red Pitaya, bachelor thesis by Jonas Lehnen (2018)