# Dipolar quantum gases## Magnetic quantum gases and liquids of Dysprosium atomsAbstract | Highlights | Research | The Crew | Refs & Publications | Open Positions | Bachelor- and Master-Topics ## Abstract
In this project we experimentally investigate the dipole-dipole interactions in quantum gases like Bose-Einstein condensates (BECs).
Even though an atomic Bose-Einstein-Condensate (BEC) is a very dilute system, the most fascinating experimental results arise from the weak interactions between the particles. In most experiments the dominating interaction in a BEC is the isotropic and short-range contact interaction that can be characterized by the s-wave scattering length a. Magnitude and sign of a can be modified using Feshbach resonances. In dipolar gases a large dipole moment results in additional long range and anisotropic interactions giving rise to novel states of matter and collective phenomena. Among those are so-called super-solids and two dimensional solitons.
In 2005 we produced the first Chromium-BEC [2] and we showed that it exhibits non-negligible dipolar effects [3]. Nevertheless, the dipole-dipole interactions were only a small perturbation compared to the usual contact interactions. After exploring a lossless cooling method for a thermal cloud [4], which exploits the fast dipolar relaxation [1], we focused on the strong-dipolar effects in the quantum degenerate regime. In 2007 we achieved the first quantum ferrofluid [5], i.e. a BEC with comparable contact and dipole-dipole interactions, and shortly afterwards a purely dipolar BEC [6]. Due to the anisotropic character of the dipole-dipole interactions the stability the BEC depends on the trap geometry [6] and shows an universal behavior in the large N regime. We have observed instabilities - the so-called dipolar collapse involving an anisotropic d-wave symmetry [7]. We have then investigated the phase coherence of collapsed condensates [8]. We induced the collapse in several condensates simultaneously and let them interfere. By observing high fringe contrast, we proved that the collapsed cloud contains a remnant condensate. Also dipolar interaction in optical lattices was observed.
Since 2014 we have replaced the Chromium quantum gas by a Dysprosium BEC. Dysprosium has the highest magnetic moment of all atoms in the periodic table and due to its larger mass a reduced contact interaction. It is therefore best suited to study the dipolar phenomena. With this new system we have observed the equivalent of the Rosensweig instability which happens for classical ferrofluids, with our dipolar BEC, a quantum ferrofluid. This instability is characterized by the formation of regular patterns of peaks and valleys in the denity of the BEC. As opposed to what was previously thought, the resulting state is actually stable, characterized by the formation of droplets [30]. These droplets are actually made of a novel quantum liquid, where the attraction due to the dipolar interaction, which tends to drive a collapse, is balanced by quantum fluctuations of the BEC field as dictated by Heisenberg uncertainty principle [31].
**A quantum ferrofluid**: The contact interaction, symbolized by the grey “hard sphere” on the schematic representation of the atoms (top), is reduced from top to bottom, while the magnetic dipole moment of the atoms (green and red arrow) stays constant. One thus enhances the effects of the dipolar interactions between the magnetic moments of the atoms. This manifests itself in an elongation of the condensate along the magnetization direction, yielding a clear change in the condensate ellipticity (false colour experimental images at the bottom).
**I****l****lustration of the behaviour of a quantum ferrofluid**: When the quantum ferrofluid is placed in a flat "pancake" trap (a, left), decreasing the contact interaction such that the gas becomes dominated by the dipolar interaction results in the formation of stable density waves and eventually of droplets (a, right and b). This is the equivalent of the rosensweig instability of classical ferrofluid.
Abstract | Highlights | Research | The Crew | Refs & Publications | Open Positions | Bachelor- and Master-Topics ## Highlights | 27.11.2017 | Striped states with tilted dipoles more... | | 10.11.2016 | Self-bound droplets of a quantum liquid more... | | 23.05.2016 | Making a quantum liquid from a quantum gas more... | | 01.02.2016 | New state of matter: Quantum gas, liquid and crystal all-in-one more... | | 14.10.2015 | Emergence of chaotic scattering in ultracold Er and Dy more... | | 29.04.2014 | Narrow-line magneto-optical trap for dysprosium atoms more... | | 16.01.2013 | Spectroscopy of a narrow-line optical pumping transition in dysprosium more... | | 09.11.2012 | Deconfinement-induced collapse of a coherent array of dipolar Bose-Einstein condensates more... | | 19.03.2009 | Artist Brigitte Simon inspired by Chrom Bose Nova more... | | 18.02.2009 | Coherent collapse of a dipolar BEC in different trapping potentials ... more... | | 19.06.2008 | On the cover of PRL: d-wave collapse and explosion of a dipolar BEC more... | | 13.05.2008 | Akademiepreis für Stuttgarter Physiker more... | | 27.02.2008 | Stabilization of a purely dipolar quantum gas against collapse more... | | 27.07.2007 | A quantum ferrofluid - creation of the first quantum gas with strong dipolar interactions, by using a Feshbach resonance in a Chromium BEC. more... | | 15.11.2006 | Demagnetization cooling of a gas more... | | 10.11.2005 | Observation of dipole-dipole interaction in a degenerate quantum gas more... | | 03.03.2005 | Here we update our previous report on Bose-Einstein condensation in a gas of chromium more... | | 26.11.2004 | Bose-Einstein Condensation of Chromium atoms more... | | 26.07.2004 | Feshbach resonances in ^{52}Cr collisions more... | | 14.07.2004 | Piet Schmidt wins prize for the best PhD thesis more... | | 15.10.2003 | Dipolar Relaxation in a magnetically trapped gas of Chromium atoms more... | | 17.04.2003 | Measurement of the scattering lengths of ^{52}Cr and ^{50}Cr more... | | 10.12.2002 | A combined trap for chromium and rubidium atoms more... | | 24.04.2002 | Theory on tunable dipolar interaction in quantum gases more... | | 14.11.2001 | First Ioffe-Pritchard trap for chromium atoms more... |
Abstract | Highlights | Research | The Crew | Refs & Publications | Open Positions | Bachelor- and Master-Topics ## Research
**Figure 1: Stability of different trap geometries:** In an pancake-shaped trap (A) the dipoles mainly repel each other, whereas in a cigar-shaped trap (B) the interaction is predominantly attractive. We find good agreement between experiment and a simple model (C). For details see [6].
**Figure 2: Interfering Chromium-BECs.**
Chromium 2008 (pdf presentation)
Chromium BEC tutorial (pdf)
Abstract | Highlights | Research | The Crew | Refs & Publications | Open Positions | Bachelor- and Master-Topics ## The CrewFabian Böttcher | (Doktorand) | | Igor Ferrier-Barbut | (Group leader) | | Tim Langen | (Group leader) | | Matthias Wenzel | (Doktorand) | |
Abstract | Highlights | Research | The Crew | Refs & Publications | Open Positions | Bachelor- and Master-Topics ## Refs & Publications[1] | S. Hensler, J. Werner, A. Griesmaier, P.O. Schmidt, A. Görlitz, T. Pfau, S. Giovanazzi and K. Rzazewski
*"Dipolar Relaxation in an ultra-cold Gas of magnetically trapped chromium atoms"*
*Appl. Phys. B ***77**, 765 (2003) | [2] | A. Griesmaier, J. Werner, S. Hensler, J. Stuhler, and T. Pfau
*"Bose-Einstein condensation of chromium"*
*Phys. Rev. Lett. ***94**, 160401 (2005) | [3] | S. Hensler, A. Greiner, J. Stuhler, and T. Pfau
*"Depolarisation cooling of an atomic cloud"*
*Europhys. Lett. ***71**, 918 (2005) | [4] | J. Stuhler, A. Griesmaier, T. Koch, M. Fattori, S. Giovanazzi, P. Pedri, L. Santos, and T. Pfau
*"Observation of Dipole-Dipole Interaction in a Degenerate Quantum Gas"*
*Phys. Rev. Lett. ***95**, 150406 (2005) | [5] | L. Santos and T. Pfau
*"Spin-3 Chromium Bose-Einstein Condensates"*
*Phys. Rev. Lett. ***96**, 190404 (2006) | [6] | A. Griesmaier, J. Stuhler, and T. Pfau
*"Production of a chromium Bose-Einstein condensate"*
*Appl. Phys. B ***82**, 211 (2006) | [7] | M. Fattori, T. Koch, S. Goetz, A. Griesmaier, S. Hensler, J. Stuhler, and T. Pfau
*"Demagnetization cooling of a gas"*
*Nature Physics ***2**, 765 (2006) | [8] | K. Glaum, A. Pelster, H. Kleinert, and T. Pfau
*"Critical Temperature of Weakly Interacting Dipolar Condensates"*
*Phys. Rev. Lett. ***98**, 080407 (2007) | [9] | S. Giovanazzi, L. Santos, and T. Pfau
*"Collective oscillations of dipolar Bose-Einstein condensates and accurate comparison between contact and dipolar interaction"*
*Phys. Rev. A ***75**, 015604 (2007); ; *arXiv:cond-mat/0608291 (2007)* | [10] | Th. Lahaye, T. Koch, B. Fröhlich, M. Fattori, J. Metz, A. Griesmaier, S. Giovanazzi, and T. Pfau
*"Strong dipolar effects in a quantum ferrofluid"*
*Nature ***448**, 672 (2007) | [11] | J. Stuhler, A. Griesmaier, J. Werner, T. Koch, M. Fattori, and T. Pfau
*"Ultracold chromium atoms: From Feshbach resonances to a dipolar Bose-Einstein condensate"*
*J. Mod Opt. ***54**, 647 (2007) | [12] | B. Fröhlich, T. Lahaye, B. Kaltenhäuser, H. Kübler, S. Müller, T. Koch, M. Fattori, and T. Pfau
*"A two-frequency acousto-optic modulator driver to improve the beam pointing stability during intensity ramps"*
*Rev. Sci. Instrum. ***78**, 043101 (2007); ; *arXiv: physics/0701183 (2007)* | [13] | L. Santos, M. Fattori, J. Stuhler, and T. Pfau
*"Spinor condensates with a laser-induced quadratic Zeeman effect"*
*Phys. Rev. A ***75**, 053606 (2007); ; *arXiv: cond-mat/0612191, (2007)* | [14] | T. Koch, T. Lahaye, J. Metz, B. Fröhlich, A. Griesmaier, and T. Pfau
*"Stabilizing a purely dipolar quantum gas against collapse"*
*Nature Physics ***4**, 218 (2008) | [15] | A. Muramatsu and T. Pfau (eds.)
*"Focus on Quantum Correlations in Tailored Matter"*
*New J. Phys. ***10**, 045001 (2008) | [16] | T. Lahaye, J. Metz, B. Fröhlich, T. Koch, M. Meister, A. Griesmaier, T. Pfau, H. Saito, Y. Kawaguchi, and M. Ueda
*"d-Wave Collapse and Explosion of a Dipolar Bose-Einstein Condensate"*
*Phys. Rev. Lett ***101**, 080401 (2008) | [17] | J. Metz, T. Lahaye, B. Fröhlich, A. Griesmaier, T. Pfau, H. Saito, Y. Kawaguchi, and M. Ueda
*"Coherent collapse of a dipolar Bose-Einstein condensate for different trap geometries"*
*New J. Phys. ***11**, 055032 (2009); *arXiv:0901.1300v1 [physics.atom-ph]* | [18] | S. Müller, J. Billy, E. A. L. Henn, H. Kadau, A. Griesmaier, M. Jona-Lasinio, and L. Santos
*"Stability of a dipolar Bose-Einstein condensate in a one-dimensional lattice"*
*Phys. Rev. A ***84**, 053601 (2011)*;* doi: 10.1103/PhysRevA.84.053601 | [19] | A. Maluckov, G. Gligoric, L. Hadzievski, B. A. Malomed, and T. Pfau
*"Stable periodic density waves in dipolar Bose-Einstein condensates trapped in optical
lattices"*
*Phys. Rev. Lett. ***108**, 140402 (2012) | [20] | D. Peter, K. Pawłowski, T. Pfau, and K. Rzążewski
*"Mean-field description of dipolar bosons in triple-well potentials"*
*J. Phys. B: At. Mol. Opt. Phys. ***45**, 225302 (2012)*;* doi: 10.1088/0953-4075/45/22/225302 | [21] | J. Billy, E. A. L. Henn, S. Müller, T. Maier, H. Kadau, A. Griesmaier, M. Jona-Lasinio, L. Santos, and T. Pfau
*"Deconfinement-induced collapse of a coherent array of dipolar Bose-Einstein condensates"*
*Phys. Rev. A ***86**, 051603(R) (2012)*;* doi: 10.1103/PhysRevA.86.051603 | [22] | D. Peter, A. Griesmaier, T. Pfau, and H. P. Büchler
*"Driving dipolar fermions into the quantum Hall regime by spin-flip induced insertion of angular momentum"* Phys. Rev. Lett. **110**, 145303 (2013); *arXiv:1302.1308 [cond-mat.quant-gas]*; **(editor** | [23] | A. Maluckov, G. Gligoric, L. Hadzievski, B. A. Malomed, and T. Pfau
*"High- and low-frequency phonon modes in dipolar quantum gases trapped in deep lattices"*
*Phys. Rev. A *** 87**, 023623 (2013); *arXiv:1302.2410* | [24] | M. Schmitt, E. A. L. Henn, J. Billy, H. Kadau, T. Maier, A. Griesmaier, and T. Pfau
*"Spectroscopy of a narrow-line optical pumping transition in dysprosium"*
*Opt. Lett. ***38**, 637 (2013)*;* doi: 10.1364/OL.38.000637 | [25] | K. Pawlowski, P. Bienias, T. Pfau and K. Rzazewski
*"Correlations of a quasi-two-dimensional dipolar ultracold gas at finite temperatures"*
*Phys. Rev. A ***87**, 043620*;* doi: 10.1103/PhysRevA.87.043620 | [26] | P. Bienias, K. Pawlowski, T. Pfau and K. Rzazewski
*"Ground state of a two component dipolar Fermi gas in a harmonic
potential"* Phys. Rev. A **88**, 043604 (2013); arXiv:1308.0567*;* doi: 10.1103/PhysRevA.88.043604 | [27] | T. Maier, H. Kadau, M. Schmitt, A. Griesmaier, and T. Pfau
*"Narrow-line magneto-optical trap for dysprosium atoms"*
*Optics Letters ***39 **, 3138(2014); *arXiv:1403.1499* | [28] | T. Maier, H. Kadau, M. Schmitt, M. Wenzel, I. Ferrier-Barbut, T. Pfau, A. Frisch, S. Baier, K. Aikawa, L. Chomaz, M. J. Mark, F. Ferlaino, C. Makrides, E. Tiesinga, A. Petrov, S. Kotochigova
*"Emergence of chaotic scattering in ultracold Er and Dy"*
*Phys. Rev. X ***5**, 041029 (2015) | [29] | T. Maier, I. Ferrier-Barbut, H. Kadau, M. Schmitt, M. Wenzel, C. Wink, T. Pfau, K. Jachymski, P. S. Julienne
*"Broad Feshbach resonances in collisions of ultracold Dysprosium atoms"* Phys. Rev. A **92**, 060702(R) (2015); arXiv:1506.01875 | [30] | H. Kadau, M. Schmitt, M. Wenzel, C. Wink, T. Maier, I. Ferrier-Barbut, T. Pfau
*"Observing the Rosensweig instability of a quantum ferrofluid"*
*Nature ***530**, 194 (2016), arXiv:1508.05007*;* doi: 10.1038/nature16485 | [31] | I. Ferrier-Barbut, H. Kadau, M. Schmitt, M. Wenzel, T. Pfau
*"Observation of quantum droplets in a strongly dipolar Bose gas"* Phys. Rev. Lett. 116, 215301 (2016)*;* doi: 10.1103/PhysRevLett.116.215301 | [32] | I. Ferrier-Barbut, M. Schmitt, M. Wenzel, H. Kadau and T. Pfau
*"Liquid quantum droplets of ultracold magnetic atoms"*
*J. Phys. B: At. Mol. Opt. Phys *** 49, **214004 (2016), arXiv:1607.07355*;* doi: 10.1088/0953-4075/49/21/214004 | [33] | M. Schmitt, M. Wenzel, B. Böttcher, I. Ferrier-Barbut, T. Pfau
*"Self-bound droplets of a dilute magnetic quantum liquid"*
*Nature ***539**, 259 (2016), arXiv:1607.07355*;* doi: 10.1038/nature20126 | [34] | Igor Ferrier-Barbut
*"Smashing magnets"*
*New Journal of Physics ***18**, 111004 (2016)*;* doi: 10.1088/1367-2630/18/11/111004 | [35] | M. Wenzel, F. Böttcher, T. Langen, I. Ferrier-Barbut, T. Pfau
*"Striped states in a many-body system of tilted dipoles"*
*Phys. Rev. A *** ****96** 053630 (2017)*;* doi: 10.1103/PhysRevA.96.053630 | [36] | Igor Ferrier-Barbut and Tilman Pfau
*"Quantum liquids get thin"*
*Science*** 359,**274 (2018)*;* doi: 10.1126/science.aar3785 |
Abstract | Highlights | Research | The Crew | Refs & Publications | Open Positions | Bachelor- and Master-Topics ## Open Positions
We are planning a new setup for a degenerate Dysprosium gases and are looking for
If you are interested in working on exotic states of matter and like to setup new things, contact Igor Ferrier-Barbut or Tilman Pfau via email/phone or just come by and visit our labs.
Interested? Here you find the recent papers on our quantum liquid
M. Schmitt, M. Wenzel, B. Böttcher, I. Ferrier-Barbut, T. Pfau
*Self-bound droplets of a dilute magnetic quantum liquid*
*Nature ***539**, 259 (2016), arXiv:1607.07355
I. Ferrier-Barbut, H. Kadau, M. Schmitt, M. Wenzel, T. Pfau
*"Observation of quantum droplets in a strongly dipolar Bose gas"*
Phys. Rev. Lett. 116, 215301 (2016)
Abstract | Highlights | Research | The Crew | Refs & Publications | Open Positions | Bachelor- and Master-Topics ## Bachelor- and Master-Topics
**Collimation cooling and Zeeman slower for a new dysprosium quantum gas experiment (Master thesis)**
We are hiring a Master student for the design and setup of a colimation cooling stage as well as a Zeeman slower for Dy atoms. These two parts will be the first blocks of a new experimental setup aimed at cooling and trapping deeply degenerate Dy atoms in optical lattices. The atoms initially come out of an effusion cell at very high temperatures (1200 degrees Celsius) in a directed jet. Due to the high temperature, the jet has a high divergence so the first essential tool is a laser cooling stage which cools-down the motion of the atoms in the transverse direction. This is done by Doppler cooling, red-detuned from the main transition of Dy at 421nm. The following block is a Zeeman slower which slows-down the axial velocity of the jet, so that it can be trapped and further cooled in a magneto-optical trap. This Zeeman slower uses magnetic coils to keep the atoms close to resonance with the light frequency, compensating for the Doppler effect via the Zeeman effect. The laser light used will be also on the 421nm transition.
The goal of the Master thesis is to design the colimation cooling scheme as well as the whole Zeeman slower, including the magnetic field configuration in order to obtain the highest flux of cold Dy atoms. Ingenious designs allowing for instance to trap two isotopes simultaneously will be explored.
**Optical lattice for long-range interacting dysprosium quantum many-body systems (Master thesis)**
In this project we are planning to design and build an optical lattice: a standing wave that acts as a periodic potential for ultracold atoms. This will allows us to experimentally study lattice models of condensed matter physics. The goal of the thesis will be to design the optical lattice in terms of wavelength(s), power and geometry. Dysprosium atoms, thanks to their strong magnetic moment offer the possibility to study long-range interacting many-body systems. The candidate will design the lattice based on the knowledge of atom-light interaction with dysprosium, influenced by its rich level structure, in order to realize strongly interacting long-range systems. The thesis will go on with the building and implementation of the optical lattice on the experimental setup.
**Simulations of beyond-mean-field dipolar Bose-Eintein condensates (Master thesis)**
The current state-of-the-art theoretical model to describe strongly dipolar Bose-Eintein condensates of Dysprosium is the so-called extended Gross-Pitaevskii equation. This equation is based on the mean-field Gross-Pitaevskii equation including the long-range anisotropic dipole-dipole interaction, to which an effective term is added to take into account the effect of beyond-mean-field corrections. These correction arise from quantum fluctuations in the fluid and act as an effective extra non-linearity. The goal of this project is to perform simulations of this equation to compare to experiments in order to test the thepry at the current level and make useful predictions for our experiments on Dysprosium. These numerical simultions are developped in our group, and they allow to implement the exact experimental conditions.
Contacts:
I. Ferrier-Barbut, T.Pfau
**Modulation of an optical dipole trap to optimize loading of Dy atoms from a magneto-optical trap (Bachelor thesis)**
In this project, we will use an accousto-optic deflector to control the spatial mode of an optical dipole trap. This will be designed to maximize the loading of the atoms from a magneto-optical trap into the optical dipole trap. Once the atoms are loaded in the optical dipole trap the beam will be used to transport the atoms from the magneto-optica trap region to a science glass cell. The whole design will combine the spatial mode control using the accousto-optical deflector, and the mobile optics to control the position of the focus to transport the atoms.
**Optical dipole trap for dysprosium atoms (Bachelor thesis)**
We are looking for a Bachelor student to design and build an optical dipole trap for dyprosium atoms. This trap will be based on a narrow-line infrared (1064nm) laser with high power (50W). The goal of the project is to design the optical system to split the dipole trap in two and fiber-couple it in order to create a crossed beam configuration to provide a trap with tighter confinement for the atoms. The project will involve optics and electronics for the control and active feedback of the beam powers.
Contacts:
I. Ferrier-Barbut, T.Pfau
Abstract | Highlights | Research | The Crew | Refs & Publications | Open Positions | Bachelor- and Master-Topics |