
A. Patscheider, B. Yang, G. Natale, D. Petter, L. Chomaz, M. J. Mark, G. Hovhannesyan, M. Lepers, F. Ferlaino Observation of a narrow innershell orbital transition in atomic erbium at 1299 nm,
Phys. Rev. Research 3 33256 (20210917),
http://dx.doi.org/10.1103/PhysRevResearch.3.033256 doi:10.1103/PhysRevResearch.3.033256 (ID: 720648)
Toggle Abstract
We report on the observation and coherent excitation of atoms on the narrow innershell orbital transition, connecting the erbium ground state [Xe]4f12(3H6)6s2 to the excited state [Xe]4f11(4I15/2)05d(5D3/2)6s2(15/2,3/2)07. This transition corresponds to a wavelength of 1299 nm and is optically closed. We perform highresolution spectroscopy to extract the gJfactor of the 1299nm state and to determine the frequency shift for four bosonic isotopes. We further demonstrate coherent control of the atomic state and extract a lifetime of 178(19) ms which corresponds to a linewidth of 0.9(1) Hz. The experimental findings are in good agreement with our semiempirical model. In addition, we present theoretical calculations of the atomic polarizability, revealing several different magicwavelength conditions. Finally, we make use of the vectorial polarizability and confirm a possible magic wavelength at 532 nm.

A. Trautmann, M. J. Mark, P. Ilzhöfer, H. Edri, A. El Arrach, J. G. Maloberti, C. Greene, F. Robicheaux, F. Ferlaino Spectroscopy of Rydberg States in Erbium using Electromagnetically Induced Transparency,
Phys. Rev. Research 3 33165 (20210819),
http://dx.doi.org/10.1103/PhysRevResearch.3.033165 doi:10.1103/PhysRevResearch.3.033165 (ID: 720647)
Toggle Abstract
We present a study of the Rydberg spectrum in 166Er for series connected to the 4f12(3H6)6s, Jc=13/2 and Jc=11/2 ionic core states using an alloptical detection based on electromagnetically induced transparency in an effusive atomic beam. Identifying approximately 550 individual states, we find good agreement with a multichannel quantum defect theory (MQDT) which allows assignment of most states to ns or nd Rydberg series. We provide an improved accuracy for the lowest two ionization thresholds to EIP,Jc=13/2=49260.750(1)cm1 and EIP,Jc=11/2=49701.184(1)cm1 as well as the corresponding quantum defects for all observed series. We identify Rydberg states in five different isotopes, and states between the two lowest ionization thresholds. Our results open the way for future applications of Rydberg states for quantum simulation using erbium and exploiting its special openshell structure.

M. Norcia, C. Politi, L. Klaus, E. Poli, M. Sohmen, M. J. Mark, R. N. Bisset, L. Santos, F. Ferlaino Twodimensional supersolidity in a dipolar quantum gas,
Nature 596 361 (20210818),
http://dx.doi.org/10.1038/s41586021037257 doi:10.1038/s41586021037257 (ID: 720624)

D. Petter, A. Patscheider, G. Natale, M. J. Mark, M. Baranov, R. van Bijnen, S. M. Roccuzzo, A. Recati, B. Blakie, D. Baillie, L. Chomaz, F. Ferlaino Bragg scattering of an ultracold dipolar gas across the phase transition from BoseEinstein condensate to supersolid in the freeparticle regime,
Phys. Rev. A 104 L011302 (20210722),
http://dx.doi.org/10.1103/PhysRevA.104.L011302 doi:10.1103/PhysRevA.104.L011302 (ID: 720484)
Toggle Abstract
We present an experimental and theoretical study of the highenergy excitation spectra of a dipolar supersolid. Using Bragg spectroscopy, we study the scattering response of the system to a highenergy probe, enabling measurements of the dynamic structure factor. We experimentally observe a continuous reduction of the response when tuning the contact interaction from an ordinary BoseEinstein condensate to a supersolid state. Yet the observed reduction is faster than the one theoretically predicted by the BogoliubovdeGennes theory. Based on an intuitive semianalytic model and realtime simulations, we primarily attribute such a discrepancy to the outofequilibrium phase dynamics, which although not affecting the system global coherence, reduces its response.

M. Sohmen, C. Politi, L. Klaus, L. Chomaz, M. J. Mark, M. Norcia, F. Ferlaino Birth, life, and death of a dipolar supersolid,
Phys. Rev. Lett. 126 233401 (20210607),
http://dx.doi.org/10.1103/PhysRevLett.126.233401 doi:10.1103/PhysRevLett.126.233401 (ID: 720625)
Toggle Abstract
In the short time since the first observation of supersolid states of ultracold dipolar atoms, substantial progress has been made in understanding the zerotemperature phase diagram and lowenergy excitations of these systems. Less is known, however, about their finitetemperature properties, particularly relevant for supersolids formed by cooling through direct evaporation. Here, we explore this realm by characterizing the evaporative formation and subsequent decay of a dipolar supersolid by combining highresolution intrap imaging with timeofflight observables. As our atomic system cools toward quantum degeneracy, it first undergoes a transition from thermal gas to a crystalline state with the appearance of periodic density modulation. This is followed by a transition to a supersolid state with the emergence of longrange phase coherence. Further, we explore the role of temperature in the development of the modulated state.

P. Ilzhöfer, M. Sohmen, G. Durastante, C. Politi, A. Trautmann, G. Morpurgo, T. Giamarchi, L. Chomaz, M. J. Mark, F. Ferlaino Phase coherence in outofequilibrium supersolid states of ultracold dipolar atoms,
Nature Phys. 17 361 (20210104),
http://dx.doi.org/10.1038/s41567020011003 doi:10.1038/s41567020011003 (ID: 720437)

M. Norcia, E. Poli, C. Politi, L. Klaus, T. Bland, M. J. Mark, L. Santos, R. N. Bisset, F. Ferlaino Can angular oscillations probe superfluidity in dipolar supersolids?,
(20211115),
arXiv:2111.07768 arXiv:2111.07768 (ID: 720697)
Toggle Abstract
Angular oscillations can provide a useful probe of the superfluid properties of a system. Such
measurements have recently been applied to dipolar supersolids, which exhibit both density modulation and phase coherence, and for which robust probes of superfluidity are particularly interesting.
So far, these investigations have been confined to linear droplet arrays. Here, we explore angular
oscillations in systems with 2D structure, which in principle have greater sensitivity to superfluidity.
Surprisingly, in both experiment and simulation, we find that the frequency of angular oscillations
remains nearly unchanged even when the superfluidity of the system is altered dramatically. This
indicates that angular oscillation measurements do not always provide a robust experimental probe
of superfluidity with typical experimental protocols.

C. Politi, A. Trautmann, P. Ilzhöfer, G. Durastante, M. J. Mark, M. Mondugno, F. Ferlaino Study of the interspecies interactions in an ultracold dipolar mixture,
(20211019),
arXiv:2110.09980 arXiv:2110.09980 (ID: 720698)
Toggle Abstract
We experimentally and theoretically investigate the influence of the dipoledipole interactions
(DDIs) on the total interspecies interaction in an erbiumdysprosium mixture. By rotating the
dipole orientation we are able to tune the effect of the longrange and anisotropic DDI, and therefore
the intrap clouds displacement. We present a theoretical description for our binary system based
on an extended GrossPitaevskii (eGP) theory, including the singlespecies beyond meanfield terms,
and we predict a lower and an upper bound for the interspecies scattering length a12. Our work is a
first step towards the investigation of the experimentally unexplored dipolar miscibilityimmiscibility
phase diagram and the realization of quantum droplets and supersolid states with heteronuclear
dipolar mixtures.

E. Poli, T. Bland, C. Politi, L. Klaus, M. Norcia, F. Ferlaino, R. N. Bisset, L. Santos Maintaining supersolidity in one and two dimensions,
(20210805),
arXiv:2108.02682 arXiv:2108.02682 (ID: 720679)
Toggle Abstract
We theoretically investigate supersolidity in threedimensional dipolar BoseEinstein condensates. We focus on the role of trap geometry in determining the dimensionality of the resulting droplet arrays, which range from onedimensional to zigzag, through to twodimensional supersolids in circular traps. Supersolidity is well established in onedimensional arrays, and may be just as favorable in twodimensional arrays provided that one appropriately scales the atom number to the trap volume. We develop a tractable variational modelwhich we benchmark against full numerical simulationsand use it to study droplet crystals and their excitations. We also outline how exotic ring and stripe states may be created with experimentallyfeasible parameters. Our work paves the way for future studies of twodimensional dipolar supersolids in realistic settings.

T. Bland, E. Poli, C. Politi, L. Klaus, M. Norcia, F. Ferlaino, L. Santos, R. N. Bisset Twodimensional supersolidity in a circular trap,
(20210714),
arXiv:2107.06680 arXiv:2107.06680 (ID: 720678)

L. R. Picard, M. J. Mark, F. Ferlaino, R. van Bijnen Deep LearningAssisted Classification of SiteResolved Quantum Gas Microscope Images,
Measurement Science and Technology 31 25201 (20191105),
http://dx.doi.org/10.1088/13616501/ab44d8 doi:10.1088/13616501/ab44d8 (ID: 720262)

G. Natale, R. van Bijnen, A. Patscheider, D. Petter, M. J. Mark, L. Chomaz, F. Ferlaino Excitation spectrum of a trapped dipolar supersolid and its experimental evidence,
Phys. Rev. Lett. 123 50402 (20190801),
http://dx.doi.org/10.1103/PhysRevLett.123.050402 doi:10.1103/PhysRevLett.123.050402 (ID: 720313)
Toggle Abstract
We study the spectrum of elementary excitations of a trapped dipolar Bose gas across the superfluidsupersolid phase transition. Our calculations, accounting for the experimentally relevant case of confined systems, show that, when entering the supersolid phase, two distinct excitation branches appear, respectively connected to crystal or superfluid orders. These results confirm infinitesystem predictions, showing that finitesize effects play only a small qualitative role. Experimentally, we probe compressional excitations in an Er quantum gas across the phase diagram. While in the BEC regime the system exhibits an ordinary quadrupole oscillation, in the supersolid regime, we observe a striking twofrequency response of the system, related to the two spontaneously broken symmetries.

D. Petter, G. Natale, R. van Bijnen, A. Patscheider, M. J. Mark, L. Chomaz, F. Ferlaino Probing the roton excitation spectrum of a stable dipolar Bose gas,
Phys. Rev. Lett. 122 183401 (20190508),
http://dx.doi.org/10.1103/PhysRevLett.122.183401 doi:10.1103/PhysRevLett.122.183401 (ID: 720098)
Toggle Abstract
We measure the excitation spectrum of a stable dipolar BoseEinstein condensate over a wide momentumrange via Bragg spectroscopy. We precisely control the relative strength, εdd, of the dipolar to the contact interactions and observe that the spectrum increasingly deviates from the linear phononic behavior for increasing εdd. Reaching the dipolar dominated regime εdd>1, we observe the emergence of a roton minimum in the spectrum and its softening towards instability. We characterize how the excitation energy and the strength of the densitydensity correlations at the roton momentum vary with εdd. Our findings are in excellent agreement with numerical calculations based on meanfield Bogoliubov theory. When including beyondmeanfield corrections, in the form of a LeeHuangYang potential, we observe a quantitative deviation from the experiment, questioning the validity of such a description in the roton regime.

L. Chomaz, D. Petter, P. Ilzhöfer, G. Natale, A. Trautmann, C. Politi, G. Durastante, R. van Bijnen, A. Patscheider, M. Sohmen, M. J. Mark, F. Ferlaino LongLived and Transient Supersolid Behaviors in Dipolar Quantum Gases,
Phys. Rev. X 9 21012 (20190419),
http://dx.doi.org/10.1103/PhysRevX.9.021012 doi:10.1103/PhysRevX.9.021012 (ID: 720203)
Toggle Abstract
By combining theory and experiments, we demonstrate that dipolar quantum gases of both 166Er and 164Dy support a state with supersolid properties, where a spontaneous density modulation and a global phase coherence coexist. This paradoxical state occurs in a well defined parameter range, separating the phases of a regular BoseEinstein condensate and of an insulating droplet array, and is rooted in the roton mode softening, on the one side, and in the stabilization driven by quantum fluctuations, on the other side. Here, we identify the parameter regime for each of the three phases. In the experiment, we rely on a detailed analysis of the interference patterns resulting from the free expansion of the gas, quantifying both its density modulation and its global phase coherence. Reaching the phases via a slow interaction tuning, starting from a stable condensate, we observe that 166Er and 164Dy exhibit a striking difference in the lifetime of the supersolid properties, due to the different atom loss rates in the two systems. Indeed, while in 166Er the supersolid behavior only survives a few tens of milliseconds, we observe coherent density modulations for more than 150ms in 164Dy. Building on this long lifetime, we demonstrate an alternative path to reach the supersolid regime, relying solely on evaporative cooling starting from a thermal gas.

A. Trautmann, P. Ilzhöfer, G. Durastante, C. Politi, M. Sohmen, M. J. Mark, F. Ferlaino Dipolar Quantum Mixtures of Erbium and Dysprosium Atoms,
Phys. Rev. Lett. 121 213601 (20181121),
http://dx.doi.org/10.1103/PhysRevLett.121.213601 doi:10.1103/PhysRevLett.121.213601 (ID: 720050)
Toggle Abstract
We report on the first realization of heteronuclear dipolar quantum mixtures of highly magnetic erbium and dysprosium atoms. With a versatile experimental setup, we demonstrate binary BoseEinstein condensation in five different ErDy isotope combinations, as well as one ErDy BoseFermi mixture. Finally, we present first studies of the interspecies interaction between the two species for one mixture.

S. Baier, D. Petter, J. H. Becher, A. Patscheider, G. Natale, L. Chomaz, M. J. Mark, F. Ferlaino Realization of a Strongly Interacting Fermi Gas of Dipolar Atoms,
Phys. Rev. Lett. 121 93602 (20180829),
http://dx.doi.org/10.1103/PhysRevLett.121.093602 doi:10.1103/PhysRevLett.121.093602 (ID: 720004)
Toggle Abstract
We realize a twocomponent dipolar Fermi gas with tunable interactions, using erbium atoms. Employing a latticeprotection technique, we selectively prepare deeply degenerate mixtures of the two lowest spin states and perform highresolution Feshbach spectroscopy in an optical dipole trap. We identify a comparatively broad Feshbach resonance and map the interspin scattering length in its vicinity. The Fermi mixture shows a remarkable collisional stability in the strongly interacting regime, providing a first step towards studies of superfluid pairing, crossing from Cooper pairs to bound molecules, in presence of dipoledipole interactions.

V. Veljic, A. R. Lima, L. Chomaz, S. Baier, M. J. Mark, F. Ferlaino, A. Pelster, A. Balaz Ground state of an ultracold Fermi gas of tilted dipoles,
New J. Phys. 20 93016 (20180614),
http://dx.doi.org/10.1088/13672630/aade24 doi:10.1088/13672630/aade24 (ID: 720051)
Toggle Abstract
Manybody dipolar effects in Fermi gases are quite subtle as they energetically compete with the large kinetic energy at and below the Fermi surface (FS). Recently it was experimentally observed that its FS is deformed from a sphere to an ellipsoid due to the presence of the anisotropic and longrange dipoledipole interaction. Moreover, it was suggested that, when the dipoles are rotated by means of an external field, the Fermi surface follows their rotation, thereby keeping the major axis of the momentumspace ellipsoid parallel to the dipoles. Here we generalise a previous HartreeFock meanfield theory to systems confined in an elongated triaxial trap with an arbitrary orientation of the dipoles relative to the trap. With this we study for the first time the effects of the dipoles' arbitrary orientation on the groundstate properties of the system. Furthermore, taking into account the geometry of the system, we show how the ellipsoidal FS deformation can be reconstructed, assuming ballistic expansion, from the experimentally measurable realspace aspect ratio after a free expansion. We compare our theoretical results with new experimental data measured with erbium Fermi gas for various trap parameters and dipole orientations. The observed remarkable agreement demonstrates the ability of our model to capture the full angular dependence of the FS deformation. Moreover, for systems with even higher dipole moment, our theory predicts an additional unexpected effect: the FS does not simply follow rigidly the orientation of the dipoles but softens showing a change in the aspect ratio depending on the dipoles' orientation relative to the trap geometry, as well as on the trap anisotropy itself. Our theory provides the basis for understanding and interpreting phenomena in which the investigated physics depends on the underlying structure of the FS, such as fermionic pairing and superfluidity.

L. Chomaz, R. van Bijnen, D. Petter, G. Faraoni, S. Baier, M. J. Mark, F. Wächtler, L. Santos, F. Ferlaino Observation of roton mode population in a dipolar quantum gas,
Nature Phys. 14 446 (20180305),
http://dx.doi.org/10.1038/s4156701800547 doi:10.1038/s4156701800547 (ID: 719813)
Toggle Abstract
The concept of a roton, a special kind of elementary excitation, forming a minimum of energy at finite momentum, has been essential to understand the properties of superfluid 4He. In quantum liquids, rotons arise from strong interparticle interactions, whose microscopic description remains debated. In the realm of highlycontrollable quantum gases, a roton mode has been predicted to emerge due to dipolar interparticle interactions despite of their weaklyinteracting character. Yet it has remained elusive to observations. Here we report momentumdistribution measurements in dipolar quantum gases of highlymagnetic erbium atoms, revealing the existence of the longsought roton. We observe the appearance of peculiar peaks at welldefined momentum matching the inverse of the tight confinement length as expected for dipolar rotons. Our combined theoretical and experimental work demonstrates unambiguously the roton softening of the excitation spectrum and provides a further step in the quest towards supersolidity.

P. Ilzhöfer, G. Durastante, A. Patscheider, A. Trautmann, M. J. Mark, F. Ferlaino Twospecies fivebeam magnetooptical trap for erbium and dysprosium,
Phys. Rev. A 97 23633 (20180226),
http://dx.doi.org/10.1103/PhysRevA.97.023633 doi:10.1103/PhysRevA.97.023633 (ID: 719919)
Toggle Abstract
We report on the first realization of a twospecies magnetooptical trap (MOT) for erbium and dysprosium. The MOT operates on an intercombination line for the respective species. Owing to the narrowline character of such a cooling transition and the action of gravity, we demonstrate a novel trap geometry employing only five beams in orthogonal configuration. We observe that the mixture is cooled and trapped very efficiently, with up to \num{5e8} Er atoms and \num{e9} Dy atoms at temperatures of about 10μK. Our results offer an ideal starting condition for the creation of a dipolar quantum mixture of highly magnetic atoms.

J. H. Becher, S. Baier, K. Aikawa, M. Lepers, J. Wyart, O. Dulieu, F. Ferlaino Anisotropic polarizability of erbium atoms,
Phys. Rev. A 97 12509 (20180119),
http://dx.doi.org/10.1103/PhysRevA.97.012509 doi:10.1103/PhysRevA.97.012509 (ID: 719925)
Toggle Abstract
We report on the determination of the dynamical polarizability of ultracold erbium atoms in the ground and in one excited state at three different wavelengths, which are particularly relevant for optical trapping. Our study combines experimental measurements of the light shift and theoretical calculations. In particular, our experimental approach allows us to isolate the different contributions to the polarizability, namely the isotropic scalar and anisotropic tensor part. For the latter contribution, we observe a clear dependence of the atomic polarizability on the angle between the laserfieldpolarization axis and the quantization axis, set by the external magnetic field. Such an angledependence is particularly pronounced in the excitedstate polarizability. We compare our experimental findings with the theoretical values, based on semiempirical electronicstructure calculations and we observe a very good overall agreement. Our results pave the way to exploit the anisotropy of the tensor polarizability for spinselective preparation and manipulation.

F. Ferlaino The Discreet Charm of Dipolar Quantum Matter near Absolute Zero Temperature,
Colloquium Università Trento (Trento, 20160928) URL (20160928),
(ID: 719665)
Toggle Abstract
Approaching temperatures near the absolute zero, i.e. the lowest temperature in the whole universe, the atoms develop extreme behaviors, which challenge our understanding. In this extreme regime, the atoms assume exceptional behaviors and form a new
type of matter, which is now governed by the laws of quantum mechanics. In this regime, the interactions between many particles dictate the quantum behavior. Highly magnetic atoms, such as Erbium, endow a novel type of interaction to the quantum behavior, namely the dipoledipole interaction (DDI). Because of its longrange nature and an isotropic character, the DDI imprints genuinely novel properties at the many and fewbody level. The behavior of these systems is fundamental to understand nature and quantum physics, and reserves us every day fascinating and counterintuitive surprises. This talk will provide an overview of the fascinating dipolar phenomena from the Innsbruck prospective, ranging from the firstproduced quantum gas of Er, to the observation of chaotic scattering and the formation of a quasi selfbound droplet state in analogy with the He case.

F. Ferlaino The Discreet Charm of Dipolar Quantum Matter near Absolute Zero Temperature,
Physikkolloquium Universität Heidelberg (Heidelberg) (20160701),
(ID: 719623)

F. Ferlaino The Fascination of Lanthanides for Ultracold Quantum Physics,
Kolloquium HumboldtUniversitaet zu Berlin (Berlin) (20160112),
(ID: 719447)
Toggle Abstract
Given their strong magnetic moment and exotic electronic configuration, lanthanide atomic species disclose a plethora of intriguing phenomena in ultracold quantum physics. The large magnetic moment of these atoms reflects on a strong interparticle dipoledipole interaction, which has both a longrange nature and an isotropic character. Here, we report on our latest results on quantum manybody and fewbody physics based on a stronglymagnetic lanthanide, erbium (Er). Particular emphasis will be given to the scattering properties of bosonic and fermionic Er, in which unconventional and fascinating phenomena appear as a result of both the magnetic and the orbital anisotropy of the underlying the native interactions between atoms.

K. Aikawa, A. Frisch, M. Mark, S. Baier, R. Grimm, J. L. Bohn, D. S. Jin, G. Bruun, F. Ferlaino Anisotropic Relaxation Dynamics in a Dipolar Fermi Gas Driven Out of Equilibrium,
Phys. Rev. Lett. 113 263201 (20141223),
http://dx.doi.org/10.1103/PhysRevLett.113.263201 doi:10.1103/PhysRevLett.113.263201 (ID: 718908)
Toggle Abstract
We report on the observation of a large anisotropy in the rethermalization dynamics of an ultracold dipolar Fermi gas driven out of equilibrium. Our system consists of an ultracold sample of strongly magnetic Er167 fermions, spin polarized in the lowest Zeeman sublevel. In this system, elastic collisions arise purely from universal dipolar scattering. Based on crossdimensional rethermalization experiments, we observe a strong anisotropy of the scattering, which manifests itself in a large angular dependence of the thermal relaxation dynamics. Our result is in good agreement with recent theoretical predictions. Furthermore, we measure the rethermalization rate as a function of temperature for different angles and find that the suppression of collisions by Pauli blocking is not influenced by the dipole orientation.

T. Takekoshi, L. Reichsöllner, A. Schindewolf, J. M. Hutson, C. Le Sueur, O. Dulieu, F. Ferlaino, R. Grimm, H. Nägerl Ultracold dense samples of dipolar RbCs molecules in the rovibrational and hyperfine ground state,
Phys. Rev. Lett. 113 205301 (20141112),
http://dx.doi.org/10.1103/PhysRevLett.113.205301 doi:10.1103/PhysRevLett.113.205301 (ID: 718921)
Toggle Abstract
We produce ultracold dense trapped samples of Rb87Cs133 molecules in their rovibrational ground state, with full nuclear hyperfine state control, by stimulated Raman adiabatic passage (STIRAP) with efficiencies of 90%. We observe the onset of hyperfinechanging collisions when the magnetic field is ramped so that the molecules are no longer in the hyperfine ground state. A strong quadratic shift of the transition frequencies as a function of applied electric field shows the strongly dipolar character of the RbCs groundstate molecule. Our results open up the prospect of realizing stable bosonic dipolar quantum gases with ultracold molecules.

K. Aikawa, S. Baier, A. Frisch, M. Mark, C. Ravensbergen, F. Ferlaino Observation of Fermi surface deformation in a dipolar quantum gas,
Science 345 6203 (20140919),
http://dx.doi.org/10.1126/science.1255259 doi:10.1126/science.1255259 (ID: 718909)
Toggle Abstract
In the presence of isotropic interactions, the Fermi surface of an ultracold Fermi gas is spherical. Introducing anisotropic interactions can deform the Fermi surface, but the effect is subtle and challenging to observe experimentally. Here, we report on the observation of a Fermi surface deformation in a degenerate dipolar Fermi gas of erbium atoms. The deformation is caused by the interplay between strong magnetic dipoledipole interaction and the Pauli exclusion principle. We demonstrate the manybody nature of the effect and its tunability with the Fermi energy. Our observation provides a basis for future studies on anisotropic manybody phenomena in normal and superfluid phases.

F. Ferlaino Molecular physics: Complexity trapped by simplicity,
Nature 512 262 (20140820),
http://dx.doi.org/10.1038/512261a doi:10.1038/512261a (ID: 718990)
Toggle Abstract
Devices known as magnetooptical traps have long been used to cool and confine atoms, but not molecules — until now. This new ability should enable many studies and applications of the physics of ultracold molecules.

A. Zenesini, B. Huang, M. Berninger, H. Nägerl, F. Ferlaino, R. Grimm Resonant atomdimer collisions in cesium: Testing universality at positive scattering lengths,
Phys. Rev. A 90 022704 (20140811),
http://dx.doi.org/10.1103/PhysRevA.90.022704 doi:10.1103/PhysRevA.90.022704 (ID: 718945)
Toggle Abstract
We study the collisional properties of an ultracold mixture of cesium atoms and dimers close to a Feshbach resonance near 550 G in the regime of positive swave scattering lengths. We observe an atomdimer loss resonance that is related to Efimov's scenario of trimer states. The resonance is found at a value of the scattering length that is different from a previous observation at low magnetic fields. This indicates nonuniversal behavior of the Efimov spectrum for positive scattering lengths. We compare our observations with predictions from effective field theory and with a recent model based on the van der Waals interaction. We present additional measurements on pure atomic samples in order to check for the presence of a resonant loss feature related to an avalanche effect, as suggested by observations in other atomic species. We could not confirm the presence of such a feature.

A. Frisch, M. Mark, K. Aikawa, F. Ferlaino, J. L. Bohn, C. Makrides, A. Petrov, S. Kotochigova Quantum Chaos in Ultracold Collisions of Erbium,
Nature 507 479 (20140327),
http://dx.doi.org/10.1038/nature13137 doi:10.1038/nature13137 (ID: 718682)
Toggle Abstract
Atomic and molecular samples reduced to temperatures below 1 microkelvin, yet still in the gas phase, afford unprecedented energy resolution in probing and manipulating how their constituent particles interact with one another. For simple atoms, such as alkalis, scattering resonances are extremely wellcharacterized. However, ultracold physics is now poised to enter a new regime, where far more complex species can be cooled and studied, including magnetic lanthanide atoms and even molecules. For molecules, it has been speculated that a dense forest of resonances in ultracold collision cross sections will likely express essentially random fluctuations, much as the observed energy spectra of nuclear scattering do. According to the BohigasGiannoniSchmit conjecture, these fluctuations would imply chaotic dynamics of the underlying classical motion driving the collision. This would provide a paradigm shift in ultracold atomic and molecular physics, necessitating new ways of looking at the fundamental interactions of atoms in this regime, as well as perhaps new chaosdriven states of ultracold matter. In this report we provide the first experimental demonstration that random spectra are indeed found at ultralow temperatures. In the experiment, an ultracold gas of erbium atoms is shown to exhibit many FanoFeshbach resonances, for bosons on the order of 3 per gauss. Analysis of their statistics verifies that their distribution of nearestneighbor spacings is what one would expect from random matrix theory. The density and statistics of these resonances are explained by fullyquantum mechanical scattering calculations that locate their origin in the anisotropy of the atoms' potential energy surface. Our results therefore reveal for the first time chaotic behavior in the native interaction between ultracold atoms.

K. Aikawa, A. Frisch, M. Mark, S. Baier, R. Grimm, F. Ferlaino Reaching Fermi Degeneracy via Universal Dipolar Scattering,
Phys. Rev. Lett. 112 010404 (20140106),
http://dx.doi.org/10.1103/PhysRevLett.112.010404 doi:10.1103/PhysRevLett.112.010404 (ID: 718621)
Toggle Abstract
We report on the creation of a degenerate dipolar Fermi gas of erbium atoms. We force evaporative cooling in a fully spinpolarized sample down to temperatures as low as 0.2 times the Fermi temperature. The strong magnetic dipoledipole interaction enables elastic collisions between identical fermions even in the zeroenergy limit. The measured elastic scattering cross section agrees well with the predictions from the dipolar scattering theory, which follow a universal scaling law depending only on the dipole moment and on the atomic mass. Our approach to quantum degeneracy proceeds with very high cooling efficiency and provides large atomic densities, and it may be extended to various dipolar systems.

A. Frisch, K. Aikawa, F. Ferlaino, E. Berseneva, S. Kotochigova Hyperfine structure of lasercooling transitions in fermionic erbium167,
Phys. Rev. A 88 032508 (20130913),
http://dx.doi.org/10.1103/PhysRevA.88.032508 doi:10.1103/PhysRevA.88.032508 (ID: 718495)
Toggle Abstract
We have measured and analyzed the hyperfine structure of two lines, one at 583 nm and one at 401 nm, of the only stable fermionic isotope of atomic erbium as well as determined its isotope shift relative to the four mostabundant bosonic isotopes. Our work focuses on the J→J+1 laser cooling transitions from the [Xe]4f126s2(3H6) ground state to two levels of the excited [Xe]4f126s6p configuration, which are of major interest for experiments on quantum degenerate dipolar Fermi gases. From a fit to the observed spectra of the strong optical transition at 401 nm we find that the magnetic dipole and electric quadrupole hyperfine constants for the excited state are Ae/h=−100.1(3) MHz and Be/h=−3079(30) MHz, respectively. The hyperfine spectrum of the narrow transition at 583 nm, was previously observed and accurate Ae and Be coefficients are available. A simulated spectrum based on these coefficients agrees well with our measurements. We have also determined the hyperfine constants using relativistic configurationinteraction ab initio calculations. The agreement between the ab initio and fitted data for the ground state is better than 0.1%, while for the two excited states the agreement is 1% and 11% for the Ae and Be constants, respectively.

A. Zenesini, B. Huang, M. Berninger, S. Besler, H. Nägerl, F. Ferlaino, R. Grimm, C. Greene, J. von Stecher Resonant fivebody recombination in an ultracold gas of bosonic atoms,
New J. Phys. 15 043040 (20130422),
http://dx.doi.org/10.1088/13672630/15/4/043040 doi:10.1088/13672630/15/4/043040 (ID: 718073)
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We combine theory and experiment to investigate fivebody recombination in an ultracold gas of atomic cesium at negative scattering length. A refined theoretical model, in combination with extensive laboratory tunability of the interatomic interactions, enables the fivebody resonant recombination rate to be calculated and measured. The position of the new observed recombination feature agrees with a recent theoretical prediction and supports the prediction of a family of universal cluster states at negative a that are tied to an Efimov trimer.

M. Berninger, A. Zenesini, B. Huang, W. Harm, H. Nägerl, F. Ferlaino, R. Grimm, P. S. Julienne, J. M. Hutson Feshbach resonances, weakly bound molecular states, and coupledchannel potentials for cesium at high magnetic fields,
Phys. Rev. A 87 032517 (20130325),
http://dx.doi.org/10.1103/PhysRevA.87.032517 doi:10.1103/PhysRevA.87.032517 (ID: 718341)
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We explore the scattering properties of ultracold groundstate Cs atoms at magnetic fields between 450 G (45 mT) and 1000 G. We identify 17 new Feshbach resonances, including two very broad ones near 549 G and 787 G. We measure the binding energies of several different dimer states by magnetic field modulation spectroscopy. We use leastsquares fitting to these experimental results, together with previous measurements at lower field, to determine a new 6parameter model of the longrange interaction potential, designated M2012. Coupledchannels calculations using M2012 provide an accurate mapping between the swave scattering length and the magnetic field over the entire range of fields considered. This mapping is crucial for experiments that rely on precise tuning of the scattering length, such as those on Efimov physics.

K. Aikawa, A. Frisch, M. Mark, S. Baier, A. Rietzler, R. Grimm, F. Ferlaino BoseEinstein Condensation of Erbium,
Phys. Rev. Lett. 108 210401 (20120521),
http://dx.doi.org/10.1103/PhysRevLett.108.210401 doi:10.1103/PhysRevLett.108.210401 (ID: 718048)
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We report on the achievement of BoseEinstein condensation of erbium atoms and on the observation of
magnetic Feshbach resonances at low magnetic field. By means of evaporative cooling in an optical dipole
trap, we produce pure condensates of 168Er, containing up to 7×104 atoms. Feshbach spectroscopy reveals
an extraordinary rich loss spectrum with six loss resonances already in a narrow magneticfield range up to
3 G. Finally, we demonstrate the application of a lowfield Feshbach resonance to produce a tunable dipolar
BoseEinstein condensate and we observe its characteristic dwave collapse.

A. Frisch, K. Aikawa, M. Mark, A. Rietzler, J. Schindler, E. Zupanic, R. Grimm, F. Ferlaino Narrowline magnetooptical trap for erbium: Simple approach for a complex atom,
Phys. Rev. A 85 051401 (20120507),
http://dx.doi.org/10.1103/PhysRevA.85.051401 doi:10.1103/PhysRevA.85.051401 (ID: 718027)
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We report on the experimental realization of a robust and efficient magnetooptical trap for erbium atoms, based on a narrow cooling transition at 583 nm. We observe up to N=2×108 atoms at a temperature of about T=15 μK. This simple scheme provides better starting conditions for direct loading of dipole traps as compared to approaches based on the strong cooling transition alone, or on a combination of a strong and a narrow kHz transition. Our results on Er point to a general, simple, and efficient approach to laser cool samples of other lanthanide atoms (Ho, Dy, and Tm) for the production of quantumdegenerate samples.

T. Takekoshi, M. Debatin, R. Rameshan, F. Ferlaino, R. Grimm, H. Nägerl, C. Le Sueur, J. M. Hutson, P. S. Julienne, S. Kotochigova, E. Tiemann Towards the production of ultracold groundstate RbCs molecules: Feshbach resonances, weakly bound states, and the coupledchannel model,
Phys. Rev. A 85 032506 (20120305),
http://dx.doi.org/10.1103/PhysRevA.85.032506 doi:10.1103/PhysRevA.85.032506 (ID: 718010)
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We have studied interspecies scattering in an ultracold mixture of 87Rb and 133Cs atoms, both in their lowestenergy spin states. The threebody loss signatures of 30 incoming s and pwave magnetic Feshbach resonances over the range 0 to 667 G have been cataloged. Magnetic field modulation spectroscopy was used to observe molecular states bound by up to 2.5 MHz×h. We have created RbCs Feshbach molecules using two of the resonances. Magnetic moment spectroscopy along the magnetoassociation pathway from 197 to 182 G gives results consistent with the observed and calculated dependence of the binding energy on magnetic field strength. We have set up a coupledchannel model of the interaction and have used direct leastsquares fitting to refine its parameters to fit the experimental results from the Feshbach molecules, in addition to the Feshbach resonance positions and the spectroscopic results for deeply bound levels. The final model gives a good description of all the experimental results and predicts a large resonance near 790 G, which may be useful for tuning the interspecies scattering properties. Quantum numbers and vibrational wave functions from the model can also be used to choose optimal initial states of Feshbach molecules for their transfer to the rovibronic ground state using stimulated Raman adiabatic passage.

F. Ferlaino, A. Zenesini, M. Berninger, B. Huang, H. C. Nägerl, R. Grimm Efimov Resonances in Ultracold Quantum Gases,
FewBody Syst. 51 133 (20111009),
http://dx.doi.org/10.1007/s0060101102607 doi:10.1007/s0060101102607 (ID: 717751)
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Ultracold atomic gases have developed into prime systems for experimental studies of Efimov threebody physics and related fewbody phenomena, which occur in the universal regime of resonant interactions. In the last few years, many important breakthroughs have been achieved, confirming basic predictions of universal fewbody theory and deepening our understanding of such systems. We review the basic ideas along with the fast experimental developments of the field, focussing on ultracold cesium gases as a wellinvestigated model system. Triatomic Efimov resonances, atomdimer Efimov resonances, and related fourbody resonances are discussed as central observables. We also present some new observations of such resonances, supporting and complementing the set of available data.

M. Debatin, T. Takekoshi, R. Rameshan, L. Reichsöllner, F. Ferlaino, R. Grimm, R. Vexiau, N. Bouloufa, O. Dulieu, H. C. Nägerl Molecular spectroscopy for groundstate transfer of ultracold RbCs molecules,
Phys. Chem. Chem. Phys. 13 18935 (20110724),
http://dx.doi.org/10.1039/C1CP21769K doi:10.1039/C1CP21769K (ID: 717719)
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We perform one and twophoton high resolution spectroscopy on ultracold samples of RbCs Feshbach molecules with the aim to identify a suitable route for efficient groundstate transfer in the quantumgas regime to produce quantum gases of dipolar RbCs groundstate molecules. Onephoton loss spectroscopy allows us to probe deeply bound rovibrational levels of the mixed excited (A1{\Sigma}+  b3{\Pi}0) 0+ molecular states. Twophoton dark state spectroscopy connects the initial Feshbach state to the rovibronic ground state. We determine the binding energy of the lowest rovibrational level v"=0,J"=0> of the X1{\Sigma}+ ground state to be DX 0 = 3811.5755(16) 1/cm, a 300fold improvement in accuracy with respect to previous data. We are now in the position to perform stimulated twophoton Raman transfer to the rovibronic ground state.

M. Berninger, A. Zenesini, B. Huang, W. Harm, H. C. Nägerl, F. Ferlaino, R. Grimm, P. S. Julienne, J. M. Hutson Universality of the ThreeBody Parameter for Efimov States in Ultracold Cesium,
Phys. Rev. Lett. 107 120401 (20110620),
http://dx.doi.org/10.1103/PhysRevLett.107.120401 doi:10.1103/PhysRevLett.107.120401 (ID: 717706)
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We report on the observation of triatomic Efimov resonances in an ultracold gas of cesium atoms. Exploiting the wide tunability of interactions resulting from three broad Feshbach resonances in the same spin channel, we measure magneticfield dependent threebody recombination loss. The positions of the loss resonances yield corresponding values for the threebody parameter, which in universal fewbody physics is required to describe threebody phenomena and in particular to fix the spectrum of Efimov states. Our observations show a robust universal behavior with a threebody parameter that stays essentially constant.

A. Lercher, T. Takekoshi, M. Debatin, B. Schuster, R. Rameshan, F. Ferlaino, R. Grimm, H. C. Nägerl Production of a dualspecies BoseEinstein condensate of Rb and Cs atoms,
Eur. Phys. J. D 65 9 (20110325),
http://dx.doi.org/10.1140/epjd/e2011200156 doi:10.1140/epjd/e2011200156 (ID: 717717)
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We report the simultaneous production of BoseEinstein condensates (BECs) of and atoms in separate optical traps. The two samples are mixed during laser cooling and loading but are separated by 400 μm for the final stage of evaporative cooling. This is done to avoid considerable interspecies threebody recombination, which causes heating and evaporative loss. We characterize the BEC production process, discuss limitations, and outline the use of the dualspecies BEC in future experiments to produce rovibronic ground state molecules, including a scheme facilitated by the superfluidtoMottinsulator (SFMI) phase transition.

S. Knoop, F. Ferlaino, M. Berninger, M. Mark, H. Nägerl, R. Grimm, J. P. D'Incao, B. Esry Magnetically Controlled Exchange Process in an Ultracold AtomDimer Mixture,
Phys. Rev. Lett. 104 053201 (20091110),
http://dx.doi.org/10.1103/PhysRevLett.104.053201 doi:10.1103/PhysRevLett.104.053201 (ID: 717092)
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We report on the observation of an elementary exchange process in an optically trapped ultracold sample of atoms and Feshbach molecules. We can magnetically control the energetic nature of the process and tune it from endoergic to exoergic, enabling the observation of a pronounced threshold behavior. In contrast to relaxation to more deeply bound molecular states, the exchange process does not lead to trap loss. We find excellent agreement between our experimental observations and calculations based on the solutions of threebody Schr\"odinger equation in the adiabatic hyperspherical representation. The high efficiency of the exchange process is explained by the halo character of both the initial and final molecular states.
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K. Pilch, A. D. Lange, A. Prantner, G. Kerner, F. Ferlaino, H. Nägerl, R. Grimm Observation of interspecies Feshbach resonances in an ultracold RbCs mixture,
Phys. Rev. A 79 042718 (20090430),
http://dx.doi.org/10.1103/PhysRevA.79.042718 doi:10.1103/PhysRevA.79.042718 (ID: 660704)
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We report on the observation of interspecies Feshbach resonances in an ultracold, optically trapped mixture of Rb and Cs atoms. In a magnetic field range up to 300 G we find 23 interspecies Feshbach resonances in the lowest spin channel and 2 resonances in a higher channel of the mixture. The extraordinarily rich Feshbach spectrum suggests the importance of different partial waves in both the open and closed channels of the scattering problem along with higherorder coupling mechanisms. Our results provide, on one hand, fundamental experimental input to characterize the RbCs scattering properties and, on the other hand, identify possible starting points for the association of ultracold heteronuclear RbCs molecules.
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F. Ferlaino, S. Knoop, M. Berninger, W. Harm, J. P. D'Incao, H. Nägerl, R. Grimm Evidence for universal fourbody states tied to an Efimov trimer,
Phys. Rev. Lett. 102 140401 (20090409),
http://dx.doi.org/10.1103/PhysRevLett.102.140401 doi:10.1103/PhysRevLett.102.140401 (ID: 717103)
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We report on the measurement of fourbody recombination rate coefficients in an atomic gas. Our results obtained with an ultracold sample of cesium atoms at negative scattering lengths show a resonant enhancement of losses and provide strong evidence for the existence of a pair of fourbody states, which is strictly connected to Efimov trimers via universal relations. Our findings confirm recent theoretical predictions and demonstrate the enrichment of the Efimov scenario when a fourth particle is added to the generic threebody problem.
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S. Knoop, F. Ferlaino, M. Mark, M. Berninger, H. Schöbel, H. Nägerl, R. Grimm Observation of an Efimovlike trimer resonance in ultracold atomdimer scattering,
Nature Phys. 5 227230 (20090209),
http://dx.doi.org/10.1038/nphys1203 doi:10.1038/nphys1203 (ID: 717105)
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The field of fewbody physics has originally been motivated by understanding nuclear matter. New model systems to experimentally explore fewbody quantum systems can now be realized in ultracold gases with tunable interactions. Albeit the vastly different energy regimes of ultracold and nuclear matter (peV as compared to MeV), fewbody phenomena are universal for nearresonant twobody interactions. Efimov states represent a paradigm for universal threebody states, and evidence for their existence has been obtained in measurements of threebody recombination in an ultracold gas of caesium atoms. Interacting samples of halo dimers can provide further information on universal fewbody phenomena. Here we study interactions in an optically trapped mixture of such halo dimers with atoms, realized in a caesium gas at nanokelvin temperatures. We observe an atomdimer scattering resonance, which we interpret as being due to a trimer state hitting the atomdimer threshold. We discuss the close relation of this observation to Efimov's scenario, and in particular to atomdimer Efimov resonances.
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A. D. Lange, K. Pilch, A. Prantner, F. Ferlaino, B. Engeser, H. Nägerl, R. Grimm, C. Chin Determination of atomic scattering lengths from measurements of molecular binding energies near Feshbach resonances,
Phys. Rev. A 79 013622 (20090123),
http://dx.doi.org/10.1103/PhysRevA.79.013622 doi:10.1103/PhysRevA.79.013622 (ID: 660710)
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We present an analytic model to calculate the atomic scattering length near a Feshbach resonance from data on the molecular binding energy. Our approach considers finiterange squarewell potentials and can be applied near broad, narrow, or even overlapping Feshbach resonances. We test our model on Cs2 Feshbach molecules. We measure the binding energy using magneticfield modulation spectroscopy in a range where one broad and two narrow Feshbach resonances overlap. From the data we accurately determine the Cs atomic scattering length and the positions and widths of two particular resonances.
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