5th Institute of Physics
 
 
 
 

Giant Rydberg Atoms in ultracold quantum gases

Rydberg Impurities in a Bose-Einstein Condensate

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Abstract

Rydberg atoms are highly excited atoms with one valence electron of principal quantum number n>>10.  The size of the Rydberg atom scales as n², making them extremely large atoms that are nearly as big as bacteria.  Our experimental apparatus combines the ability of producing Bose-Einstein-condensates of ultracold Rubidium atoms with high resolution optical access for the controlled creation of individual Rydberg atoms.  This allows us to study in detail static and dynamic properties of single or few Rydberg atoms, which act as impurities in the condensate.

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Research

 

Fig. 1: Precision spectroscopy of nS Rydberg states excited in a Bose-Einstein condensate. The line is shifted and broadened due to neutral atoms inside the Rydberg orbit. The line shape is compared to simulations including s-wave scattering only (blue) and both s- and p-wave scattering (red)

Currently we are studying the interaction of Rydberg atoms within a dense background gas. This includes both Bose-Einstein condensates and ultracold clouds slightly above the critical temperature.

We have performed high resolution spectroscopy of a single Rydberg atom in the BEC. Due to high atom number density, when Rydberg states in the range n=40…111 are excited, up to atoms 8000 are located inside the Rydberg orbit. The Rydberg electron scatters of this neutral perturbers, leading to a shift and broadening of the spectroscopic line. Not only a frequency shift proportional to the density is observed, as discovered by Amaldi and Segre in 1934, but an asymmetric broadening, which depends on the principal quantum number n. The line broadening depends on the interaction potential energy curves of the Rydberg electron scatterer with the neutral atom perturber. In Rb there is a shape resonance for the triplet p-wave scattering of e--Rb(5s) at 0.02 eV leading to a potential with a large energy shift, which crosses the lower lying nS, (n-2)D, and (n-1)P states. When a nS +N x 5S1/2 state is photoassociated, neutral atom perturbers near the crossing with the shape resonance potential become relevant, leading to large n-dependent line broadenings.

 

We developed a simple microscopic model for the spectroscopic line shape by treating the atoms overlapped with the Rydberg orbit as zero-velocity, independent, point-like particles, with binding energies associated with their ion-neutral separation. In Fig. 1 this model is displayed with using s-wave scattering only (blue) and using both s- and p-wave scattering (red). One can see, that the model including the p-wave scattering fit the data (black) quite well.

 

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Our setups

Sketch of the experimental setup. A Bose-Einstein condensate is trapped and one Rydberg atom is excited in its center via 2-Photon excitation. The excitation scheme is shown in the upper right corner.

 

Fig. 2: Sketch of the experimental setup. A Bose-Einstein condensate is trapped and one Rydberg atom is excited in its center via 2-Photon excitation. The excitation scheme is shown in the upper right corner.

The setup uses a dual chamber approach, where the MOT loading region is separated from the experiment region by a distance of ~50cm. Due to differential pumping, a pressure difference of 3 orders of magnitude is achieved. With this configuration we can load the MOT directly from background Rubidium while still maintaining the ultra-high vacuum required for BEC experiments in the main chamber.  After the magnetical transport from the MOT to the science chamber, the atoms are held and cooled close to or below the Bose-Einstein condensation temperature in a QUIC-trap. Diffraction-limited high NA aspherical lenses which are placed inside the vacuum chamber enable us to focus down the Rydberg excitations lasers to below the Rydberg blockade radius which is typically a couple of µm in size.

 

The excitation energies to Rydberg states are hardly reached in a one-photon process, e.g. the excitation of the n=40 state in 87Rb requires a laser at 297 nm. Therefore we use a two-photon excitation scheme: A blue laser (420 nm) is nearly resonant with the transition to the 6P3/2 state and together with a red laser (~1020 nm) resonant with the Rydberg level. The principal quantum number n can be varied between n=30...200 by tuning the red laser. We can address both S and D rydberg states. After excitation the Rydberg atoms are field ionized by an electric field pulse and the produced ions are detected by a microchannel plate (MCP) or a channeltron.

 

 

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The Crew

Carolin Dietrich(Master Student)
Felix Engel(Doktorand)
Kathrin Kleinbach(Doktorandin)
Robert Löw(Group leader)
Florian Meinert(Post Doc)
Tilman Pfau(Institutsleiter)

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Open Positions

quantum mechanical wave function of a giant Rydberg moleculeWe are always looking for motivated people to join our Rydberg team.

We are currently trying to spatially image an electron wave function directly by its interaction with a Rb Bose-Einstein Condensate. For this we are looking for

  • a Master student
  • a PhD student

If you are interested in our effort to understand the physics of Rydberg atoms in quantum gases , contactmailto icon F. Meinert, mailto icon R. Löw, or mailto icon T. Pfau via email/phone or just come by and visit our labs (5.125).

 

Abstract | Research | Our setups | The Crew | Open Positions | Refs & Publications | Bachelor- and Master-Topics

Refs & Publications

[1]M. Schlagmüller, T. C. Liebisch, H. Nguyen, G. Lochead, F. Engel, F. Böttcher, K. M. Westphal, K. S. Kleinbach, R. Löw, S. Hofferberth, T. Pfau, J. Pérez-Ríos, C. H. Greene
"Probing an Electron Scattering Resonance using Rydberg Molecules within a Dense and Ultracold Gas"
Phys. Rev. Lett. 116, 053001 (2016) , arXiv:1510.07003; doi: 10.1103/PhysRevLett.116.053001
[2]F. Böttcher, A. Gaj, K. M. Westphal, M. Schlagmüller, K. S. Kleinbach, R. Löw, T. Cubel Liebisch, T. Pfau, and S. Hofferberth
"Observation of mixed singlet-triplet Rb2 Rydberg molecules"
Phys. Rev. A 93, 032512 (2016) , arXiv:1510.01097; doi: 10.1103/PhysRevA.93.032512
[3]M. Schlagmüller, T. C. Liebisch, F. Engel, K. S. Kleinbach, F. Böttcher, U. Hermann, K. M. Westphal, A. Gaj, R. Löw, S. Hofferberth, T. Pfau, J. Pérez-Ríos, and C. H. Greene
"Ultracold Chemical Reactions of a Single Rydberg Atom in a Dense Gas"
Phys. Rev. X 6, 031020 (2016) , arXiv:1605.04883; doi: 10.1103/PhysRevX.6.031020
[4]T. C. Liebisch, M. Schlagmüller, F. Engel, H. Nguyen, J. Balewski, G. Lochead, F. Böttcher, K. M. Westphal, K. S. Kleinbach, T. Schmid, A. Gaj, R. Löw, S. Hofferberth, T. Pfau, J. Pérez-Ríos and C. H. Greene
"Controlling Rydberg atom excitations in dense background gases"
J. Phys. B: At. Mol. Opt. Phys. 49, (2016) 182001 , arXiv:1607.01325; doi: 10.1088/0953-4075/49/18/182001
[5]Kathrin S. Kleinbach, Florian Meinert, Felix Engel, Woo Jin Kwon, Robert Löw, Tilman Pfau, Georg Raithel
"Photo-association of trilobite Rydberg molecules via resonant spin-orbit coupling"
Phys. Rev. Lett. 118, 223001 (2017) ; arXiv:1703.01096; doi: 10.1103/PhysRevLett.118.223001
[6]Krzysztof Jachymski, Florian Meinert, Hagar Veksler, Paul S. Julienne, and Shmuel Fishman
"Ultracold atoms in quasi-1D traps: a step beyond the Lieb-Liniger model"
Phys. Rev. A 95, 052703 (2017) ; arXiv:1703.08364; doi: 10.1103/PhysRevA.95.052703
[7]Florian Meinert
"Riesenmoleküle am absoluten Nullpunkt"
Phys. Unserer Zeit 48, 236–242 (2017) ; doi: 10.1002/piuz.201701478

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Bachelor- and Master-Topics

Characterization of an optical tweezer for far-detuned trapping of ions  (Bachelor Thesis)

In our experiment, we plan to implement optical trapping of single ionic impurities that we create by photo-ionization of parent Rydberg impurities. For this to realize it is of importance to characterize the beam profile of a micro-meter sized optical tweezer. You will develop a fully automized Piezo-based scanning system to measure the point-spread-function of a micro-tweezer generated from a high-NA aspheric lens. The goal is to identify optimal laser beam parameters to balance diffraction limited performance and undesired geometric aberrations in view of realizing minimal spot sizes.

Contacts:

  F.Meinert, K.Kleinbach, F. Engel, T. Pfau

 

High resolution Rydberg spectroscopy in a Bose-Einstein Condensate (Master Thesis) 

A  Rydberg atom provide a single electron in a well defined quantum state that can cover thousands of atoms in a Bose-Einstein Condensate. We are interested to watch this single quantum imersed in a sea of atoms. On the one hand it can bind atoms into molecular states on the other habe it can backact on the collective excitations of a quantum gas. The interaction between the electron and the quantum gas is mediated by low energy scattering. The interaction depends on the electron spin and we have recently studies how this spin dependence can be used to excite very exotic "trilobite" molecules (see figure above) [2]. In this thesis we want to understand the transition from molecular physics to many-body physics [3] including the spin degree of freedom. In addition we want to study the depencence on the orbital angular momentum [4]. The experimenatl tool is high resolution spectroscopy on a BEC sample including single ion detection.

  

[1] Kathrin S. Kleinbach, Florian Meinert, Felix Engel, Woo Jin Kwon, Robert Löw, Tilman Pfau, Georg Raithel
"Photo-association of trilobite Rydberg molecules via resonant spin-orbit coupling"
Phys. Rev. Lett. 118, 223001 (2017)

[2] A. Gaj, A. T. Krupp, J. B. Balewski, R. Löw, S. Hofferberth, and T. Pfau
"From molecular spectra to a density shift in dense Rydberg gases"
Nature Comm. 5, 4546 (2014)

[3] A.T. Krupp, A. Gaj, J.B. Balewski, P. Ilzhöfer, S. Hofferberth, R. Löw, T. Pfau, M. Kurz, and P. Schmelcher,
"Alignment of D-state Rydberg molecules"
Phys. Rev. Lett. 112, 143008 (2014)

Contacts:

  F.Meinert, K.Kleinbach, F. Engel, T. Pfau

 

Characterizing and optmizing a high resolution ion microscope for ultra cold ions (Master Thesis, beginning: from summer 2018)

Strongly interacting Rydberg atoms are predicted to undergo quantum phase transitons We are currently setting up an ion microscope for the detection of single ions from a Rydberg excited quantum gas with a spatial resolution better than the size of a Rydberg atom. Within this thesis the microscope will be characterized via various abberation correction mechanisms. For this the experimental control and data aquisition system will be extended to include automatic feedback. The goal is to reach a spatial resolution such that the microscopic structure of strongly interacting quantum matter can be resolved.

 

Contacts:

  Th. Schmid, Ch. Veit, N. Zuber,  R. Löw, T. Pfau

Abstract | Research | Our setups | The Crew | Open Positions | Refs & Publications | Bachelor- and Master-Topics