Welcome to the Quantum gases group at the Eindhoven University of Technology

Below you find some highlights

Ronen Kroeze and Jaron Sanders prize winners at the TU/e Academic Awards, for best Master thesis and best PhD thesis - May 2017

Ronen got the award for his Master thesis research on few-body physics in our group last year (2016), with title "Finite range corrections to the universal Efimov spectrum". Jaron, who did his Master thesis research in our group on Rydberg crystals and continued to work with us on this topic when doing his PhD with Johan van Leeuwaarden and Sem Borst in the department of Mathematics, got the award for his PhD thesis "Stochastic optimization of large-scale complex systems".
Ronen Kroeze Jaron Sanders

Sub-Poissonian Statistics of Jamming Limits in Ultracold Rydberg Gases - July 2015

Several recent experiments have established by measuring the Mandel Q parameter that the number of Rydberg excitations in ultracold gases exhibits sub-Poissonian statistics. This effect is attributed to the Rydberg blockade that occurs due to the strong interatomic interactions between highly excited atoms.

Because of this blockade effect, the system can end up in a state in which all particles are either excited or blocked: a jamming limit. We analyze appropriately constructed random-graph models that capture the blockade effect, and derive formulae for the mean and variance of the number of Rydberg excitations in jamming limits. This yields an explicit relationship between the Mandel Q parameter and the blockade effect, and comparison to measurement data shows strong agreement between theory and experiment. Phys. Rev. Lett. 115, 043002 (2015). FOM published a Press release on our work with the title Simple model explains crystal formation of exceptional atoms.

Three-body recombination at vanishing scattering lengths in an ultracold Bose gas - July 2014

We report on measurements of three-body recombination rates in an ultracold gas of 7Li atoms in the extremely non-universal regime where the two-body scattering length vanishes.

We show that the rate is well-defined and can be described by two-body parameters only: the scattering length a and the effective range Re. We find the rate to be energy-independent, and by connecting our results with previously reported measurements in the universal limit we cover three-body recombination in the whole range from weak to strong two-body interactions. We identify a non-trivial magnetic field value in the non-universal regime where the rate should be strongly reduced. Phys. Rev. Lett. 113, 053202 (2014).

Why an ultracold gas is like a wireless network - April 2014

We found a surprising similarity between ultracold gases of Rydberg atoms and wireless telecommunications networks.

After publication, several articles appeared in the news on this research: The Institute of Physics published PhysicsWorld.com, FOM came with a Press release Wireless networks show the same patterns as interacting Rydberg atoms, and our own university devoted a news item Draadloze netwerken verbonden aan atomaire gassen. The original paper can be found here: Phys. Rev. Lett. 112, 163001 – Published 22 April (2014).

Universal three-body parameter in ultracold 4He∗ - Dec 2012

We have analyzed our recently measured three-body loss rate coefficient for a Bose-Einstein condensate of spin-polarized metastable triplet 4He atoms in terms of Efimov physics. The large value of the scattering length for these atoms, which provides access to the Efimov regime, arises from a nearby potential resonance. We find the loss coefficient to be consistent with the three-body parameter (3BP) found in alkali-metal experiments, where Feshbach resonances are used to tune the interaction.

This provides evidence for a universal 3BP outside the group of alkali-metal elements. In addition, we give examples of other atomic systems without Feshbach resonances but with a large scattering length that would be interesting to analyze once precise measurements of three-body loss are available. Phys. Rev. A 86, 062705 (2012).

Efimov Trimers in a Harmonic Potential - Nov 2011

We study the Efimov effect in a harmonic oscillator in the hyperspherical formulation, and show how a reduced model allows for a description that is a generalization of the Efimov effect in free space and leads to results that are easily interpreted. Efimov physics may be observed by varying the value of the scattering length, since in the regime where the trimers have a mixed harmonic oscillator and Efimov character, the inelastic properties of these states are still manageable. The model also allows for the study of non-universal Efimov trimers by including the effective range scattering parameter. While we find that in a certain regime the effective range parameter can take over the role of the three-body parameter, interestingly, we obtain a numerical relationship between these two parameters different from what was found in other models:  Few-Body Syst. 51, 219 (2011).

Creation of Rydberg Crystals - Sept 2011

Ultracold atomic gases have been used extensively in recent years to realize textbook examples of condensed matter phenomena. Recently, phase transitions to ordered structures have been predicted for gases of highly excited, 'frozen' Rydberg atoms. Such Rydberg crystals are a model for dilute metallic solids with tunable lattice parameters, and provide access to a wide variety of fundamental phenomena.

We investigate theoretically how such structures can be created in four distinct cold atomic systems, by using tailored laser-excitation in the presence of strong Rydberg-Rydberg interactions. We study in detail the experimental requirements and limitations for these systems, and characterize the basic properties of small crystalline Rydberg structures in one, two and three dimensions: J. Phys. B: At. Mol. Opt. Phys. 44 184008 (2011).

Magical Trios Disentangled - March2011

The weekly journal Cursor of Eindhoven University of Technology devoted an article to our work on Universal Efimov Physics. It can be found here (in dutch).

Universal Efimov Physics - January 2011

We observed the existence of a universal regime for Efimov trimers through three-body recombination loss in the vicinity of a Feshbach resonance, for ultracold Li-7 atoms. We characterize two broad Feshbach resonances in different spin states by fitting the binding energies of weakly bound molecules, created by radio-frequency association, by making use of a theoretical coupled channels analysis. This gives rise to a very precise determination of the absolute positions of the Feshbach resonances, and to very precise values of the singlet and triplet scattering length that characterize the molecular potentials of lithium. N Gross et al, C. R. Physique 12, 4 (2011)

Asymptotic Bound state Model - October 2010

1 It took some time, but now we have written up an overview paper on the Asymptotic Bound state Model, T. Tiecke et al, Phys. Rev. A 82, 042712 (2010), an easy to use model to study weakly-bound states of an ultracold two-body system, and predict the positions of Feshbach resonances. This model is already used by several experimental and theoretical groups, in particular for ultracold mixtures where no precise interaction potentials are known. In the same issue of Phys. Rev. A, we also applied this model to study Feshbach resonances in 3He*-4He* mixtures: M. Goosen et al, Phys. Rev. A 82, 042713 (2010)


Figure (left): Energies of coupled bound states for Li-40 K with total spin MF = ±3. The black solid line indicates the threshold energy of the entrance channel  The gray area represents the scattering continuum and the (colored) lines indicate the coupled bound states. Feshbach resonances occur when a bound state crosses the threshold energy. The color scheme indicates the admixture of singlet and triplet contributions in the bound states. Figure (right): Threshold behavior in a two-channel version of the dressed ABM. The threshold behavior is determined by the coupling between the least bound level in the open channel in P space and the resonant bound level in Q space. The uncoupled levels are shown as the blue (P) and red (Q) dash-dotted lines, with Q crossing the threshold at B0. The solid black lines represent the dressed levels, with the upper branch crossing the threshold at B0 . Near the threshold, the dressed level shows the
characteristic quadratic dependence on (B − B0 ) (see inset). For pure ABM levels (dotted gray) no threshold effects occur and the coupled bound state crosses the threshold at B0.

Nuclear-Spin-Independent Short-Range Three-Body Physics in Ultracold Atoms - September (2010)

We investigate three-body recombination loss across a Feshbach resonance in a gas of ultracold 7Li atoms prepared in the absolute ground state and perform a comparison with previously reported results of a different nuclear-spin state [N. Gross et al., Phys. Rev. Lett. 103, 163202 (2009)]. We extend the previously reported universality in three-body recombination loss across a Feshbach resonance to the absolute ground state. We show that the positions and widths of recombination minima and Efimov resonances are identical for both states which indicates that the short-range physics is nuclear-spin independent.

Figure: Experimentally measured three-body loss coefficient K3 as a function of scattering length (in units of Bohr radius a0 ) for the mF=1 state (red solid circles) and
the mF=0 state (blue open diamonts). The solid lines represent fits to the analytical expressions of universal theory.

Nuclear-Spin-Independent Short-Range Three-Body Physics in Ultracold Atoms
Noam Gross, Zav Shotan, Servaas Kokkelmans, and Lev Khaykovich,
Phys. Rev. Lett. 105, 103203 (2010)

Search for broad Feshbach resonances in the 6Li-40K mixture - August (2009)

We explore the widths of interspecies Feshbach resonances in a mixture of the fermionic quantum gases 6Li and 40K. Experimentally, we obtain the asymmetric lineshape of the interspecies elastic cross section by measuring the distillation rate of 6Li atoms from an optically-trapped 6Li/40K mixture as a function of magnetic field. This provides us with the first experimental determination of the width of a resonance in this mixture, ΔB=1.5(5) G, being one of the broadest. Our results offer good perspectives for the observation of universal crossover physics in this mixture.

Feshbach widths Fano profile

Figure (left): The width and position of all s-wave Feshbach resonances in stable two-component 6Li/40K mixtures below 500 G as calculated with the Asymptotic Bound-state Model.
Figure (right): Measurement of the Feshbach resonance width. The red solid line indicates the best fit obtained for B0=114.47(5) G and ΔB = 1.5(5) G. The gray shaded area indicates the combined error in B0 and ΔB.
Broad Feshbach Resonance in the 6Li-40K mixture
T.G. Tiecke, M. Goosen, A. Ludewig, S.D. Gensemer, S. Kraft, S. J. J. M. F. Kokkelmans, and J.T.M. Walraven,
Physical Review Letters 104 (2010) 053202 

Interspecies Feshbach Resonances in a Fermi-Fermi Mixture - February (2008)

The Fermionic Lithium Potassium Experiment (FeLiKx) at the Institute of Quantum Optics and Quantum Information (IQOQ) has been built to study a new class of strongly interacting many-body quantum systems. In our ultracold mixtures of the fermionic atoms Li-6 and K-40 we could characterize the interactions by measuring 13 heteronuclear Feshbach resonances. This breaks the ground for fundamental experiments exploring fermionic mixtures with different atoms.

Figure: Feshbach spectroscopy data of Li and K in one combination of spin states. In total, over 15000 data points with four different spin states have been taken during many weeks. In close collaboration between the experimental group in Innsbruck, the theory groups at NIST/Gaithersburg (USA), the Eindhoven University of Technology (The Netherlands) and the University of Amsterdam (The Netherlands) we were able to fully understand the interaction properties of Li-6 K-40 mixtures.

Exploring an Ultracold Fermi-Fermi Mixture: Interspecies Feshbach Resonances and Scattering Properties of 6Li and 40K.
E. Wille, F. M. Spiegelhalder, G. Kerner, D. Naik, A. Trenkwalder, G. Hendl, F. Schreck, R. Grimm, T. G. Tiecke, J. T. M. Walraven, S. J. J. M. F. Kokkelmans, E. Tiesinga, and P. S. Julienne.
Phys. Rev. Lett. 100, 053201 (2008)

(top, from left to right) Gerhard Hendl, Rudi Grimm, Frederik Spiegelhalder, Eric Wille, Devang Naik, Andreas Trenkwalder, Gabriel Kerner, and Florian Schreck (www.ultracold.at and IQOQI

(bottom, from left to right): Tobias Tiecke and Jook Walraven (Quantum Gases Group Amsterdam), Servaas Kokkelmans (Eindhoven), Eite Tiesinga and Paul Julienne ( Quantum Processes and Metrology Group NIST/USA) German press release from IQOQI:

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Trapped electrons in the quantum degenerate regime

A full strength Coulomb interaction between trapped electrons can be felt only in absence of a neutralizing background. In order to study quantum degenerate electrons without such a background, an external trap is needed to compensate for the strong electronic repulsion. As a basic model for such a system, we study a trapped electron pair in a harmonic trap with an explicit inclusion of its Coulomb interaction. We find the eigenenergies of the system for any value of the trapping strength. The problem is solved either numerically or by using approximate methods. As function of the trapping strength a crossover can be made from the strongly to the weakly-coupled regime, and we show that in both regimes perturbative methods based on a pair-wise electron description would be effective for a many-particle trapped electron system.   arXiv:1508.00365

Stability of triplet rubidium ground-state molecules

Experiments involving ultracold molecules require sufficiently long lifetimes, which can be very short for excited rovibrational states in the molecular potentials. For alkali atoms such as rubidium, molecular, rovibrational ground-states can both be found in the electronic singlet and triplet configurations. The molecular singlet ground state is absolutely stable, however, the triplet ground state can decay to a deeper bound singlet molecule due to a radiative decay mechanism that involves the interatomic spin-orbit interaction. We investigate this mechanism, and find the lifetime of rubidium molecules in the triplet rovibrational ground-state to be about 13 minutes. This is sufficiently long for experimental purposes.   arXiv:1412.5799