Martin Berninger

Evidence for universal four-body states tied to an Efimov trimer.

We report on the measurement of four-body 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 four-body 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 three-body problem.

Observation of an Efimov-like trimer resonance in ultracold atom–dimer scattering

The observation of a trimer resonance in an ultracold mixture of caesium atoms and dimers confirms one of the key predictions of three-body physics in the limit of resonant two-body interactions, with possible implications for understanding few-body states in nuclear matter.

Universality of the three-body parameter for Efimov states in ultracold cesium.

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 magnetic-field dependent three-body recombination loss. The positions of the loss resonances yield corresponding values for the three-body parameter, which in universal few-body physics is required to describe three-body phenomena and, in particular, to fix the spectrum of Efimov states. Our observations show a robust universal behavior with a three-body parameter that stays essentially constant.

Polarizability measurements of a molecule via a near-field matter-wave interferometer

We present a method for measuring the scalar polarizability of molecules by means of near-field matter-wave interferometry. This technique is applicable to a wide range of complex neutral molecules. A key feature of our experiment is the combination of good transmission and high spatial resolution, gained by imprinting a submicrometer spatial modulation onto a wide molecular beam. Our method significantly improves the measurement sensitivity of the polarizability-dependent shifts. We have successfully performed measurements on fullerenes and report the polarizability to be {alpha}=88.9{+-}0.9{+-}5.1 A{sup 3} for C{sub 60} and {alpha}=108.5{+-}2.0{+-}6.2 A{sup 3} for C{sub 70}.

Magnetically controlled exchange process in an ultracold atom-dimer mixture

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 three-body Schrödinger 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.

Efimov Resonances in Ultracold Quantum Gases

Ultracold atomic gases have developed into prime systems for experimental studies of Efimov three-body physics and related few-body 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 few-body 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 well-investigated model system. Triatomic Efimov resonances, atom-dimer Efimov resonances, and related four-body resonances are discussed as central observables. We also present some new observations of such resonances, supporting and complementing the set of available data.

Collisions between Tunable Halo Dimers : Exploring an Elementary Four-Body Process with Identical Bosons

We study inelastic collisions in a pure, trapped sample of Feshbach molecules made of bosonic cesium atoms in the quantum halo regime. We measure the relaxation rate coefficient for decay to lower-lying molecular states and study the dependence on scattering length and temperature. We identify a pronounced loss minimum with varying scattering length along with a further suppression of loss with decreasing temperature. Our observations provide insight into the physics of a few-body quantum system that consists of four identical bosons at large values of the two-body scattering length.

Resonant five-body recombination in an ultracold gas of bosonic atoms

We combine theory and experiment to investigate five-body 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 five-body 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.

Feshbach resonances, weakly bound molecular states, and coupled-channel potentials for cesium at high magnetic fields.

We explore the scattering properties of ultracold ground-state Cs atoms at magnetic fields between 450 G (45 mT) and 1000 G. We identify 17 previously unreported Feshbach resonances, including two very broad ones near 549 and 787 G. We measure the binding energies of several different dimer states by magnetic field modulation spectroscopy. We use least-squares fitting to these experimental results, together with previous measurements at lower field, to determine a six-parameter model of the long-range interaction potential, designated M2012. Coupled-channels calculations using M2012 provide an accurate mapping between the s-wave 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.

Resonant atom-dimer collisions in cesium: Testing universality at positive scattering lengths

Efimov’s solution to the problem of three resonantly interacting particles [1] is widely considered to be the most prominent example of a universal few-body system, where the knowledge of the two-body scattering length a and an additional three-body parameter is sufficient to define the who le energy spectrum and to locate all the bound states. The details of the interparticle potential become irrelevant and diffe rent systems very far apart in energy and length scales can be described in the same way. The famous discrete scaling of the Efimov spectrum (scaling factor of 22 .7) and the precise ratios that link its different parts have attracted large interest in the scientific community. Universal behavior arises from the presence of resonant interactions leading to collisions on a length scale exceedin g the typical size of the interparticle potential. In trimer s ystems, the contributions of the short-range details are commonly included in the “three-body parameter”. This parameter fixes the starting point of the spectrum and can be expressed in terms of the scattering length a− at which the most deeply bound Efimov state crosses the zero-energy threshold [2]. Within the ideal Efimov scenario, the positions of all th e other features of the spectrum are uniquely determined, both at positive and negative values of a. The validation of Efimov’s scenario had remained elusive for decades until experiments on ultracold atoms provided evidence for its existence [3‐13]. The appearance of trimer bound states has been shown by measuring inelastic collisional rates in atomic samples or atom-dimer mixtures by tuning the scattering length via magnetically controlled Fesh bach resonances [14]. The presence of trimer bound states leads to enhancement and suppression of losses [15‐17]. In particular the loss resonances represent a “smoking gun” for Efimov’s spectrum and occur where the trimer energy state crosses the atomic threshold (at a−, in the region of negative a) or merges into the state of a dimer plus a free atom (at a∗, in the region of positive a).