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Few-body collisions in a quantum gas mixture of 40K and 87Rb atoms

Ruth Bloom

Published Jan 1, 2014• 1 citation
OpenAlex

Abstract

This thesis describes experiments on few-body interactions in a mixture of ultracold bosonic 87Rb and fermionic 40K atoms. Ultracold atoms are celebrated as a platform to explore fundamental quantum physics because their internal states, external potentials, and interactions with each other can be controlled by straightforward electrical and optical tools. In the case of Bose-Fermi mixtures, control comes at the cost of stability when strong interactions give rise to inelastic collisions that destroy the atomic samples. I present detailed measurements of the cross sections for these inelastic processes and discuss how they fit into an emerging pattern of resonances that is leading to better models of few-atom interactions. These phenomena have their roots in universal Efimov physics, which is a paradigm of quantum three-body interactions that was originally inspired by exotic nuclear states. I also discuss several methods to control the formation of KRb Feshbach molecules within the finite lifetime of the interacting K and Rb atomic mixture.

Related papers

Bose polarons and rotating gases in an ultracold Bose-Fermi gas mixture of 40K and 87Rb atoms

Ming-Guang Hu

An ultracold Bose-Fermi gas mixture of 40K and 87Rb atoms has tunable interspecies interactions and therefore provides a fantastic platform for exploring not only few-body physics such as Feshbach molecule formation and Efimov trimers, but also many-body physics including Bose polarons, quantum Hall physics and so on. In this thesis, I present experimental evidence of Bose polarons in cold atoms obtained using radio-frequency spectroscopy to measure the excitation spectrum of fermionic 40K impurities resonantly interacting with a BEC of 87Rb atoms. These Bose polarons originate from the dressing of an impurity coupled to its environment, which is an important paradigm in quantum many-body physics. I also present initial work that launches an exciting new direction for our experiment, which is exploring rotating quantum gases. Goals for this work include studying both rotating Bose and Fermi superfluids with tunable interactions as well as working toward rapidly rotating quantum gases in the quantum Hall regime. For these goals, a new all-optical trap for rotating gases was designed, implemented, and tested using a 87Rb Bose-Einstein condensate.

•Jan 1, 2016•0 citations

Exploring Few-body Scattering Resonances in a Bose-Fermi Mixture: From Feshbach Dimers to Efimov Trimers

Tyler D. Cumby

This thesis presents experiments exploring Feshbach molecules and Efimov trimers in an ultracold Bose-Fermi atomic gas mixture of 40K and 87Rb. These bound states are accessed via scattering resonances, called Fano-Feshbach resonances, which arise as a consequence of a bound state being tuned through degeneracy with the threshold energy of colliding atoms. In addition to allowing the tunability of interactions, the Fano-Feshbach resonance allows for a weakly bound molecule state, called the Feshbach molecule state, to be populated. The efficiency of conversion to the Feshbach molecule state depends upon the rate with which the bound state energy is swept through degeneracy with atomic threshold energy. In regions of large scattering lengths, three-body bound states, called Efimov states, also influence inelastic Bose-Bose-Fermi collisions and boson+molecule collisions in our system. The work presented in this thesis utilizes a magnetically tunable Fano-Feshbach resonance to study Feshbach molecule creation in the perturbative limit (the limit of fast magnetic-field sweeps) and the saturated limit (the limit of slow sweeps), where a phenomenological model is applied to understand the conversion efficiency as well as the quantum degeneracy of the molecules. In addition, I present a study of boson+molecule collision rates and Bose-Bose-Fermi collision rates in order to observe signatures of Efimov states and compare against results in the gas mixture 41K-87Rb to address the question of universality of the three-body parameter in heteronuclear gas mixtures.

•Jan 1, 2012•0 citations

Tests of Universal Three-Body Physics in an Ultracold Bose-Fermi Mixture

Ruth Bloom, Ming-Guang Hu, Tyler D. Cumby +1 more

Recent measurements of Efimov resonances for a number of ultracold atom species have revealed an unexpected universality, in which three-body scattering properties are determined by the van der Waals length of the two-body interaction potential. To investigate whether this universality extends to heteronuclear mixtures, we measure loss rate coefficients in an ultracold trapped gas of 40K and 87Rb atoms. We find an Efimov-like resonance in the rate of inelastic collisions between 40K87Rb Feshbach molecules and 87Rb atoms. However, we do not observe any Efimov-related resonances in the rates of inelastic collisions between three atoms. These observations are compared to previous measurements by the LENS group of Efimov resonances in a 41K and 87Rb mixture as well as to recent predictions.

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Precise Measurements of Few-Body Physics in Ultracold 39k Bose Gas

Roman Chapurin

Ultracold atomic gases with tunable interactions offer an ideal platform for studying interacting quantum matter. While the few- and many-body physics are generally complex and intractable, the problem can be greatly simplified in an atomic gas by a controlled separation of relevant length and energy scales. Precise control of experimental parameters, via Feshbach resonances, optical potentials and radio-frequency radiation, enables deterministic measurements of few-body physics, including universal physics and the Efimov effect. In this thesis, I present our recent studies on precisely measuring two- and three-body physics in an ultracold Bose gas. I begin by describing our new apparatus used for generating and studying ultracold 39K gas samples. Then, I focus on our precision spectroscopy of Feshbach dimer binding energies, spanning three orders of magnitude and with sub-kilohertz resolution. These measurements enable us to locate a Feshbach resonance and determine scattering length values with unprecedented accuracy. Finally, I present our precise measurements locating an exotic three-body state, specifically the Efimov ground state. We find that the trimer state location significantly deviates from the value predicted by van der Waals universality. Due to small experimental and systematic uncertainties, our measurement is the strongest evidence of departure from the universal value and is the first observed deviation near a Feshbach resonance of intermediate strength.

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Collisional Stability of Fermionic Feshbach Molecules

J. J. Zirbel, Kang-Kuen Ni, Silke Ospelkaus +4 more

Using a Feshbach resonance, we create ultracold fermionic molecules starting from a Bose-Fermi atom gas mixture. The resulting mixture of atoms and weakly bound molecules provides a rich system for studying few-body collisions because of the variety of atomic collision partners for molecules; either bosonic, fermionic, or distinguishable atoms. Inelastic loss of the molecules near the Feshbach resonance is dramatically affected by the quantum statistics of the colliding particles and the scattering length. In particular, we observe a molecule lifetime as long as 100 ms near the Feshbach resonance.

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Efimov Physics in a three-component Fermi gas

Thomas Lompe

This thesis reports on experiments studying the few-body physics of three distinguishable fermionic atoms with large scattering lengths. The experiments were performed with ultracold gases of fermionic 6Li atoms in three different hyperfine states. By tuning the strength of the interactions between the atoms with Feshbach resonances and measuring the rate constants for inelastic three-atom and atom-dimer collisions the intersections of two universal trimer states with the three-atom and atom-dimer continuum could be located. Subsequently, one of these Efimov states was directly observed with RF-association spectroscopy. Using this technique the binding energy of this Efimov state was measured as a function of the strength of the interparticle interactions. The experiments presented in this thesis provide a nearly complete understanding of the universal few-body physics of three-component Fermi gases of 6Li atoms. This understanding will be the foundation for future studies of the many-body physics of three-component Fermi gases.

•Jan 1, 2011•1 citation
This thesis describes experiments on few-body interactions in a mixture of ultracold bosonic 87Rb and fermionic 40K atoms. Ultracold atoms are celebrated as a platform to explore fundamental quantum physics because their internal states, external potentials, and interactions with each other can be controlled by straightforward electrical and optical tools. In the case of Bose-Fermi mixtures, control comes at the cost of stability when strong interactions give rise to inelastic collisions that destroy the atomic samples. I present detailed measurements of the cross sections for these inelastic processes and discuss how they fit into an emerging pattern of resonances that is leading to better models of few-atom interactions. These phenomena have their roots in universal Efimov physics, which is a paradigm of quantum three-body interactions that was originally inspired by exotic nuclear states. I also discuss several methods to control the formation of KRb Feshbach molecules within the finite lifetime of the interacting K and Rb atomic mixture.
An ultracold Bose-Fermi gas mixture of 40K and 87Rb atoms has tunable interspecies interactions and therefore provides a fantastic platform for exploring not only few-body physics such as Feshbach molecule formation and Efimov trimers, but also many-body physics including Bose polarons, quantum Hall physics and so on. In this thesis, I present experimental evidence of Bose polarons in cold atoms obtained using radio-frequency spectroscopy to measure the excitation spectrum of fermionic 40K impurities resonantly interacting with a BEC of 87Rb atoms. These Bose polarons originate from the dressing of an impurity coupled to its environment, which is an important paradigm in quantum many-body physics. I also present initial work that launches an exciting new direction for our experiment, which is exploring rotating quantum gases. Goals for this work include studying both rotating Bose and Fermi superfluids with tunable interactions as well as working toward rapidly rotating quantum gases in the quantum Hall regime. For these goals, a new all-optical trap for rotating gases was designed, implemented, and tested using a 87Rb Bose-Einstein condensate.
This thesis presents experiments exploring Feshbach molecules and Efimov trimers in an ultracold Bose-Fermi atomic gas mixture of 40K and 87Rb. These bound states are accessed via scattering resonances, called Fano-Feshbach resonances, which arise as a consequence of a bound state being tuned through degeneracy with the threshold energy of colliding atoms. In addition to allowing the tunability of interactions, the Fano-Feshbach resonance allows for a weakly bound molecule state, called the Feshbach molecule state, to be populated. The efficiency of conversion to the Feshbach molecule state depends upon the rate with which the bound state energy is swept through degeneracy with atomic threshold energy. In regions of large scattering lengths, three-body bound states, called Efimov states, also influence inelastic Bose-Bose-Fermi collisions and boson+molecule collisions in our system. The work presented in this thesis utilizes a magnetically tunable Fano-Feshbach resonance to study Feshbach molecule creation in the perturbative limit (the limit of fast magnetic-field sweeps) and the saturated limit (the limit of slow sweeps), where a phenomenological model is applied to understand the conversion efficiency as well as the quantum degeneracy of the molecules. In addition, I present a study of boson+molecule collision rates and Bose-Bose-Fermi collision rates in order to observe signatures of Efimov states and compare against results in the gas mixture 41K-87Rb to address the question of universality of the three-body parameter in heteronuclear gas mixtures.
Recent measurements of Efimov resonances for a number of ultracold atom species have revealed an unexpected universality, in which three-body scattering properties are determined by the van der Waals length of the two-body interaction potential. To investigate whether this universality extends to heteronuclear mixtures, we measure loss rate coefficients in an ultracold trapped gas of 40K and 87Rb atoms. We find an Efimov-like resonance in the rate of inelastic collisions between 40K87Rb Feshbach molecules and 87Rb atoms. However, we do not observe any Efimov-related resonances in the rates of inelastic collisions between three atoms. These observations are compared to previous measurements by the LENS group of Efimov resonances in a 41K and 87Rb mixture as well as to recent predictions.
Ultracold atomic gases with tunable interactions offer an ideal platform for studying interacting quantum matter. While the few- and many-body physics are generally complex and intractable, the problem can be greatly simplified in an atomic gas by a controlled separation of relevant length and energy scales. Precise control of experimental parameters, via Feshbach resonances, optical potentials and radio-frequency radiation, enables deterministic measurements of few-body physics, including universal physics and the Efimov effect. In this thesis, I present our recent studies on precisely measuring two- and three-body physics in an ultracold Bose gas. I begin by describing our new apparatus used for generating and studying ultracold 39K gas samples. Then, I focus on our precision spectroscopy of Feshbach dimer binding energies, spanning three orders of magnitude and with sub-kilohertz resolution. These measurements enable us to locate a Feshbach resonance and determine scattering length values with unprecedented accuracy. Finally, I present our precise measurements locating an exotic three-body state, specifically the Efimov ground state. We find that the trimer state location significantly deviates from the value predicted by van der Waals universality. Due to small experimental and systematic uncertainties, our measurement is the strongest evidence of departure from the universal value and is the first observed deviation near a Feshbach resonance of intermediate strength.
Using a Feshbach resonance, we create ultracold fermionic molecules starting from a Bose-Fermi atom gas mixture. The resulting mixture of atoms and weakly bound molecules provides a rich system for studying few-body collisions because of the variety of atomic collision partners for molecules; either bosonic, fermionic, or distinguishable atoms. Inelastic loss of the molecules near the Feshbach resonance is dramatically affected by the quantum statistics of the colliding particles and the scattering length. In particular, we observe a molecule lifetime as long as 100 ms near the Feshbach resonance.
This thesis reports on experiments studying the few-body physics of three distinguishable fermionic atoms with large scattering lengths. The experiments were performed with ultracold gases of fermionic 6Li atoms in three different hyperfine states. By tuning the strength of the interactions between the atoms with Feshbach resonances and measuring the rate constants for inelastic three-atom and atom-dimer collisions the intersections of two universal trimer states with the three-atom and atom-dimer continuum could be located. Subsequently, one of these Efimov states was directly observed with RF-association spectroscopy. Using this technique the binding energy of this Efimov state was measured as a function of the strength of the interparticle interactions. The experiments presented in this thesis provide a nearly complete understanding of the universal few-body physics of three-component Fermi gases of 6Li atoms. This understanding will be the foundation for future studies of the many-body physics of three-component Fermi gases.