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Non-Equilbrium Fermi Gases

John E. Thomas

Published Feb 2, 2016• 0 citations
OpenAlex

Abstract

Abstract : During this period, we made important breakthroughs in the optical control of two-body interactions in ultra-cold gases. We have studied magnetic Feshbach resonances in an optically-trapped mixture of the two lowest hyperfine states of a 6Li Fermi gas, using two optical fields to create a darkstate in the closed molecular channel. In the experiments, the narrow Feshbach resonance is tuned by up to 3 G. For the broad resonance, thespontaneous lifetime is increased from 0.5 ms for single field tuning to 0.4 s at the dark state resonance, despite the large background scatteringlength.A major breakthrough in our understanding is the experimental verification of a new model of light-induced loss spectra, employing continuum-dressed basis states, which agrees in shape and magnitude with all of our loss measurements for both broad and narrow resonances.Using this model, we show that our method substantially reduces the two-body loss rate compared to single field methods for same tuning range.

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Optical Control of Magnetic Feshbach Resonances by Closed-Channel Electromagnetically Induced Transparency

Arunkumar Jagannathan, N. Arunkumar, J. A. Joseph +1 more

We control magnetic Feshbach resonances in an optically trapped mixture of the two lowest hyperfine states of a ^{6}Li Fermi gas, using two optical fields to create a dark state in the closed molecular channel. In the experiments, the narrow Feshbach resonance is tuned by up to 3 G. For the broad resonance, the spontaneous lifetime is increased to 0.4 s at the dark-state resonance, compared to 0.5 ms for single-field tuning. We present a new model of light-induced loss spectra, employing continuum-dressed basis states, which agrees in shape and magnitude with loss measurements for both broad and narrow resonances. Using this model, we predict the trade-off between tunability and loss for the broad resonance in ^{6}Li, showing that our two-field method substantially reduces the two-body loss rate compared to single-field methods for the same tuning range.

arXiv:1511.01403•Physical Review Letters•Feb 17, 2016•40 citations

Feshbach resonances in cold atomic gases

E. G. M. van Kempen

The research described in this thesis is part of an international effort with the aim of studying ultracold quantum degenerate gas samples like Bose-Einstein condensates and Fermi degenerate systems, consisting of atoms confined in magnetic or optical traps. The behavior of such samples is governed by the inter-atomic interactions and the resulting properties of the atom-atom scattering. At the relevant temperatures of 1nK to 10µK a key property is the atom-atom scattering length a. In chapter 2 a theoretical method is presented which enables one to describe the interaction and scattering of (ultra)cold atoms to unprecedented precision. It is also unparalleled in comprehensiveness: it allows the prediction of a large and varied set of experimental data for all isotopes of the same element. The method relies on the extraction from experiments of a few (phase) parameters which completely summarize the behavior of the atoms in the (ultra)cold regime. In chapter 3 the method is applied to the "workhorses" of cold-atom physics: the atomic species 85Rb and 87Rb. We extract the foregoing parameters to a very high precision from several recent high precision experiments, allowing us to predict e.g. the 87Rb spinor condensate to be ferromagnetic: a prediction for which the scattering length has to be calculated with a precision better than 1%. We also predict Feshbach resonances at experimentally accessible magnetic field strengths; resonances searched for and found by the experimental group of Rempe. In close collaboration with his group we "fine-tune" the interaction parameters found previously, by making use of only one of the observed resonances. We then obtain agreement with 42 out of the observed 43 resonance field strengths and are able to identify bound states inducing the Feshbach resonances at these locations. Chapter 4 describes the results of this research. With a thorough understanding of the rubidium interactions, we then switch to lithium which has a fermionic (6Li) and a bosonic (7Li) isotope. Both are being used in cold-atom experiments. In chapter 5 we evaluate the interaction parameters for lithium allowing us to predict magnetic field strengths for which a sample of fermionic 6Li atoms can be regarded as strongly interacting. Furthermore, a three-level method for measuring mean-field shifts, based on radio-frequency techniques, is introduced. For weak interactions we find proportionality of resonance shifts to interaction strengths. In the strongly interacting regime, however, these shifts become very small reflecting the quantum unitarity limit and many-body effects. Most interesting is the fact that in this regime the shifts are small both for large positive a and for large negative a, likely reflecting the universality of the interaction energy. In chapters 6 and 7 the interactions between lithium atoms are reinvestigated, making use of newly available experimental data and with the updated interaction parameters special attention is paid to locating field strengths at which magnetically tunable Feshbach resonances occur in the scattering of lithium atoms. In chapter 6 scattering events in a gas of (fermionic) 6Li atoms are studied. In chapter 7 we show that a consistent description of the 6Li+6Li system can be given. We discuss theoretical uncertainties for the position of the wide 6Li Feshbach resonance and present an analytic scattering model for this resonance, based on the inclusion of a field-dependent virtual open-channel state. We predict new Feshbach resonances for the 6Li-7Li system, and their importance for different types of crossover superfluidity models is discussed. Molecules created by magnetically sweeping over these resonances will have a fermionic character. One magnetic field strength is predicted at which two different fermionic molecules can be created simultaneously.

•Jan 1, 2006•1 citation

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Haibin Wu, John E. Thomas

We propose a general method for optical control of magnetic Feshbach resonances in ultracold atomic gases with more than one molecular state in an energetically closed channel. Using two optical frequencies to couple two states in the closed channel, inelastic loss arising from spontaneous emission is greatly suppressed by destructive quantum interference at the two-photon resonance, i.e., dark-state formation, while the scattering length is widely tunable by varying the frequencies and/or intensities of the optical fields. This technique is of particular interest for a two-component atomic Fermi gas, which is stable near a Feshbach resonance.

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Few-body physics in ultracold Fermi gases

T. B. Ottenstein

This thesis reports on experimental studies of few-body physics in ultracold Fermi gases. The main part is devoted to the first realisation of an ultracold three-component Fermi gas in thermal equilibrium that consists of atoms in the three lowest spin states of 6Li. In first experiments the collisional stability of the gas was investigated in dependence of the interparticle interaction. The interaction strength was tuned by an external magnetic field employing Feshbach resonances in all three two-body scattering channels. At two magnetic field values a resonant enhancement of three-body loss was observed that can be explained by an Efimov-like trimer consisting of one atom in each of the three states which crosses the continuum twice in the studied magnetic field region. The behaviour of three-body loss rates could be understood by taking into account a variation of the lifetime of the trimer with the magnetic field. These experiments were performed with large ensembles of atoms. A finite deeply degenerate Fermi gas consisting of a controlled number of atoms may serve as a clean model for finite Fermi systems as for example atomic nuclei. An approach for the preparation of such a gas is discussed and first experimental steps are presented.

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Interspecies Feshbach Resonances in an Ultracold, Optically Trapped Bose-Fermi Mixture of Cesium and Lithium

Marc Repp

This thesis reports on the tunability of interactions in ultracold Bose-Fermi mixtures of Cesium and Lithium. The \nfirst realization of an optically trapped \n6Li -133Cs mixture enabled to perform trap loss spectroscopy measurements to identify \nmagnetic Feshbach resonances. A total of 19 interspecies Feshbach resonances, all in the magnetic field range between 650 G and 950 G, were observed for the two energetically lowest spin states of each species. Two 5 G broad and especially two 60 G broad s-wave resonances give perspectives to produce a dipolar quantum gas of LiCs ground state molecules as well as to study the Efimov effect in highly mass imbalanced systems. In addition, a unique relative tunability of intra- and interspecies \nscattering lengths was found which makes the 6Li -133Cs system also well suited for the investigation of polarons. Evaporative cooling was performed on optically trapped samples which contained only one of the species. In this way, Bose-Einstein condensates of 6Li molecules as well as 133Cs samples at a phase-space density of p = 4 · 10^2 were prepared. All experiments were performed in a new apparatus, which has been designed and set up during this thesis.

•Jan 1, 2013•5 citations
Abstract : During this period, we made important breakthroughs in the optical control of two-body interactions in ultra-cold gases. We have studied magnetic Feshbach resonances in an optically-trapped mixture of the two lowest hyperfine states of a 6Li Fermi gas, using two optical fields to create a darkstate in the closed molecular channel. In the experiments, the narrow Feshbach resonance is tuned by up to 3 G. For the broad resonance, thespontaneous lifetime is increased from 0.5 ms for single field tuning to 0.4 s at the dark state resonance, despite the large background scatteringlength.A major breakthrough in our understanding is the experimental verification of a new model of light-induced loss spectra, employing continuum-dressed basis states, which agrees in shape and magnitude with all of our loss measurements for both broad and narrow resonances.Using this model, we show that our method substantially reduces the two-body loss rate compared to single field methods for same tuning range.
We control magnetic Feshbach resonances in an optically trapped mixture of the two lowest hyperfine states of a ^{6}Li Fermi gas, using two optical fields to create a dark state in the closed molecular channel. In the experiments, the narrow Feshbach resonance is tuned by up to 3 G. For the broad resonance, the spontaneous lifetime is increased to 0.4 s at the dark-state resonance, compared to 0.5 ms for single-field tuning. We present a new model of light-induced loss spectra, employing continuum-dressed basis states, which agrees in shape and magnitude with loss measurements for both broad and narrow resonances. Using this model, we predict the trade-off between tunability and loss for the broad resonance in ^{6}Li, showing that our two-field method substantially reduces the two-body loss rate compared to single-field methods for the same tuning range.
The research described in this thesis is part of an international effort with the aim of studying ultracold quantum degenerate gas samples like Bose-Einstein condensates and Fermi degenerate systems, consisting of atoms confined in magnetic or optical traps. The behavior of such samples is governed by the inter-atomic interactions and the resulting properties of the atom-atom scattering. At the relevant temperatures of 1nK to 10µK a key property is the atom-atom scattering length a. In chapter 2 a theoretical method is presented which enables one to describe the interaction and scattering of (ultra)cold atoms to unprecedented precision. It is also unparalleled in comprehensiveness: it allows the prediction of a large and varied set of experimental data for all isotopes of the same element. The method relies on the extraction from experiments of a few (phase) parameters which completely summarize the behavior of the atoms in the (ultra)cold regime. In chapter 3 the method is applied to the "workhorses" of cold-atom physics: the atomic species 85Rb and 87Rb. We extract the foregoing parameters to a very high precision from several recent high precision experiments, allowing us to predict e.g. the 87Rb spinor condensate to be ferromagnetic: a prediction for which the scattering length has to be calculated with a precision better than 1%. We also predict Feshbach resonances at experimentally accessible magnetic field strengths; resonances searched for and found by the experimental group of Rempe. In close collaboration with his group we "fine-tune" the interaction parameters found previously, by making use of only one of the observed resonances. We then obtain agreement with 42 out of the observed 43 resonance field strengths and are able to identify bound states inducing the Feshbach resonances at these locations. Chapter 4 describes the results of this research. With a thorough understanding of the rubidium interactions, we then switch to lithium which has a fermionic (6Li) and a bosonic (7Li) isotope. Both are being used in cold-atom experiments. In chapter 5 we evaluate the interaction parameters for lithium allowing us to predict magnetic field strengths for which a sample of fermionic 6Li atoms can be regarded as strongly interacting. Furthermore, a three-level method for measuring mean-field shifts, based on radio-frequency techniques, is introduced. For weak interactions we find proportionality of resonance shifts to interaction strengths. In the strongly interacting regime, however, these shifts become very small reflecting the quantum unitarity limit and many-body effects. Most interesting is the fact that in this regime the shifts are small both for large positive a and for large negative a, likely reflecting the universality of the interaction energy. In chapters 6 and 7 the interactions between lithium atoms are reinvestigated, making use of newly available experimental data and with the updated interaction parameters special attention is paid to locating field strengths at which magnetically tunable Feshbach resonances occur in the scattering of lithium atoms. In chapter 6 scattering events in a gas of (fermionic) 6Li atoms are studied. In chapter 7 we show that a consistent description of the 6Li+6Li system can be given. We discuss theoretical uncertainties for the position of the wide 6Li Feshbach resonance and present an analytic scattering model for this resonance, based on the inclusion of a field-dependent virtual open-channel state. We predict new Feshbach resonances for the 6Li-7Li system, and their importance for different types of crossover superfluidity models is discussed. Molecules created by magnetically sweeping over these resonances will have a fermionic character. One magnetic field strength is predicted at which two different fermionic molecules can be created simultaneously.
We propose a general method for optical control of magnetic Feshbach resonances in ultracold atomic gases with more than one molecular state in an energetically closed channel. Using two optical frequencies to couple two states in the closed channel, inelastic loss arising from spontaneous emission is greatly suppressed by destructive quantum interference at the two-photon resonance, i.e., dark-state formation, while the scattering length is widely tunable by varying the frequencies and/or intensities of the optical fields. This technique is of particular interest for a two-component atomic Fermi gas, which is stable near a Feshbach resonance.
We describe two-field optical techniques to control interactions in Feshbach resonances for two-body scattering in ultracold gases. These techniques create a molecular dark state in the closed channel of a magnetically tunable Feshbach resonance, greatly suppressing optical scattering compared to single optical field methods. The dark-state method enables control of the effective range, by creating narrow features that modify the energy dependence of the scattering phase shift, as well as control of the elastic and inelastic parts of the zero-energy sss-wave scattering amplitude. We determine the scattering length and the effective range from an effective range expansion, by calculating the momentum-dependent scattering phase shift from the two-body scattering state.
This thesis reports on experimental studies of few-body physics in ultracold Fermi gases. The main part is devoted to the first realisation of an ultracold three-component Fermi gas in thermal equilibrium that consists of atoms in the three lowest spin states of 6Li. In first experiments the collisional stability of the gas was investigated in dependence of the interparticle interaction. The interaction strength was tuned by an external magnetic field employing Feshbach resonances in all three two-body scattering channels. At two magnetic field values a resonant enhancement of three-body loss was observed that can be explained by an Efimov-like trimer consisting of one atom in each of the three states which crosses the continuum twice in the studied magnetic field region. The behaviour of three-body loss rates could be understood by taking into account a variation of the lifetime of the trimer with the magnetic field. These experiments were performed with large ensembles of atoms. A finite deeply degenerate Fermi gas consisting of a controlled number of atoms may serve as a clean model for finite Fermi systems as for example atomic nuclei. An approach for the preparation of such a gas is discussed and first experimental steps are presented.
This thesis reports on the tunability of interactions in ultracold Bose-Fermi mixtures of Cesium and Lithium. The \nfirst realization of an optically trapped \n6Li -133Cs mixture enabled to perform trap loss spectroscopy measurements to identify \nmagnetic Feshbach resonances. A total of 19 interspecies Feshbach resonances, all in the magnetic field range between 650 G and 950 G, were observed for the two energetically lowest spin states of each species. Two 5 G broad and especially two 60 G broad s-wave resonances give perspectives to produce a dipolar quantum gas of LiCs ground state molecules as well as to study the Efimov effect in highly mass imbalanced systems. In addition, a unique relative tunability of intra- and interspecies \nscattering lengths was found which makes the 6Li -133Cs system also well suited for the investigation of polarons. Evaporative cooling was performed on optically trapped samples which contained only one of the species. In this way, Bose-Einstein condensates of 6Li molecules as well as 133Cs samples at a phase-space density of p = 4 · 10^2 were prepared. All experiments were performed in a new apparatus, which has been designed and set up during this thesis.