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Above-threshold scattering about a Feshbach resonance for ultracold atoms in an optical collider

Milena S. J. Horvath, Ryan Thomas, Eite Tiesinga, Amita B. Deb, Niels Kjærgaard

Published Jan 1, 2017• 5 citations
OpenAlex

Abstract

Abstract Ultracold atomic gases have realized numerous paradigms of condensed matter physics, where control over interactions has crucially been afforded by tunable Feshbach resonances. So far, the characterization of these Feshbach resonances has almost exclusively relied on experiments in the threshold regime near zero energy. Here, we use a laser-based collider to probe a narrow magnetic Feshbach resonance of rubidium above threshold. By measuring the overall atomic loss from colliding clouds as a function of magnetic field, we track the energy-dependent resonance position. At higher energy, our collider scheme broadens the loss feature, making the identification of the narrow resonance challenging. However, we observe that the collisions give rise to shifts in the center-of-mass positions of outgoing clouds. The shifts cross zero at the resonance and this allows us to accurately determine its location well above threshold. Our inferred resonance positions are in excellent agreement with theory.

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Above-threshold scattering about a Feshbach resonance for ultracold atoms in an optical collider

Milena S. J. Horvath, Ryan Thomas, Eite Tiesinga +2 more

Ultracold atomic gases have realized numerous paradigms of condensed matter physics, where control over interactions has crucially been afforded by tunable Feshbach resonances. So far, the characterization of these Feshbach resonances has almost exclusively relied on experiments in the threshold regime near zero energy. Here, we use a laser-based collider to probe a narrow magnetic Feshbach resonance of rubidium above threshold. By measuring the overall atomic loss from colliding clouds as a function of magnetic field, we track the energy-dependent resonance position. At higher energy, our collider scheme broadens the loss feature, making the identification of the narrow resonance challenging. However, we observe that the collisions give rise to shifts in the center-of-mass positions of outgoing clouds. The shifts cross zero at the resonance and this allows us to accurately determine its location well above threshold. Our inferred resonance positions are in excellent agreement with theory.Studies on energy-dependent scattering of ultracold atoms were previously carried out near zero collision energies. Here, the authors observe a magnetic Feshbach resonance in ultracold Rb collisions for above-threshold energies and their method can also be used to detect higher partial wave resonances.

arXiv:1704.07109•Nature Communications•Aug 31, 2017•14 citations

Energy dependent scattering of ultracold atoms about a Feshbach resonance

Milena S. J. Horvath

In this thesis we explore ultracold atomic collisions in the vicinity of a magnetic Feshbach resonance located at 9.1 G, between 87Rb atoms in the |F = 2, mF = 1> and the |F =1, mF = 1> hyperfine states. We experimentally investigate the energy dependence of the enhancement of inelastic interaction in the neighborhood of this Feshbach resonance by analyzing the temperature dependence of a loss spectroscopy feature. An asymmetric widening of the resonance feature with respect to temperature is observed. We develop a simple model which captures these asymmetric line shapes through thermal averaging over the collision energy dependence of the Feshbach resonance. The energy dependence of the Feshbach interactions is further explored by colliding ultracold atomic samples at a fixed energy significantly above threshold using an optical trapping system; the elastic scattering enhancement analyzed by colliding the samples in free space, while for the inelastic measurements the collisions were carried out in trap. No conclusive resonant behavior could obtained for the elastic component of the 9.1 G resonance, however the inelastic scattering enhancement exhibited an anticipated shift.

•Jan 1, 2016•1 citation

Quantum Scattering in an Optical Collider for Ultracold Atoms

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Optical control enables new high resolution probes of narrow collisional (Feshbach) resonances, which are strongly dependent on the relative momentum of colliding atom pairs, and important for simulating neutron matter with ultracold atomic gases. We demonstrate a two-field optical vernier, which expands kHz (mG) magnetic field detunings near a narrow resonance into MHz optical field detunings, enabling precise control and characterization of the momentum-dependent scattering amplitude. Two-photon loss spectra are measured for the narrow resonance in ^{6}Li, revealing rich structure in very good agreement with our theoretical model. However, anomalous overall frequency shifts between the measured and predicted two-photon spectra are not yet explained.

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We employ a quantum defect theory framework to provide a detailed analysis of the interplay between a magnetic Feshbach resonance and a shape resonance in cold collisions of ultracold $^{87}\mathrm{Rb}$ atoms as captured in recent experiments using a laser-based collider [M. Chilcott et al., Phys. Rev. Research 3, 033209 (2021)]. By exerting control over a parameter space spanned by both collision energy and magnetic field, the width of a Feshbach resonance can be tuned over several orders of magnitude. We apply a quantum defect theory specialized for ultracold atomic collisions to fully describe of the experimental observations. While the width of a Feshbach resonance generally increases with collision energy, its coincidence with a shape resonance leads to a significant additional boost. By conducting experiments at a collision energy matching the shape resonance and using the shape resonance as a magnifying lens, we demonstrate a feature broadening to a magnetic width of 8 G compared to a predicted Feshbach resonance width much less than $0.1$ mG.

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Experimental observation of the avoided crossing of two S -matrix resonance poles in an ultracold atom collider

Matthew Chilcott, Ryan Thomas, Niels Kjærgaard

In quantum mechanics, collisions between two particles are captured by a scattering matrix which describes the transfer from an initial entrance state to an outgoing final state. Analyticity of the elements of this $S$ matrix enables their continuation onto the complex energy plane and opens up a powerful and widely used framework in scattering theory, where bound states and scattering resonances for a physical system are ascribed to $S$-matrix poles. In the Gedankenexperiment of gradually changing the potential parameters of the system, the complex energy poles will begin to move, and in their ensuing flow, two poles approaching will interact. An actual observation of this intriguing interaction between scattering poles in a collision experiment has, however, been elusive. Here, we expose the interplay between two scattering poles relating to a shape resonance and a magnetically tunable Feshbach resonance by studying ultracold atoms with a laser-based collider. We exploit the tunability of the Feshbach resonance to observe a compelling avoided crossing of the poles in their energies which is the hallmark of a strongly coupled system.

arXiv:2103.05278•Physical Review Research•Sep 2, 2021•10 citations
Abstract Ultracold atomic gases have realized numerous paradigms of condensed matter physics, where control over interactions has crucially been afforded by tunable Feshbach resonances. So far, the characterization of these Feshbach resonances has almost exclusively relied on experiments in the threshold regime near zero energy. Here, we use a laser-based collider to probe a narrow magnetic Feshbach resonance of rubidium above threshold. By measuring the overall atomic loss from colliding clouds as a function of magnetic field, we track the energy-dependent resonance position. At higher energy, our collider scheme broadens the loss feature, making the identification of the narrow resonance challenging. However, we observe that the collisions give rise to shifts in the center-of-mass positions of outgoing clouds. The shifts cross zero at the resonance and this allows us to accurately determine its location well above threshold. Our inferred resonance positions are in excellent agreement with theory.
Ultracold atomic gases have realized numerous paradigms of condensed matter physics, where control over interactions has crucially been afforded by tunable Feshbach resonances. So far, the characterization of these Feshbach resonances has almost exclusively relied on experiments in the threshold regime near zero energy. Here, we use a laser-based collider to probe a narrow magnetic Feshbach resonance of rubidium above threshold. By measuring the overall atomic loss from colliding clouds as a function of magnetic field, we track the energy-dependent resonance position. At higher energy, our collider scheme broadens the loss feature, making the identification of the narrow resonance challenging. However, we observe that the collisions give rise to shifts in the center-of-mass positions of outgoing clouds. The shifts cross zero at the resonance and this allows us to accurately determine its location well above threshold. Our inferred resonance positions are in excellent agreement with theory.Studies on energy-dependent scattering of ultracold atoms were previously carried out near zero collision energies. Here, the authors observe a magnetic Feshbach resonance in ultracold Rb collisions for above-threshold energies and their method can also be used to detect higher partial wave resonances.
In this thesis we explore ultracold atomic collisions in the vicinity of a magnetic Feshbach resonance located at 9.1 G, between 87Rb atoms in the |F = 2, mF = 1> and the |F =1, mF = 1> hyperfine states. We experimentally investigate the energy dependence of the enhancement of inelastic interaction in the neighborhood of this Feshbach resonance by analyzing the temperature dependence of a loss spectroscopy feature. An asymmetric widening of the resonance feature with respect to temperature is observed. We develop a simple model which captures these asymmetric line shapes through thermal averaging over the collision energy dependence of the Feshbach resonance. The energy dependence of the Feshbach interactions is further explored by colliding ultracold atomic samples at a fixed energy significantly above threshold using an optical trapping system; the elastic scattering enhancement analyzed by colliding the samples in free space, while for the inelastic measurements the collisions were carried out in trap. No conclusive resonant behavior could obtained for the elastic component of the 9.1 G resonance, however the inelastic scattering enhancement exhibited an anticipated shift.
We report on experiments investigating the collisional properties of atoms at ultralow collision energies using an all-optical atom collider. By using a pair of optical tweezers, we can manipulate two ultracold atom clouds and collide them together at energies up to three orders of magnitude larger than their thermal energy. Our experiments measure the scattering of 87 Rb, 40 K, and 40 K- 87 Rb collisions. The versatility of our collider allows us to probe both shape resonances and Feshbach resonances in any partial wave. As examples, we present experiments demonstrating p-wave scattering with indistinguishable fermions, inelastic scattering at non-zero energies near a homonuclear Feshbach resonance, and partial wave interference in heteronuclear collisions.
Optical control enables new high resolution probes of narrow collisional (Feshbach) resonances, which are strongly dependent on the relative momentum of colliding atom pairs, and important for simulating neutron matter with ultracold atomic gases. We demonstrate a two-field optical vernier, which expands kHz (mG) magnetic field detunings near a narrow resonance into MHz optical field detunings, enabling precise control and characterization of the momentum-dependent scattering amplitude. Two-photon loss spectra are measured for the narrow resonance in ^{6}Li, revealing rich structure in very good agreement with our theoretical model. However, anomalous overall frequency shifts between the measured and predicted two-photon spectra are not yet explained.
We employ a quantum defect theory framework to provide a detailed analysis of the interplay between a magnetic Feshbach resonance and a shape resonance in cold collisions of ultracold 87Rb^{87}\mathrm{Rb}87Rb atoms as captured in recent experiments using a laser-based collider [M. Chilcott et al., Phys. Rev. Research 3, 033209 (2021)]. By exerting control over a parameter space spanned by both collision energy and magnetic field, the width of a Feshbach resonance can be tuned over several orders of magnitude. We apply a quantum defect theory specialized for ultracold atomic collisions to fully describe of the experimental observations. While the width of a Feshbach resonance generally increases with collision energy, its coincidence with a shape resonance leads to a significant additional boost. By conducting experiments at a collision energy matching the shape resonance and using the shape resonance as a magnifying lens, we demonstrate a feature broadening to a magnetic width of 8 G compared to a predicted Feshbach resonance width much less than 0.10.10.1 mG.
In quantum mechanics, collisions between two particles are captured by a scattering matrix which describes the transfer from an initial entrance state to an outgoing final state. Analyticity of the elements of this SSS matrix enables their continuation onto the complex energy plane and opens up a powerful and widely used framework in scattering theory, where bound states and scattering resonances for a physical system are ascribed to SSS-matrix poles. In the Gedankenexperiment of gradually changing the potential parameters of the system, the complex energy poles will begin to move, and in their ensuing flow, two poles approaching will interact. An actual observation of this intriguing interaction between scattering poles in a collision experiment has, however, been elusive. Here, we expose the interplay between two scattering poles relating to a shape resonance and a magnetically tunable Feshbach resonance by studying ultracold atoms with a laser-based collider. We exploit the tunability of the Feshbach resonance to observe a compelling avoided crossing of the poles in their energies which is the hallmark of a strongly coupled system.