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arXiv:1712.07873 · Physical Review A

Dynamics of a single ion-spin impurity in a spin-polarized atomic bath

Henning A. Fürst, Thomas Feldker, Norman Vincenz Ewald, J. Joger, Michał Tomza, R. Gerritsma

Published Jul 27, 2018• 39 citations
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Abstract

We report on observations of spin dynamics in single ${\mathrm{Yb}}^{+}$ ions immersed in a cold cloud of spin-polarized $^{6}\mathrm{Li}$ atoms. This species combination has been proposed to be the most suitable system to reach the quantum regime in atom-ion experiments. For $^{174}\mathrm{Yb}^{+}$, we find that the atomic bath polarizes the spin of the ion by 93(4)% after a few Langevin collisions, pointing to strong spin-exchange rates. For the hyperfine ground states of $^{171}\mathrm{Yb}^{+}$, we also find strong rates towards spin polarization. However, relaxation towards the $F=0$ ground state occurs after 7.7(1.5) Langevin collisions. We investigate spin impurity atoms as a possible source of apparent spin-relaxation leading us to interpret the observed spin-relaxation rates as an upper limit. Using ab initio electronic structure and quantum scattering calculations, we explain the observed rates and analyze their implications for the possible observation of Feshbach resonances between atoms and ions once the quantum regime is reached.

DOI
10.1103/physreva.98.012713

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We report on observations of spin dynamics in single Yb+{\mathrm{Yb}}^{+}Yb+ ions immersed in a cold cloud of spin-polarized 6Li^{6}\mathrm{Li}6Li atoms. This species combination has been proposed to be the most suitable system to reach the quantum regime in atom-ion experiments. For 174Yb+^{174}\mathrm{Yb}^{+}174Yb+, we find that the atomic bath polarizes the spin of the ion by 93(4)% after a few Langevin collisions, pointing to strong spin-exchange rates. For the hyperfine ground states of 171Yb+^{171}\mathrm{Yb}^{+}171Yb+, we also find strong rates towards spin polarization. However, relaxation towards the F=0F=0F=0 ground state occurs after 7.7(1.5) Langevin collisions. We investigate spin impurity atoms as a possible source of apparent spin-relaxation leading us to interpret the observed spin-relaxation rates as an upper limit. Using ab initio electronic structure and quantum scattering calculations, we explain the observed rates and analyze their implications for the possible observation of Feshbach resonances between atoms and ions once the quantum regime is reached.
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We present a joint experimental and theoretical study of spin dynamics of a single ^{88}Sr^{+} ion colliding with an ultracold cloud of Rb atoms in various hyperfine states. While spin exchange between the two species occurs after 9.1(6) Langevin collisions on average, spin relaxation of the Sr^{+} ion Zeeman qubit occurs after 48(7) Langevin collisions, which is significantly slower than in previously studied systems due to a small second-order spin-orbit coupling. Furthermore, a reduction of the endothermic spin-exchange rate is observed as the magnetic field is increased. Interestingly, we find that while the phases acquired when colliding on the spin singlet and triplet potentials vary largely between different partial waves, the singlet-triplet phase difference, which determines the spin-exchange cross section, remains locked to a single value over a wide range of partial waves, which leads to quantum interference effects.
Hybrid systems of single, trapped ions embedded in quantum gases are a promising platform for quantum simulations and the study of long-range interactions in the ultracold regime. Feshbach resonances allow for experimental control over the character and strength of the atom-ion interaction. However, the complexity of atom-ion Feshbach spectra, e.g., due to second-order spin-orbit coupling, requires a detailed experimental understanding of the resonance properties—such as the contributing open-channel partial waves. In this work, we immerse a single barium ( Ba + ) ion in a bath of lithium (Li) atoms spin polarized in their hyperfine ground state to investigate the collision energy dependence of magnetically tunable atom-ion Feshbach resonances. We demonstrate fine control over the kinetic energy of the Ba + ion and employ it to explore three-body recombination in the transition from the many- to the few-partial wave regime, marked by a sudden increase of resonant loss. In a dense spectrum—with on average 0.58(1) resonances per Gauss—we select a narrow, isolated feature and characterize it as an s -wave resonance. We introduce a quantum recombination model that allows us to distinguish it from higher-partial-wave resonances. Furthermore, in a magnetic field range with no significant loss at the lowest collision energies, we identify a higher-partial-wave resonance that appears and peaks only when we increase the energy to around the s -wave limit. Our results demonstrate that hybrid atom-ion traps can reach collision energies well in the ultracold regime and that the ion’s kinetic energy can be employed to tune the collisional complex to resonance, paving the way for fast control over the interaction in settings where magnetic field variations are detrimental to coherence.
Hybrid atom-ion systems combine the precise control of a single radio-frequency-trapped atomic ion with the rich few- to many-body phenomena of ultracold degenerate quantum gases trapped in light fields. The associated long-range interaction enables studies of impurity physics and the implementation of a single-particle quantum gas microscope, sensitive to quantities beyond the atomic density, including energy distributions, atom-atom couplings, and many-body exchange symmetry. This perspective requires atom-ion coupling, which can be tuned through resonantly enhanced scattering. While atom-ion Feshbach resonances have recently become experimentally accessible, the role of quantum statistics in resonantly enhanced three-body recombination remains largely unexplored. This thesis investigates the role of fermionic quantum statistics in atom-atom-ion three-body recombination. The experiment employs a single 138Ba+ ion immersed in an ultracold two-component Fermi gas of 6Li. An atom-ion Feshbach resonance enables resonantly enhanced recombination for one spin component, while the other component acts as a non-resonant spectating collision partner. This allows a direct comparison between recombination processes involving identical and distinguishable fermions. Experimentally, recombination rates are extracted from bath polarization-dependent ion-loss spectroscopy. Expressing the results as relative rate constants largely detaches them from the total atomic density and results in suppression of recombination in the identical fermion channel. Additional measurements of two-body reactive collisions are combined with simulations of the Fermi gas, demonstrating that density modifications arising from Fermi-Dirac statistics or mutual atom-atom attraction are insufficient to explain the observed recombination behavior. To interpret the measurements, an extension of the conventional two-step model is developed. Using a hyperspherical treatment that includes all pairwise interactions and fermionic exchange symmetry, effective relaxation rates for the loosely-bound atom-ion complex are obtained. The calculations predict a suppression of recombination involving identical fermions relative to distinguishable collision partners. The experimental observations are consistent with this prediction and provide evidence that fermionic exchange symmetry significantly affects resonantly enhanced atom-atom-ion recombination. These results establish three-body recombination as a sensitive probe of short-range quantum correlations in degenerate gases and demonstrate that a single trapped ion can be employed as a localized sensor of many-body quantum statistics.
Abstract Quantum control of chemical reactions is an important goal in chemistry and physics. Ultracold chemical reactions are often controlled by preparing the reactants in specific quantum states. Here we demonstrate spin-controlled atom–ion inelastic (spin-exchange) processes and chemical (charge-exchange) reactions in an ultracold Rb-Sr+ mixture. The ion’s spin state is controlled by the atomic hyperfine spin state via spin-exchange collisions, which polarize the ion’s spin parallel to the atomic spin. We achieve ~ 90% spin polarization due to the absence of strong spin-relaxation channel. Charge-exchange collisions involving electron transfer are only allowed for (RbSr)+ colliding in the singlet manifold. Initializing the atoms in various spin states affects the overlap of the collision wave function with the singlet molecular manifold and therefore also the reaction rate. Our observations agree with theoretical predictions.