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arXiv:1511.08237 · Physical Review Letters

Spin-Orbit Interactions and Quantum Spin Dynamics in Cold Ion-Atom Collisions

Timur V. Tscherbul, Paul W. Brumer, Alexei A. Buchachenko

Published Sep 27, 2016• 28 citations
arXivHTMLPDF

Abstract

We present accurate ab initio and quantum scattering calculations on a prototypical hybrid ion-atom system Yb^{+}-Rb, recently suggested as a promising candidate for the experimental study of open quantum systems, quantum information processing, and quantum simulation. We identify the second-order spin-orbit (SO) interaction as the dominant source of hyperfine relaxation in cold Yb^{+}-Rb collisions. Our results are in good agreement with recent experimental observations [L. Ratschbacher et al., Phys. Rev. Lett. 110, 160402 (2013)] of hyperfine relaxation rates of trapped Yb^{+} immersed in an ultracold Rb gas. The calculated rates are 4 times smaller than is predicted by the Langevin capture theory and display a weak T^{-0.3} temperature dependence, indicating significant deviations from statistical behavior. Our analysis underscores the deleterious nature of the SO interaction and implies that light ion-atom combinations such as Yb^{+}-Li should be used to minimize hyperfine relaxation and decoherence of trapped ions in ultracold atomic gases.

DOI
10.1103/physrevlett.117.143201

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We present accurate ab initio and quantum scattering calculations on a prototypical hybrid ion-atom system Yb^{+}-Rb, recently suggested as a promising candidate for the experimental study of open quantum systems, quantum information processing, and quantum simulation. We identify the second-order spin-orbit (SO) interaction as the dominant source of hyperfine relaxation in cold Yb^{+}-Rb collisions. Our results are in good agreement with recent experimental observations [L. Ratschbacher et al., Phys. Rev. Lett. 110, 160402 (2013)] of hyperfine relaxation rates of trapped Yb^{+} immersed in an ultracold Rb gas. The calculated rates are 4 times smaller than is predicted by the Langevin capture theory and display a weak T^{-0.3} temperature dependence, indicating significant deviations from statistical behavior. Our analysis underscores the deleterious nature of the SO interaction and implies that light ion-atom combinations such as Yb^{+}-Li should be used to minimize hyperfine relaxation and decoherence of trapped ions in ultracold atomic gases.
Ultracold hybrid ion–atom traps offer the possibility of microscopic manipulation of quantum coherences in the gas using the ion as a probe. However, inelastic processes, particularly charge transfer can be a significant process of ion loss and has been measured experimentally for the ion immersed in a Rb vapour. We use first-principles quantum chemistry codes to obtain the potential energy curves and dipole moments for the lowest-lying energy states of this complex. Calculations for the radiative decay processes cross sections and rate coefficients are presented for the total decay processes; and . Comparing the semi-classical Langevin approximation with the quantum approach, we find it provides a very good estimate of the background at higher energies. The results demonstrate that radiative decay mechanisms are important over the energy and temperature region considered. In fact, the Langevin process of ion–atom collisions dominates cold ion–atom collisions. For spin-dependent processes [ 1 ] the anisotropic magnetic dipole–dipole interaction and the second-order spin–orbit coupling can play important roles, inducing coupling between the spin and the orbital motion. They measured the spin-relaxing collision rate to be approximately five orders of magnitude higher than the charge-exchange collision rate [ 1 ]. Regarding the measured radiative charge transfer collision rate, we find that our calculation is in very good agreement with experiment and with previous calculations. Nonetheless, we find no broad resonances features that might underly a strong isotope effect. In conclusion, we find, in agreement with previous theory that the isotope anomaly observed in experiment remains an open question.
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.
Charge-transfer cold Yb+{}^{+}++++ Rb collision dynamics is investigated theoretically using high-level ab initio potential energy curves, dipole moment functions, and nonadiabatic coupling matrix elements. Within the scalar-relativistic approximation, the radiative transitions from the entrance $A{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}$ to the ground $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}$ state are found to be the only efficient charge-transfer pathway. The spin-orbit coupling does not open other efficient pathways, but alters the potential energy curves and the transition dipole moment for the $A\ensuremath{-}X$ pair of states. The radiative, as well as the nonradiative, charge-transfer cross sections calculated within the ${10}^{\ensuremath{-}3}--10$ cm${}^{\ensuremath{-}1}$ collision energy range exhibit all features of the Langevin ion-atom collision regime, including a rich structure associated with centrifugal barrier tunneling (orbiting) resonances. Theoretical rate coefficients for two Yb isotopes agree well with those measured by immersing Yb+{}^{+}+ ions in an ultracold Rb ensemble in a hybrid trap. Possible origins of discrepancy in the product distributions and relations to previously studied similar processes are discussed.
We report on the observation of cold collisions between 6Li^{6}\mathrm{Li}6Li atoms and Yb+{\mathrm{Yb}}^{+}Yb+ ions. This combination of species has recently been proposed as the most suitable for reaching the quantum limit in hybrid atom-ion systems, due to its large mass ratio. For atoms and ions prepared in the 2S1/2^{2}S_{1/2}2S1/2​ ground state, the charge-transfer and association rate is found to be at least 103{10}^{3}103 times smaller than the Langevin collision rate. These results confirm the excellent prospects of $^{6}\mathrm{Li}\ensuremath{-}{\mathrm{Yb}}^{+}$ for sympathetic cooling and quantum information applications. For ions prepared in the excited electronic states 2P1/2,2D3/2^{2}P_{1/2}, ^{2}D_{3/2}2P1/2​,2D3/2​, and 2F7/2^{2}F_{7/2}2F7/2​, we find that the reaction rate is dominated by charge transfer and does not depend on the ionic isotope or the collision energy in the range $\ensuremath{\sim}1--120\phantom{\rule{0.16em}{0ex}}\mathrm{mK}$. The low charge-transfer rate for ground-state collisions is corroborated by theory, but the 4f4f4f shell in the Yb+{\mathrm{Yb}}^{+}Yb+ ion prevents an accurate prediction for the charge-transfer rate of the 2P1/2,2D3/2^{2}P_{1/2}, ^{2}D_{3/2}2P1/2​,2D3/2​, and 2F7/2^{2}F_{7/2}2F7/2​ states. Using ab initio methods of quantum chemistry we calculate the atom-ion interaction potentials up to energies of $30\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, and use these to give qualitative explanations of the observed rates.
Electronic close-coupling scattering calculations are reported on the relaxation process Ne+(2p5 2Pj1=1/2,m1)+He→Ne+(2p5 2Pj2=3/2,m2)+He using a new interaction potential derived by combining spectroscopy and ab initio theory. Collision energies range from E/kB=300 K to less than 1 μK. As E is reduced, the total inelastic cross section σj1→j2=σ1/2→3/2 passes through a resonance-dominated regime and apparently approaches the Wigner limit below 10 μK, where the cross section is found to diverge in accord with the predicted E−1/2 dependence. A complex scattering length a=−53.0−0.04145i Å is derived from the low-energy behavior. m-dependent alignment and orientation cross sections, σj1|m1|→j2|m2| and σj1m1→j2m2, undergo rapid changes near resonances, and approach limiting behavior below 10 μK as well, while their ratios become asymptotically constant below 100 μK. Analysis of the scattering amplitude in the s-wave limit shows that the latter behavior is expected in general, with the ratios depending only on the geometry of angular momentum coupling. This geometrical effect disentangles s-wave dominance from the Wigner regime, and provides a new criterion for assessing the approach to the ultracold limit. Boltzmann averaging to simulate ultracold trapping conditions shows that a remnant of the resonance region and the characteristic limiting behavior remain observable.