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Trapping Ions in Atoms and Molecules Optically

Periodic Reporting for period 4 - TIAMO (Trapping Ions in Atoms and Molecules Optically)

Reporting period: 2020-02-01 to 2021-01-31

Isolating ions and atoms from the environment is essential in experiments on a quantum level. For decades, this has been achieved by trapping ions with radiofrequency (rf) fields and neutral particles with optical fields. Our group demonstrated the trapping of ions by interaction with light. We see these results and our proposal as starting point for finally combining the advantages of optical trapping and ions. In particular, ions provide individual addressability, high fidelities of operations and long-range Coulomb interaction, significantly larger compared to those of atoms and molecules.

The aim of this proposal is to (i) study and establish optically trapping of ions and atoms in general, to (ii) demonstrate the substantial improvement of our approach in the context of interaction and reaction at ultra-low temperatures and to (iii) explore further perspectives by adapting methodology of quantum optics to gain control and state-sensitive detection on the level of individual quanta within the merged ion-atom system.

The field of ultra cold chemistry is perfectly suited for this purpose, as a showcase as well as an area of research struggling with the fundamental quest for quantum effects. A simplifying summary of the quest might be:
How are interaction and chemical reactions proceeding at lowest temperatures? The classical concept predicts that approaching zero velocity is equivalent to standstill of any dynamics. On the one hand, deviations can be expected, since the classical model is ceasing to be appropriate when particle-wave dualism is gaining of importance. On the other hand, we might be inclined to see quantum mechanical properties as minor corrections - such as the uncertainty principle allowing for some residual but negligible interaction. However, in the regime of lowest temperatures quantum effects dominate, and chemistry is predicted to obey fundamentally different rules. Examples are:
(i) collisions of two species, necessary for a reaction, cannot be described as a billiard-like impact between hard spheres anymore, but rather as interfering waves, interacting at long range, which can coherently amplify or even completely annihilate each other.
(ii) energy barriers can mount above the kinetic energy, but can be efficiently passed by tunneling.
As a consequence and vision, quantum chemistry might permit to manipulate or even control reactions and their pathways by external fields, since the forces and related interactions become relevant compared to the kinetic energy.

Within this proposal, we aim at embedding optically trapped ions into quantum degenerate gases to reach temperatures, orders of magnitude below the current state of the art. Our approach circumvents the currently inevitable excess kinetic energy in hybrid traps, where ions are kept but also driven by rf-fields. It permits to enter the temperature regime where quantum effects are predicted to dominate, (i) in many-body physics, including the potential formation and dynamics of mesoscopic clusters of atoms of a Bose-Einstein-Condensate, binding to the “impurity ion”, as well as (ii) the subsequent two-particle s-wave collisions, the ultimate limit in ultra-cold chemistry.
Further development of our novel and generic tools for “quantum engineering can be expected to propel several other striving fields of research, such as, experimental quantum simulations.
"To address our short and long-term objectives, we select certain species (Ba+ and Rb,Li) and structure our work program into three interlinked work packages; (WP1) to realize the technological and experimental setup and work horse to provide the basic requirements of the project, to study and characterize the many-body physics (WP2) and quantum chemistry (WP3) in the ultra-cold regime.
We were capable to follow our schedule to directly address and reach the anticipated milestones.

a) Demonstrating Feshbach resonances for ion-atom ensembles for the first time [publication [t1] in preparation + conference LesHouches2021] (M3.2)
b) Controlling atom-ion interaction at ultra-cold temperatures, e.g. three- to many-body recombination versus two-body sympathetic cooling rates to reach deeper into the pure s-wave regime [publication [t1] in preparation + LesHouches2021].
c) Proving the complete set of relevant experimental data, allowing state of the art numerical approaches of leading theoreticians in the field to assign s-wave resonances [publication [t1] in preparation + invited talk Warsaw2021].
d) Accessing the short-range potentials of atom-ion interaction via uniquely high spin-orbit coupling of the collisional partners, leading to shifts and splittings of resonances of higher partial wave contributions. Exploiting the latter to allow theory to derive all relevant interaction parameters at short range [publication [t2] in preparation].
e) Studying the dependency of atom-ion interaction in the quantum regime, in dependence on the electronic states of the constituents, revealing the control of the related pathways of the chemical reactions [publication [t2], eventually separately [t3] in preparation].
f) The latter also permitting to exploit spin polarised Li-atoms and the related Pauli-exclusion principle to spin-polarize the Ba ion via two-body spin-flip collisions [publication [t1] in preparation + LesHouches2021].
g) Immerging the Doppler cooled, rf-trapped Ba ion in a bath of ultra cold Li atoms [publication [d4] in preparation] (M1.3).
h) Accessing the collisional dynamics the electronically excited states of Ba+ in the Li ensemble, revealing the branching between non-radiative charge exchange and fine-structure quenching [publication [d4] in preparation] (at M3.3).
i) Demonstrating trapping of a basic Coulomb Crystal within an optical lattice [publication [t5] in preparation] (at M3.5).

The related publications for the achievments above are in preparation. Main achievments already peer reviewed and published are listed below:

j) All optical trapping of an ion and atoms in a bi-chromatic dipole trap (M1.1) [topical review: publication #1]
k) Demonstrating sympathetic cooling by immerging the Doppler cooled, optically trapped Ba ion in a bath of ultra cold Rubidium atoms (M1.2) [PRL: publication #5] (at M2.2).
l) Measuring the effective mass of an optically trapped Ba ion in ultra cold Rb ensemble (at M2.1) [PRL: publication #5].
m) Extending the lifetime of optically trapped ions by a factor of 1000 to three seconds, that is, reaching the same order of magnitude as for state-of-the-art optically trapped atoms - at comparable trapping parameters [Nature Photonics : publication #2].
n) Realisation of a self-assembled array of up to 6 ions within a single beam dipole trap (also of substantial importance for Objective #2) [PRX: publication #3].
o) Measuring the temperature of up to 6 optically trapped ions (at M2.2) [PRX: publication #3].
p) Mitigating the impact of disturbances from the environment (heating rate) to a level small compared to the predicted sympathetic cooling rates [publication #2: Nature Photonics].
q) Accessing the motional degree of freedom within an optically trapped Coulomb crystal – demonstrating long range-interaction within a dipole trap as well as the related access to a channel permitting the exchange of information as well as simulating an effective interaction between the ions (also of substantial importance for Objective #2) [PRX: publication #3].
r) Demonstrating confinement of the ions that we control by the internal (electronic) state of the ions (gain of control – substantial importance for both objectives) [Nature Photonics: publication #2]."
Long trapping times as well as low heating and decoherence rates, essentially isolating individual particles from the environment, are crucial ingredients for controlling these particles on the quantum level. In work performed during the project so far, we demonstrated that optical trapping and isolating ions can be performed on a level comparable to neutral atoms, boosting the lifetime by three orders of magnitude, and measure an upper bound of the total heating rate.
That is, we demonstrated optical trapping of an ion for several seconds in the absence of any rf field. The achieved isolation from the environment allows entering a new regime of ultracold interactions of ions and atoms at previously inaccessible collision energies and permits a novel class of experimental quantum simulations with ions and atoms in a variety
of versatile optical trapping geometries, e.g. bichromatic traps or higher-dimensional optical lattices.
The RF-micro-motion and the related fundamental heating of an atom–ion ensemble. Left: spatially ove
Examples of applications for optically trapped ions: (a) an ion immersed in an ultra-cold bath of at