The collision of an antiproton and a hydrogen atom at ultralow energies is described in hyperspherical coordinates. An adiabatic approximation leads to potential-energy curves that can be used to describe the dynamics of capture in the $\overline{p}\ensuremath{-}\mathrm{H}(1s)$ collision. A unified formulation of the collision and capture processes leading to formation of
pp bound states emerges. Both $\mathrm{Pn}{(\mathrm{nl})\ensuremath{-}e}^{\ensuremath{-}}$ ``atomic-type'' and $\overline{p}\ensuremath{-}\mathrm{H}{(n}^{\ensuremath{'}}{l}^{\ensuremath{'}})$ ``molecular-type'' states are represented. These potential curves are calculated to more than 10 000 protonium Bohr radii, where low-energy antiprotons are stopped in collisions with hydrogen atoms. Elastic and inelastic partial and total protonium formation cross sections in the mass-scaled
p-H system are calculated. The existence of protonium negative (or positive) ion in its ground and excited states is discussed.
Non-local energy transfer between bound electronic states close to the ionisation threshold is employed for efficient state preparation in dilute atom systems from technological foundations to quantum computing. The generalisation to electronic transitions into and out of the continuum is lacking quantum simulations necessary to motivate such potential experiments. Here, we present the first development of a electron-dynamical model simulating fully three-dimensional atomic systems for this purpose. We investigate the viability of this model for the prototypical case of recombination of ultracold barium(II) by environment-assisted electron capture thanks to a rubidium atom in its vicinity. Both atomic sites are modelled as effective one-electron systems using the Multi Configuration Time Dependent Hartree (MCTDH) algorithm and can transfer energy by dipole-dipole interaction. We find that the simulations are robust enough to realise assisted capture over a dilute interatomic distance which we are able to quantify by comparing to simulations without interatomic energy exchange. For our current parameters not yet optimised for reaction likelihood, an environment-ionising assisted capture has a probability of
1.9×10−5 % over the first
15 fs of the simulation. The environment-exciting assisted-capture path to
[Ba+∗Rb∗] appears as a stable long-lived intermediate state with a probability of
8.2×10−4 % for at least
20 fs after the capture has been completed. This model shows potential to predict optimised parameters as well as to accommodate the conditions present in experimental systems as closely as possible. We put the presented setup forward as a suitable first step to experimentally realise environment-assisted electron capture with current existing technologies.
We extend the work in 2017 New J. Phys. 19 103015 by deriving a lower bound for the minimum time necessary to implement a unitary transformation on a generic, closed quantum system with an arbitrary number of classical control fields. This bound is explicitly analyzed for a specific N -level system similar to those used to represent simple models of an atom, or the first excitation sector of a Heisenberg spin chain, both of which are of interest in quantum control for quantum computation. Specifically, it is shown that the resultant bound depends on the dimension of the system, and on the number of controls used to implement a specific target unitary operation. The value of the bound determined numerically, and an estimate of the true minimum gate time are systematically compared for a range of system dimension and number of controls; special attention is drawn to the relationship between these two variables. It is seen that the bound captures the scaling of the minimum time well for the systems studied, and quantitatively is correct in the order of magnitude.