Atom–ion interaction in an optical dipole trap

Example experiment on the interaction of a neutral atom with a single ion in an optical dipole trap.

Overview

In this example we combine a lithium MOT / optical dipole trap (ODT) with the cooling, shuttling and state detection of a barium ion into one complete atom–ion interaction experiment. The experiment class interaction_optical_trapping subclasses Experiment, and the main sequence is assembled in stages by methods such as init_CMOT, absorption_imaging, expose_camera and run.

A typical sequence is roughly: load the MOT and pre-cool the ion and shuttle it “out of the way”, then load the atom cloud into the CMOT and the optical dipole trap, and after evaporation and spin polarisation move the ion back and let it interact with the atoms (loss spectroscopy or Feshbach resonance measurements are possible); finally perform optical thermometry, absorption imaging and detection of the ionic product state.

Main devices

Category	Device (excerpt)	Purpose
Lithium MOT/CMOT	`li_cooling_aom` / `li_repump_aom` / `li_zeeman_aom`, `dds_0/1`, `li_mot_det_switch`	cooling/repump/Zeeman light and frequency, detuning switching
Magnetic field	`b_quadrupole`, `b_homogen`, `quadrupole_fet_off`, `homogen_fet_off`, Feshbach coil relays	MOT gradient, homogeneous field, Feshbach/evaporation field
Ba lasers and states	`aom_1(_ttl)`, `aom_2(_ttl)`, the shutters, `trigger_cam`	ion cooling, pumping, ion state detection
Optical dipole trap	`li_odt_*` (xODT/cODT), laser power	optical trap for atoms/ions
Imaging	`li_absorption_h`, `li_imaging_freq`, `trigger_abs`	absorption imaging and vertical trigger

Timing stages (summarised from the comments in run)

MOT loading → ion pre-cooling and shuttling (making room for the atom loading)
  → CMOT (optional high-gradient/power/detuning stage) → transfer into the ODT / two-colour trap
  → evaporative cooling (optional rf association) → spin polarisation
  → ion moved back into the atom cloud → interaction / loss / Feshbach measurement
  → trap-frequency measurement / time of flight (TOF) → ion optical thermometry → absorption imaging + ion state detection

Key code (excerpt)

from quatm.experiment import NumberValue, BoolValue, Experiment, delay, realtime

class interaction_optical_trapping(Experiment):
    def build(self):
        # devices for the lithium MOT/CMOT
        self.setattr_device("li_cooling_power")
        self.setattr_device("li_cooling_aom")
        self.setattr_device("b_quadrupole")
        self.setattr_device("b_homogen")
        # ... remaining devices omitted

    def run(self):
        # keep the ion inside the atom cloud during the interaction
        with realtime():
            self.li_cooling_aom = False
            delay(0.5)
            # ... timing omitted

Running

After placing the script in the corresponding directory under experiments/, open it in the experiment browser (Browser) to see the parameter panel generated by build; clicking run executes the whole sequence according to the timing in run. The script can also serve as a template for writing custom atom–ion interaction experiments (note the ion-shuttling ramps, the CMOT loading and the ODT transfer stages).