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Deterministic loading of single atoms in optical tweezers and controlled collision

Final Activity Report Summary - SINGLE ATOM CONTROL (Deterministic loading of single atoms in optical tweezers and controlled collision)

We constructed a new, and remarkably simple, optical system, designed to capture and observe a single neutral atom in an optical tweezer, created by focusing a laser beam using a large numerical aperture (N.A.=0.5) aspheric lens. We characterised the performance of this optical system, showing that diffraction was limited over a large transverse field and a large spectral range. This optical tweezer allowed us to trap single 87 Rb atoms via a ‘collisional blockade mechanism’ that prevented two or more atoms from being trapped simultaneously due to inelastic collisions. The large collection efficiency of the lens allowed us to detect single atoms with a statistical confidence better than 99 % within 10 ms. We showed that the resolution was good enough to resolve two atoms trapped in two tweezers separated by less than 2 µm.

We investigated techniques to reduce the mean energy of the single atoms trapped within the optical tweezer, with the goal of approaching the ground vibrational state of the trapping potential. We experimentally investigated the energy distribution of single trapped atoms under various cooling regimes. Using two different methods to measure the mean energy of the atom, we showed that the energy distribution of the cooled atom was close to thermal. We then demonstrated how to reduce the energy of the single atoms, first by adiabatic cooling and then by truncating their Boltzmann distribution. These techniques provided a non-deterministic way to prepare single atoms at low micro-Kelvin temperatures, close to the vibrational ground state of the trapping potential. We showed that we could prepare single atoms in the so-called Lamb-Dicke regime, which opened up future possibilities of using two-photon Raman sideband cooling to prepare these atoms in the vibrational ground state.

Through work carried out in conjunction by the two research teams working on the two single-atom trapping apparatuses in the Grangier group, we demonstrated the coherent transport and transfer of single atomic qubits in moving optical tweezers. More specifically, we experimentally demonstrated the coherent transport of a qubit, encoded on an atom trapped in an optical tweezer over a distance of tens of microns. In addition, we demonstrated that the coherence of the qubit was also preserved during its transfer between two optical tweezers. We also showed that these transport and transfer manipulations of the qubit did not induce any change in its external degrees of freedom. This was demonstrated by comparing the mean energy of the single atoms before and after these manipulations.

Finally, we worked towards the experimental production of a Bose-Einstein condensate in a tightly focused microscopic optical tweezer. We investigated methods to optimally load several hundred atoms into the optical tweezer. We studied the expansion behaviour of the atomic cloud after it was released from the trap by observing the atomic fluorescence using an intensified charge coupled device (CCD) camera after an adjustable time of flight. We used this expansion behaviour to determine the temperature of the atomic cloud. We furthermore investigated the effect of applying different ramps to the trapping potential. By the time of the project completion we were investigating interesting and unexpected phenomena that we observed in this ultra-cold, dense atomic system.