Cooling and Manipulation of Atoms in a Microscopic Dipole Trap for Quantum Computing

Patrick, Siobhan (2024). Cooling and Manipulation of Atoms in a Microscopic Dipole Trap for Quantum Computing. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.00099170

Abstract

This thesis reports on the development and characterisation of an optical dipole trap for ultracold Rubidium-87 atoms, an important step towards our overall goal of implementing atomic qubits for a protocol known as Deterministic Quantum Computing with One Clean Qubit (DQC1). This protocol has been shown to provide a speed-up over classical algorithms despite having no entanglement present. Since the onset of decoherence is one of the major factors limiting longer algorithms from being performed, we want to investigate this alternative model of quantum computation, where entanglement plays a significantly smaller role. The protocol only requires one pure control qubit, while the ensemble atoms are in the mixed state. We therefore propose to implement DQC1 in the cold atoms platform, which is suited to being scaled up to large qubit numbers, using a scheme where a CNOT gate is performed on the control and ensemble, to create strong quantum correlations between them. The CNOT gate scheme makes use of Rydberg blockade to provide the conditional logic mechanism, and therefore atoms must be well constrained to within the blockade radius of the chosen energy level (R_b = 7.5 µm for the 43D_5/2 state), to avoid a breakdown of the blockade and the reduction of the fidelity of the gate. During this PhD an optical dipole trap for this purpose was developed and characterized. Our setup uses a magneto-optical trap to cool the atoms to sub-Doppler temperatures before they are loaded into the dipole trap. Characterisation of the trap is carried out and a trap frequency measurement is performed, which allows the distribution of atoms within the trap to be probed. This measurement technique has been used in the past for single atoms in dipole traps [1, 2] and on a large ensemble of around 10⁵ atoms [3], but here the method is demonstrated on a mesoscopic cloud for the first time (using 40-50 atoms). A model which simulates the trap frequency measurement for such an ensemble of atoms is also developed and provides useful insight into this novel experiment. Since the results of this experiment indicate that the atoms are insufficiently well localised for high-fidelity implementation of the CNOT gate, initial work on implementing grey molasses cooling, which uses dark states to reduce the warming of atoms due to spontaneous emission recoil, is also presented.

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