Measurement-selected ensembles in trapped-ion qubits

Thesis

This thesis describes the development of two segmented ion trap systems for quantum information experiments and investigations of novel quantum measurements. We report successful loading of ⁴⁰Ca⁺ ions in one of the trap systems and the successful demonstration of the partial collapse and revival of the wavefunction of an ion qubit using the system.

We describe the operation of ion trap systems with segmented electrodes and develop a practical strategy for choosing the voltages to apply to the electrode segments to obtain desired trapping potentials and correct for stray fields. The linearity of the Laplace equation results in a linear map from the configuration space of electrode voltages to the function space of electric potentials in the trap. We decompose the electrode configuration space into basis vectors whose corresponding potentials have intuitive meaning.

We describe attempts at loading Ca⁺ ions in a micro-scale segmented ion trap built at Lucent Technologies. The Lucent Trap is a planar ion trap fabricated using lithographic techniques on a silicon substrate. We experienced several problems with this trap and were unable to successfully load ions in it.

A second ion trap, built at the University of Liverpool, is described. The Liverpool trap has miniature segmented electrodes constructed using conventional machining. We describe loading of Ca⁺ ions, and the demonstration of stable trapping with long ion lifetimes and stable electric and magnetic fields.

The Liverpool trap is used to demonstrate non-projective “partial” measurements of a qubit wavefunction, which result in non-unitary, non-projective evolution of the wavefunction dubbed partial collapse. This effect can be reversed by performing another partial measurement, but only if the second measurement results in a particular outcome. When this outcome is observed, the qubit wavefunction is restored to the original state. An exact restoration occurs when both measurements are ideal, and we quantify the success of our demonstration by quantum process tomography. We obtain process fidelity >0.65 for a wide variation in strength of the partial collapse from 0−0.94 (where 0 leaves the system unaffected and 1 is the limit of a complete projective measurement).

Authors: Curtis, MJ

• DOI: 10.5287/ora-mpae1wj60
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English