Improving Circuit Performance in a Trapped-Ion Quantum Computer

A quantum circuit is a widely used model for quantum computation. It consists of quantum registers, which we refer to as qubits, and quantum gates. To build a large-scale trapped ion quantum computer, the performance of executing quantum circuits is a bottleneck. Atomic ions are great qubit candidates. However, high-fidelity two-qubit gates extending over all qubits with individual control in a large-scale trapped-ion system have not been achieved. Moreover, coherent gate errors in deep quantum circuits exaggerate the error since they accumulate quadratically. This thesis presents the effort to build a trapped-ion quantum computing system that possesses individual qubit control, scalable high-fidelity two-qubit gates, and the capability to run quantum circuits with multiple qubits. This thesis shows that we realize and characterize high-fidelity two-qubit gates in a system with up to 4 ions using radial modes. The ions are individually addressed by two tightly focused beams steered using micro-electromechanical system (MEMS) mirrors. We accomplish the highest two-qubit gate fidelity using radial motional modes to date. Two methods of robust frequency-modulated two-qubit gate pulse design are introduced. With the state-of-the-art scalable two-qubit gates, we propose a compilation technique, which we refer to as hidden inverses, that creates circuits robust to residual coherent errors. We present experimental data showing that hidden inverses suppress both overrotation and phase misalignment errors in our trapped-ion system, resulting in improved quantum circuit performance.