Development of The Fundamental Components of A Superconducting Qubit Quantum Computer

Superconducting qubits have emerged as a promising architecture for building a scalable quantum computer. In this thesis we use a particular type of superconducting qubit architecture, the flux-biased phase qubit, to build and characterize the fundamental components of a quantum computer: universal quantum gates and a scalable qubit coupling architecture.;A universal quantum gate allows for the construction of any arbitrary quantum computing operations, and is the analog of classical universal logic gates like the NAND gate. We build this gate using a pair of coupled flux-biased phase qubits where the coupling magnitude is fixed. We characterize this coupled qubit system and show how to construct the gate from the Hamiltonian of this two-qubit system. The universal quantum gate must also be characterized to verify that it has been constructed properly. However, to completely characterize a quantum gate, its output must be mapped out for any arbitrary input. Due to the infinite Hilbert space of qubits, such a characterization is more involved than simply obtaining a truth table, as would be done for classical computational logic. To achieve a complete characterization of a quantum gate we use a technique called quantum process tomography (QPT). We perform QPT on our universal gate, the "square-root of i-swap" gate, and for the first time in any solid state qubit architecture we completely characterize a universal quantum gate. As a result of this gate characterization, we discover that our gate performance is limited by qubit dephasing times. We are also able to measure noise correlations in the coupled qubit system using QPT.We find that by increasing the coupling strength between the qubits, we can build faster gates. This lets us get around the limits imposed by dephasing times by increasing the speed at which we can execute our universal gate. However, increasing the coupling strength of our fixed coupling scheme leads to increased errors during single qubit operations and measurement. In addition, we also discuss the difficulties in scaling up fixed coupling schemes to many qubits.;To address these issues, we design a tunable coupling architecture that allows us to operate at higher coupling strengths during the gate operation and near zero coupling during single-qubit operations and measurement. This minimizes single-qubit errors and measurement crosstalk while allowing for a much faster universal gate. We experimentally show that using this coupler the measurement crosstalk can be minimized and the inter-qubit coupling strength can be tuned arbitrarily, over nanosecond time scales, within a sequence of operations that mimics actual use in an algorithm. Unlike previously demonstrated tunable couplers, this novel tunable coupling circuit is designed to be modular and physically separate from the qubits. It also allows superconducting qubits to be coupled over long distances. This allows the coupler to be used as a module to connect a variety of elements such as qubits, resonators, amplifiers, and readout circuitry over distances much larger than nearest-neighbor. Such design flexibility is likely to be useful for scaling up a quantum computer and allows for the construction of new superconducting microwave circuits with tunable interactions between elements.