High-fidelity Readout Of Superconducting Qubits Based On Josephson-junction Parametric Amplifiers

Quantum computers exploit quantum mechanics and can outperform their classical counterparts in some critical tasks such as finding the prime factors of composite numbers. With the rapid progress in device performance and control precision, quantum bits (qubits) based on superconducting Josephson junctions have emerged as an important candidate for building practical quantum computers. In this thesis we first introduce the two key elements, qubits and quantum logic gates, that are necessary for quantum computing, following which we introduce the physical realizations of qubits in superconducting circuits with Josephson junctions.In particular we review the development of superconducting qubits from the perspective of the qubit readout. We discuss various readout schemes, with a focus on the newly-developed dispersive method, which has been implemented in our lab as a reliable way to detect the qubit state. In our superconducting Xmon device, each qubit is coupled to its own readout resonator, and all readout resonators are coupled in parallel to a common transmission line. Near the resonance frequency of the readout resonator, the transmission coefficient S21 of the transmitted signal is modified differently depending upon the two eigenstates of the qubit, and hence can be used to read out the qubit. However, low-noise amplifiers are necessary to raise the signal-to-noise ratio in order to achieve a true single-shot and high-fidelity readout. Working with our collaborators, we have designed and characterized two types of Josephson-junction parametric amplifiers that both function in the frequency range of 5 to 7 GHz, achieving gains up to 20 dB, bandwidths up to 800 MHz, and saturation powers around-113 dBm, while noise temperatures of these amplifiers are almost lowered to the quantum limit. Our experiments demonstrate that these Josephson-junction parametric amplifiers are effective in improving the signal-to-noise ratio of our measurement system for the qubit readout, so that we can measure the qubit with a fidelity value above 90% in less than a microsecond. These amplifiers will be highly useful in more complex experiments featuring synchronized quantum control of multiple qubits in the near future.