Digital Microwave Control of Superconducting Qubits

Abstract: We manipulate two superconducting qubits using digital microwave electronics. Starting fromtheir characterization, we develop a real-time reset scheme and implement the iSwap gate. Thequbits’ parameters are obtained using standard single-qubit characterization techniques, such asRabi and Ramsey oscillations and frequency sweep of the resonators. We also characterized theexperimental setup, including finding the working point of a Josephson Parametric Amplifierand the coupler between the two qubits. We solve the linear differential equations that modelthe resonator, in order to design a high-fidelity, single-shot qubit-measurement pulse shape,which actively empties the cavity. Using this pulse, we achieve a readout assignment fidelity of99.9%. The readout is formed in real-time using template matching. In addition, we implementa conditional reset of the qubit’s state in 1.4 μs, which resets the excited state population from5.4% to 0.5%. We simulate the cavity using QuTip to further optimize the readout pulse.Furthermore, we characterize the third energy level of the qubit to implement a qutrit readoutand observe a second excited state population of 0.3%, in accordance with theory. Finally,we implement the iSwap gate that, together with single-qubit gates, constitute a set of universalquantum gates, where we swap the 95.4% of the quantum state between the qubits in 690 ns. Allexperiments, including the pulse events and synchronization of the readout and feedback, wereperformed using a digital microwave platform based on a radio-frequency-on-a-chip system,and implemented using a Python interface.