Collaborative Research: Proximal Digital Control and Stabilization of Superconducting Qubits

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While there has been tremendous recent progress in constructing small-scale quantum circuits comprising several quantum bits (or "qubits"), a fault-tolerant quantum computer that exceeds the performance of existing classical machines will require a network of thousands or millions of qubits, far beyond current capabilities. Robust approaches to the control and measurement of next-generation quantum machines have yet to be developed. This project consists of an experimental program to develop digital control, measurement, and feedback techniques for stabilizing the quantum state of a qubit formed from superconducting circuit elements. The program will move quantum control and feedback from the analog realm to the digital realm; all of the robustness associated with digital control in the classical regime should carry over to the quantum regime. The project will involve the extensive participation of undergraduate and graduate researchers, and is rich in educational opportunities at both of these levels. Science outreach in local schools will extend a broader impact throughout the local communities of Madison and Syracuse. This project consists of an experimental program to apply optimal control theory to the manipulation of superconducting qubits and to implement low-latency feedback based on digital qubit readout for stabilizing computational basis states. The group will demonstrate high-fidelity coherent control of a superconducting qubit using complex trains of Single Flux Quantum (SFQ) voltage pulses derived from optimal control theory. Each of these pulses provides a delta function-like kick to the qubit, inducing a complex trajectory on the Bloch sphere that is tailored to minimize leakage errors. In parallel, the team will employ a recently developed approach to qubit measurement based on the preparation of appropriate microwave cavity pointer states and microwave photon counting, combined with SFQ-based control, to implement low-latency feedback. This will provide a path to monitor and correct leakage out of the computational subspace, which is particularly damaging to proposed approaches to fault-tolerant quantum computing. The program will move quantum control and feedback from the analog realm to the digital realm, and all of the robustness associated with digital control in the classical regime will carry over to the quantum regime.