Control for Programmable Superconducting Quantum Systems

Abstract

The discovery of quantum mechanics in the 20th century forms the basis of many of the technologies that define our lives today. The ability to engineer and manipulate individual quantum systems -- even create artificial atoms -- promises a similar revolutionary leap in technology. A quantum technology of particular interest is quantum computing, which has the potential to solve problems that are intractable for classical computers, opening up new domains of computation. Meanwhile, an attractive approach to creating engineered quantum systems is circuit quantum electrodynamics (cQED). Where research initially focused on understanding the physics of cQED devices, focus has shifted to building systems capable of performing useful computations. However, this remains extremely challenging, in part due to the inherently fragile nature of the individual quantum bits, but also due to difficulties in controlling and scaling up these systems. This thesis focuses on the control aspects of building an extensible full-stack quantum computer based on superconducting transmon qubits. We define the demonstration of quantum fault-tolerance as our target application to give focus to our efforts. The QuSurf architecture for a full-stack quantum computer presented in this thesis is designed with this application in mind. We provide a detailed study of the error sources present in this system and give an overview of the relevant characterization techniques. In the second part of this thesis, we address several key challenges in the control of a quantum computer. To realize high-fidelity coherence limited gates, we present a novel tuneup protocol that achieves a tenfold speedup over the state-of-the-art. This is realized by eliminating the need for qubit initialization. We demonstrate this protocol by calibrating single-qubit gates to a coherence limited Clifford fidelity of 99.9% in one minute. Performing repeated parity checks, as is required for quantum error correction, requires reusing qubits quickly after they have been measured. By introducing a numerically optimized depletion pulse we are able to speeds up the depletion of measurement photons in a readout resonator without having to rely on specific symmetry conditions. Using this technique speed up photon depletion by more than six inverse resonator linewidths, reducing the error rate in an emulated ancilla parity check by a factor 75. Flux-pulsing based two-qubit gates are the fastest two-qubit gates. However, they are also very technically demanding. The key challenge in performing these gates is addressing the distortions that control signals experience as they traverse various electrical components. We have developed Cryoscope (short for cryogenic oscilloscope) to characterize and correct these distortions. Cryoscope is an in-situ technique that uses the qubit to sample control pulses of arbitrary shape. Even when correcting distortions to within $\sim 0.1\%$ two-qubit gates are history-dependent due to the long timescale upon which some of these distortions act. We have invented Net-Zero, a new type of flux-pulsing based two-qubit gate, to address this problem. It makes use of a symmetry condition of the transmon to have net-zero integral, making the gate resilient to long-timescale distortions. The gate suppresses leakage out of the computational subspace to 0.1% by making use of leakage interference and has a built-in echo effect that enhances the coherence of the gate, achieving a two-qubit gate fidelity of 99.1%. Custom software is required to perform the physics experiments needed to build and operate a quantum computer. PycQED is an open-source software framework we have developed for this purpose. We discuss the design choices and concepts of PycQED before turning our focus to characterization and calibration. Here we introduced dependency graphs as a useful abstraction and system emulation as an essential development tool for automating the characterization and calibration process. We conclude the thesis by reflecting on the limitations of our architecture and providing an outlook on the grand challenges of building a useful kilo-qubit sized quantum computer. We define these challenges as The Application Problem, The Fabrication Problem, and The Calibration Problem.