Quantum Control and Error Correction of Grid States in a Superconducting Oscillator
The potential of quantum systems for computation is challenged by decoherence, a process in which the system interacts with its noisy, uncontrolled environment. However, with quantum error correction (QEC), robust quantum computation in the presence of noise is possible. In a QEC encoding, information is stored as non-local correlations in an engineered multi-state, quantum many-body system. One promising encoding is the Gottesman-Kitaev-Preskill (GKP) code, in which logical states are encoded as oscillator grid states. However, ever since the code was envisioned in 2001, its experimental realization had remained out of reach. The most important barrier for realization was the code sensitivity to nonlinearity, an unfortunate requirement, given that a strong nonlinearity is in general favorable to control quantum systems. Whether it is possible to control a linear oscillator such that non-local correlations in phase space can be manipulated and stabilized is an open question. The goal of the PhD work reported in this dissertation is to answer this question. Our main result is the realization of universal control of a nearly-linear superconducting oscillator by weakly coupling it to a superconducting qubit in a many-drive-photon regime that previously was not controllable. Using this weak-coupling, large-displacement architecture, we demonstrate the first realization of stabilized quantum information encoded in grid states of an oscillator, ultimately leading in a follow-up experiment to a demonstration of qubit stabilized beyond break-even, i.e. in which the quantum coherence of the protected qubit is longer than that of all components of the system.
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