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Reducing Leakage of X-Gate in Flux-Tunable Transmon Qubits (Master thesis)

Βασιλειάδης, Βύρων/ Vasileiadis, Vyron/ Quantum Computing and Quantum Technologies/ Κβαντική Υπολογιστική και Κβαντικές Τεχνολογίες

Weak anharmowenicity in transmon qubits makes fast single–qubit rotations vulnerable to spectral overlap with leakage transitions, so that improving speed often aggravates population transfer outside the computational subspace. This thesis addresses that tension within the circuit quantum electrodynamics (cQED) framework by systematically comparing and extending analytical pulse–shaping families for resonant control. Starting from the standard Gaussian and Derivative Removal by Adiabatic Gate (DRAG) pulses, we develop two analytically tractable extensions—Fourier–ansatz spectrum–tuning DRAG (FAST–DRAG) and higher–derivative DRAG (HD–DRAG)—that further suppress leakage without increasing gate duration. These pulse families are integrated into a fully scripted cQED workflow that compiles pulse–level programs, executes them on a flux–tunable transmon, and benchmarks fidelities and leakage under identical conditions across shaping strategies. The designs map cleanly onto experimental knobs, which are calibrated through 1-D and 2-D spectroscopy, Rabi oscillations, Single-Shot Readout (SSRO), and randomized benchmarking (RB), all implemented in a unified software stack that separates a pulse–programming language from instrument backends and standardizes data acquisition. The resulting pipeline enables a head–to–head, apples–to–apples comparison of Gaussian, DRAG, FAST–DRAG, and HD–DRAG at fixed gate time. Experimentally, both FAST–DRAG and HD–DRAG reduce leakage and total error relative to conventional DRAG at comparable bandwidth and approach the coherence floor at the chosen gate duration, while remaining robust under moderate parameter variations.