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Abstract: Current quantum processors operate at the intermediate scale, with tens to hundreds of qubits, but remain error-limited. This thesis studies two related sources. Environmental noise may have temporal and spatial correlations and nonclassical components. State-preparation and measurement (SPAM) errors arise in operations used to characterize this noise, a prerequisite for boosting operational fidelities. Neither source can be characterized alone. Noise spectroscopy techniques use imperfect preparation and readout, while SPAM characterization is affected by qubit decoherence. Our methods vary measurement depth, drive duration, or sequence repetition so each source changes the measured signal differently. Regression separates them without assuming either is negligible.
Specifically, we introduce quantum SPAM (QSPAM) protocols. These separate state-preparation and measurement errors in parallel across a register using only single-qubit gates and repeated measurements at a fixed cost per qubit. Tests on an existing 127-qubit IBM Quantum device validate the protocols. Analysis shows that standard SPAM-mitigation bias grows linearly with register size and preparation error. SPAM-robust spin-locking noise spectroscopy reconstructs multiaxis spectra, including nonclassical components, without assuming ideal SPAM. On hardware, neglecting SPAM shifts spectra upward by up to 26.4% and can make quantum spectra unphysical. Two-qubit frequency-comb spectroscopy further supplies spectra for noise-tailored entangling gate design. The idling and entangling gates can outperform generic protocols employing dynamical decoupling by factors of 23.7 and 9.8, respectively. At leading order, the worst-case gate error depends only on noise local to the active pair. For bounded connectivity and sequential pair operation, characterization and design costs grow only linearly with qubit number. Spectra accessible only through two-qubit spectroscopy also guide entanglement storage. When leakage invalidates the two-level SPAM model, a qutrit framework recovers all eight parameters in closed form and tests physicality. It distinguishes the coherent post-measurement rotation caused by a non-diagonal measurement from the classical state update of an inefficient detector.
Together, these results replace assumptions about device errors with measurements that guide control design. Fault-tolerance estimates depend on assumptions about the locality and independence of physical errors. These assumptions must be tested on each device. Error correction therefore increases the need for physical-layer characterization.

Graduate Advisor: Professor Lorenza Viola 

Zoom Link: https://dartmouth.zoom.us/j/7490864617?