UC Santa Barbara

Quantum computers can provide new computational abilities, but only if their intrinsic noise can be suppressed. Quantum error correction (QEC) promises to suppress errors exponentially, provided they are sufficiently uncorrelated. Large arrays of superconducting qubits are a leading platform for implementing quantum error correction, but feature several sources of error that are highly correlated; a single physical process that produces the equivalent of many independent errors to be corrected. These correlated errors must be studied and mitigated in order for quantum error correction to succeed. This thesis addresses two correlated error sources in particular; leakage and impacts from high-energy radiation.

Leakage of information out of the states selected for computation presents a significant challenge, as leakage populations spread virally through the device and induce a large pattern of errors if they are not suppressed. We develop several new techniques for removing leakage during QEC codes, eventually achieving regular leakage removal from all qubits, successfully curtailing the ability of leakage to spread.

High-energy radiation impacting the device present another problematic error source, as the energies deposited are enough to cause significant error over the entire chip at current sizes. We present time-resolved measurements of such impacts and explain the underlying physical processes with a view toward future mitigation of this error source in hardware.

This work understanding and suppressing these correlated errors in our hardware has run parallel to and been integrated into the development of the first scaling demonstrations of surface code quantum error correction.