Enhancing the Reach and Reliability of Quantum Annealers by Pruning Longer Chains

Analog Quantum Computers (QCs), such as D-Wave's Quantum Annealers (QAs) and QuEra's neutral atom platform, rival their digital counterparts in computing power. Existing QAs boast over 5,700 qubits, but their single-instruction operation model prevents using SWAP operations for making physically distant qubits adjacent. Instead, QAs use an embedding process to chain multiple physical qubits together, representing a program qubit with higher connectivity and reducing effective QA capacity by up to 33x. We observe that, post-embedding, nearly 25% of physical qubits remain unused, becoming trapped between chains. Additionally, we observe a "Power-Law" distribution in the chain lengths, where a few dominant chains possess significantly more qubits, thereby exerting a considerably more significant impact on both qubit utilization and isolation. Leveraging these insights, we propose Skipper, a software technique designed to enhance the capacity and fidelity of QAs by skipping dominant chains and substituting their program qubit with two measurement outcomes. Using a 5761-qubit QA, we observed that by skipping up to eleven chains, the capacity increased by up to 59% (avg 28%), and the error decreased by up to 44% (avg 33%).