ForeSight: Reducing SWAPs in NISQ Programs via Adaptive Multi-Candidate Evaluations

Abstract: Near-term quantum computers are noisy and have limited connectivity between qubits. Compilers are required to introduce SWAP operations in order to perform two-qubit gates between non-adjacent qubits. SWAPs increase the number of gates and depth of programs, making them even more vulnerable to errors. Moreover, they relocate qubits which affect SWAP selections for future gates in a program. Thus, compilers must select SWAP routes that not only minimize the overheads for the current operation, but also for future gates. Existing compilers tend to select paths with the fewest SWAPs for the current operations, but do not evaluate the impact of the relocations from the selected SWAP candidate on future SWAPs. Also, they converge on SWAP candidates for the current operation and only then decide SWAP routes for future gates, thus severely restricting the SWAP candidate search space for future operations. We propose ForeSight, a compiler that simultaneously evaluates multiple SWAP candidates for several operations into the future, delays SWAP selections to analyze their impact on future SWAP decisions and avoids early convergence on sub-optimal candidates. Moreover, ForeSight evaluates slightly longer SWAP routes for current operations if they have the potential to reduce SWAPs for future gates, thus reducing SWAPs for the program globally. As compilation proceeds, ForeSight dynamically adds new SWAP candidates to the solution space and eliminates the weaker ones. This allows ForeSight to reduce SWAP overheads at program-level while keeping the compilation complexity tractable. Our evaluations with a hundred benchmarks across three devices show that ForeSight reduces SWAP overheads by 17% on average and 81% in the best-case, compared to the baseline. ForeSight takes minutes, making it scalable to large programs.