Assessing Quantum Layout Synthesis Tools via Known Optimal-SWAP Cost Benchmarks

TL;DR

This paper introduces QUBIKOS, a benchmark set with known optimal SWAP gate counts for quantum layout synthesis, revealing significant optimality gaps in current algorithms and guiding future improvements.

Contribution

QUBIKOS provides the first benchmark with provable-optimal SWAP counts, enabling precise evaluation of QLS tools and highlighting their current limitations.

Findings

LightSabre has a 63x optimality gap in SWAP usage.

ML-QLS exhibits a 117x gap, indicating room for improvement.

QMAP and t|ket> have even larger gaps, 250x and 330x respectively.

Abstract

Quantum layout synthesis (QLS) is a critical step in quantum program compilation for superconducting quantum computers, involving the insertion of SWAP gates to satisfy hardware connectivity constraints. While previous works have introduced SWAP-free benchmarks with known-optimal depths for evaluating QLS tools, these benchmarks overlook SWAP count - a key performance metric. Real-world applications often require SWAP gates, making SWAP-free benchmarks insufficient for fully assessing QLS tool performance. To address this limitation, we introduce QUBIKOS, a benchmark set with provable-optimal SWAP counts and non-trivial circuit structures. For the first time, we are able to quantify the optimality gaps of SWAP gate usages of the leading QLS algorithms, which are surprisingly large: LightSabre from IBM delivers the best performance with an optimality gap of 63x, followed by ML-QLS with an optimality gap of 117x. Similarly, QMAP and t|ket> exhibit significantly larger gaps of 250x and 330x, respectively. This highlights the need for further advancements in QLS methodologies. Beyond evaluation, QUBIKOS offers valuable insights for guiding the development of future QLS tools, as demonstrated through an analysis of a suboptimal case in LightSABRE. This underscores QUBIKOS's utility as both an evaluation framework and a tool for advancing QLS research.