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Quantum-dot cellular automata (QCA) technology is a promising alternative to CMOS technology. QCA provides a novel paradigm both for communication and computation. Meanwhile, a new challenge referred to as the “layout = timing” problem is introduced due to the unique clocking and inherent pipeline nature of “wires” in QCA. As a result, feedback is intractable in sequential circuits due to the QCA timing constraints. Cut-set retiming is a technique to solve the timing issues. It can accommodate the QCA characteristics by performing delay-transfer and timing-scaling to relocate the existing delays. Cut-set retiming makes it possible to effectively design more complicated QCA circuits. In this case study, a QCA Montgomery modular multiplier is designed, which is the first application of cut-set retiming to QCA circuit design.