  Quantum circuit transformation (QCT, a.k.a. qubit mapping) is a critical step
in quantum circuit compilation. Typically, QCT is achieved by finding an
appropriate initial mapping and using SWAP gates to route the qubits such that
all connectivity constraints are satisfied. The objective of QCT can be to
minimise circuit size or depth. Most existing QCT algorithms prioritise
minimising circuit size, potentially overlooking the impact of single-qubit
gates on circuit depth. In this paper, we first point out that a single SWAP
gate insertion can double the circuit depth, and then propose a simple and
effective method that takes into account the impact of single-qubit gates on
circuit depth. Our method can be combined with many existing QCT algorithms to
optimise circuit depth. The Qiskit SABRE algorithm has been widely accepted as
the state-of-the-art algorithm for optimising both circuit size and depth. We
demonstrate the effectiveness of our method by embedding it in SABRE, showing
that it can reduce circuit depth by up to 50% and 27% on average on, for
instance, Google Sycamore and 117 real quantum circuits from MQTBench.
