Why a Quantum Circuit Changes Before It Reaches a Chip

The diagram is a description of the task

A quantum algorithm is often introduced as a clean sequence of gates acting on conveniently arranged qubits. A physical processor has its own available operations and connections. The original diagram must therefore be translated into instructions the selected device can perform. The resulting circuit can look different while aiming to implement the same intended computation.

IBM's documentation calls this process transpilation and describes stages including layout, routing, translation, optimization and scheduling. Its hardware-representation guide explains that limited connectivity may require additional operations to move quantum information. These details make circuit compilation part of the experiment. Counting gates in the first diagram alone can underestimate the work the processor will actually perform, especially when the algorithm's desired interactions fit the device poorly.