Circuit Depth
Gate count measures how much work a circuit does; depth counts the layers of gates that must run one after another. Two circuits with identical gates can have different depth, and on real hardware more layers means more time exposed to error.
Gate count or depth — what is the difference?
Two numbers summarise a circuit's size, and they answer different questions.
Gate count is the total number of operations — how much work the circuit contains.
Depth is about time. Gates that touch entirely different qubits can run simultaneously; a group of such gates forms one layer. Depth is the number of layers when every gate is packed as early as its dependencies allow. A gate depends on another only when they share a qubit — that is exactly the ordering promise a circuit diagram makes.
Count is the size of the to-do list; depth is the length of the critical path through it.
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