# Why is depth complexity revelant?

Since gate complexity correspond to the number of gate for a given quantum circuit, it seems that depth complexity bring no more information about quantum complexity than gate complexity. So does gate complexity encompass depth complexity ?

Circuit depth matters because qubits have finite coherence time. You could imagine two circuits, each with $$N$$ gates, where the first circuit can implement them all in parallel and thus finish in one time-step, where the other must implement them in series, necessitating longer coherence of the physical qubits. Thus, the gate count alone only tells you how the fidelity might degrade as a function of gate error, and the context of the gates relative to each other and the underlying hardware will determine the circuit depth.
Great answer by chrysaor4! To give another example: implementing Toffoli gates in a fault-tolerant quantum computer. The most time-consuming gate to implement in a fault-tolerant setting will be the T gate [1]. Therefore if you can run T gates in parallel when possible, that could help save a lot of time. Below are 2 different ways to implement a Toffoli gate (taken from this paper) which both have a T-count of 7 but different T-depths. The paper has many more examples, so if you are interested in this kind of stuff I would highly recommend having a quick look through the paper!
[1] This is because it is the only non-Clifford gate required to make a universal fault tolerant computer. For a short explanation, I would recommend reading the first three paragraphs of the introduction section in https://quantum-journal.org/papers/q-2019-04-30-135/