# Explanation of the function of the circuit

I once experimented with the tool "quirk" and came to a gate, whose function I can not properly tap into. I'm working in the circuit with 4 bits, the last bit is negated, so from 0 to 1. On all 4 bits then I apply a Hadamard transformation. Then this special gate is used, whose function I can not explain.

Hence my question. Can someone explain what makes the gate? How does this work? I know so far the CNOT gate and the Toffoli gate, but this one seems to be something else?

I hope the question is understandable.

This gate is closely related to the CNOT gate that you've already learnt about. Where the CNOT gate says "apply NOT to the target if the control is in the state $$|1\rangle$$", this gate says "apply NOT to the target if all 3 control qubits are in the state $$|1\rangle$$". The three control qubits are the closed black circles, and the target qubit is the other one.