Fan-in is the number of inputs a logic gate can handle. For instance the fan-in for the AND gate shown in the figure is 3. Physical logic gates with a large fan-in tend to be slower than those with a small fan-in. This is because the complexity of the input circuitry increases the input capacitance of the device. Using logic gates with higher fan-in will help reducing the depth of a logic circuit.
In quantum logic gates the fan-in always has to be equal to the number of outputs, the Fan-out. Gates for which the numbers of inputs and outputs differ would not be reversible (unitary) and are therefore not allowed.
- Fan-out, a related concept, which is the number of inputs that a given logic output drives.
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