Quantum Circuit
The standard way to write a quantum program: qubits as wires, gates as boxes applied left to right, measurements at the end. Sheet music for a quantum processor.
A quantum circuit is the standard way of writing a quantum program: qubits drawn as horizontal wires, gates as boxes applied left to right, measurements at the end (or, increasingly, in the middle). It's sheet music for a quantum processor.
Two numbers describe a circuit's difficulty: width (how many qubits) and depth (how many sequential layers of gates). Depth is the killer: every layer takes time, and qubits are decohering throughout. A machine's useful depth is roughly its coherence time divided by its gate time, which is why 'how many qubits?' is only half a question. The other half is 'how deep can you go before the answer turns to static?'