03 · Circuits

Programs

A program is gates on wires. No if, except a measurement. Depth is time against T₂. Width is how many qubits you rented.

In English

A program is gates on wires, drawn left to right. There is no “if” except a measurement at the end. How long the circuit is (depth) races the hardware’s memory of phase. How many wires (width) is what you rented.

Try this: Place a couple of gates on two wires. Run. Count depth. That number is time against decoherence.
Keep this: Width is qubits. Depth is time. Measurement is the only if.

Lab · two wires

q0 |0⟩q1 |0⟩

Click a column to remove it

Target

0.71|00⟩ + 0.71|11⟩

|00⟩
0.7150%
|10⟩
00%
|01⟩
00%
|11⟩
0.7150%

Volume II’s one-qubit wire was a 2×2 product. Two qubits are where entanglement happens: H then CNOT is the Bell factory. SWAP is three CNOTs because the chip probably cannot couple those two qubits directly.

Depth is the scarce resource. Every extra CNOT eats T₂. A compiler that adds twenty SWAPs can take a correct circuit and make it a random number generator.