08 · The system

The stack

Pulses, readout, a fridge, a compiler that inserts SWAPs, error correction that wants a thousand physical qubits per logical. The algorithm is the last layer, not the first.

In English

An algorithm is the last slide. Under it: pulses that implement gates, a freezer, a compiler that shuffles wires because qubits cannot all talk, and error correction that may spend a thousand noisy qubits to make one reliable one.

Try this: Click each layer. The “without / with” pair is why a screenshot of a circuit is not a computer.
Keep this: The circuit is a wish. The stack under it is the machine.

Lab · explode the computer

Device

The qubit

A transmon, a trapped ion, a photon, a spin. Different physics, same contract: two levels you can rotate and (barely) keep coherent.

On a slide: A warm LC circuit is just noise.
On the metal: A cooled, isolated two-level system is a register you can address.

Superconducting transmons, trapped ions, photonics, spin qubits — pick a modality and you pick a different stack: temperature, native gates, connectivity, how you measure. The programming model stays the same. The error budget does not.

Volume I’s lesson was that the model is not the product. Volume III’s is that the algorithm is not the computer. Pulses, readout, topology, and error correction are the harness. Until those land, Grover and Shor are true and not yet useful.