02 · ISA
Instructions
NAND throws information away. Quantum gates cannot: every instruction is a reversible rotation. CNOT copies a bit only if you keep the original.
In English
Ordinary computers throw information away (NAND). Quantum instructions cannot: every gate has an undo. CNOT can copy a bit onto a blank wire, but it keeps the original — no cloning of unknown states, just reversible bookkeeping.
- Try this
- Apply a gate. Hit undo. Land where you started. That is the ISA’s whole personality.
- Keep this
- Quantum instructions are reversible rotations. Erase is not allowed.
Lab · reversible instructions
1|0⟩
|0⟩1100%|1⟩00%
X is its own inverse. NAND is not.
Universality is Clifford+T (or CNOT plus a single-qubit rotation that is not Clifford). H, S, CNOT generate the Clifford group, which is efficiently simulable classically — the Gottesman–Knill theorem. T is the piece that makes it hard, and expensive.
Reverse is not a courtesy. You uncompute garbage so that interference still has a chance. A classical programmer deletes temps. A quantum programmer has to undo them.