Glossary
Every technical term this platform uses, with what it means and — the part that is harder to find elsewhere — the mistake it prevents. On this subject the intuitive reading is often the wrong one, so each entry says what a reader would otherwise get wrong.
Circuits
- Qubit
The quantum equivalent of a bit, which can be in a combination of 0 and 1 rather than only one of them.
Easy to get wrong: A qubit is not "both 0 and 1 at once" in any useful sense. It has an amplitude for each, and only the relative sizes and phases of those amplitudes affect anything you can measure.
Seen in use on /workbench
- OpenQASM
The text format for writing quantum circuits that most tools can read.
Easy to get wrong: Versions 2 and 3 are different languages. Several frameworks read only version 2, so exporting to them silently loses anything version 3 added — single-bit conditions among them.
Seen in use on /workbench
- Circuit depth
How many layers of operations run one after another, rather than side by side.
Easy to get wrong: Depth, not gate count, sets how long a circuit takes and how much time it has to decohere. A wide shallow circuit and a narrow deep one with the same gate count behave completely differently.
Seen in use on /workbench
- Two-qubit gate
An operation acting on two qubits at once, which is how entanglement is created.
Easy to get wrong: These are the expensive, error-prone operations on real hardware — typically ten times worse than single-qubit gates, so the two-qubit count matters more than the total.
- Transpilealso routing, SWAP
Rewriting a circuit so it only uses gates the device has, on qubits that are physically connected.
Easy to get wrong: It is not free. Connecting distant qubits inserts SWAP gates, and the transpiled circuit can be several times deeper than what you wrote.
Seen in use on /workbench
- Clifford
The family of gates that are easy to simulate classically — H, S, CNOT and their combinations.
Easy to get wrong: A circuit made only of Clifford gates needs no magic states and offers no quantum advantage. The T gates are what make a circuit both useful and expensive.
- Pauli
The three basic single-qubit errors and operations: X, Y and Z.
Easy to get wrong: Y is not a separate kind of error — it is X and Z together, which is why a code that catches both catches Y automatically.
Seen in use on /qec/syndromes
- Statevector
The complete description of a quantum state, as one amplitude per possible outcome.
Easy to get wrong: It doubles in size with every qubit, so a statevector simulation is exact and small only. It also does not survive measurement — measuring collapses the state.
Seen in use on /workbench