Barrier¶
A barrier isn't a quantum operation at all - it's a visual/compiler divider that marks a point in the circuit that optimizers shouldn't rearrange gates across.
| Qubits | Any number (applies across the wires it spans) |
| Parameters | None |
| Changes the quantum state? | No |
| Also called | Barrier |
What it does¶
Barriers have no effect on the mathematics of a circuit - the statevector, probabilities, and phases are exactly the same with or without one. They exist purely to (a) visually separate stages of a circuit for readability, and (b) tell a compiler or transpiler "don't reorder or merge gates across this line," which matters when gate order affects real hardware behavior even though it doesn't affect the ideal simulated math.
Example¶
Placing a barrier after a state-preparation section and before an algorithm's main loop keeps the two visually and logically distinct, without changing any simulation results.
Other notations¶
Naming conventions across ecosystems; exact syntax can vary by library version, and this does not claim to be Qompile's generated output unless otherwise noted.
| Language | Typical form |
|---|---|
| OpenQASM 2.0 | barrier q[0], q[1]; |
| Qiskit | circuit.barrier[q[0], q[1]] |
| Cirq | - |
| Q# | - |
Notes¶
Cirq has no direct equivalent instruction - its Moment structure organizes timing differently.