Annotations

Bloch uses annotations to mark functions or variables with special behavior. Annotations are prefixed with @ and appear immediately before the item they modify.

Available annotations in v1.0.0:

• @quantum — marks a function that performs quantum operations, functions with the @quantum annotation can only return void or bit.
• @tracked — collects outcome statistics for qubits across program runs.

Multiple annotations are allowed syntactically, but each has specific applicability and rules described below.

@quantum functions

Apply @quantum to functions that call quantum gates (h, x, y, z, rx, ry, rz, cx) or otherwise manipulate qubits.

Rules:

• Return type must be bit or void.
• Non-void @quantum functions must return along all paths (usual rule applies).

Example:

@quantum
function bell(qubit a, qubit b) -> void {
  h(a); cx(a, b);
}

@quantum
function sample() -> bit {
  qubit q; h(q);
  return measure q;
}

@tracked qubits

Use @tracked on qubit or qubit[] variables to aggregate measurement outcomes across runs (shots). Tracking ends at scope exit.

Rules:

• Only valid on qubit or qubit[]. Applying it to other types is a semantic error.
• If a tracked qubit/register is never measured, an outcome of "?" is recorded.

Examples:

function main() -> void {
  @tracked qubit q;
  h(q);
  measure q; // record 0/1 for this shot
}

function two() -> void {
  @tracked qubit[2] r;
  h(r[0]); cx(r[0], r[1]);
  measure r[0]; measure r[1];
}

Run with multiple shots to see an aggregate table in the CLI output:

bloch --shots=100 entangle.bloch

Tip: use --echo=all to print echo() output on every shot (otherwise suppressed when --shots > 1).