Scoped and composed noise profiles

Purpose and scope

Proposes gate- and register-scoped noise profiles separate from the global noise directive. Channel order and seeded sampling are recorded explicitly.

Core design rules

• A profile defines a local channel after a gate, an atomic correlated Pauli event or readout noise before measurement.
• Profile and register scopes are resolved at compile time; matching channels execute in source order.
• There are at most four profiles and eight channels per profile; probabilities are finite literals in 0..0.25.

Example from the original

This example illustrates the design recorded in the original. It is not by itself a claim of executable or stable support; check required options and the current version.

noise profile bell_device {
  after H on q[0] apply phase_damping(0.02);
  after CNOT on q[0..1] apply correlated_pauli("XX", 0.08);
  before measure on q[0..1] apply readout(0.03);
}

@noise_profile("bell_device");

Limits and interpretation

• This local trajectory model is not calibration, leakage, crosstalk or pulse timing.
• Existing global @noise semantics remain unchanged; a profile is not evidence that real device noise is represented.

Status and implementation boundary

The proposed source records a separate noise-profile contract. Retain channel matches and ordering with the result when making same-seed comparisons.