Density-matrix execution and Kraus channels

Purpose and scope

Proposes evolving mixed quantum states with a density matrix distinct from finite-shot trajectories. Exact terminal probabilities and a sampled histogram drawn from them remain separate.

Core design rules

• The distinct browser-density-matrix target is never selected automatically; the full gate plan and channels are resolved before execution.
• Custom channels are exact-version KrausChannel<1> or KrausChannel<2> package exports.
• Channel dimensions and trace preservation, plus state Hermiticity, unit trace and positivity, are checked with bounded numerical tolerances.

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.

@target("browser-density-matrix");
@seed(42);

density profile relaxation {
  after H on q[0] apply amplitude_damping(0.02);
  after CNOT on q[0..1]
    apply kraus("package.community.noise@1.2.0", "thermal_pair");
}

@density_noise_profile("relaxation");

qreg q[2];
sample 1024 {
  H(q[0]);
  CNOT(q[0], q[1]);
  measure(q[0]);
  measure(q[1]);
}

Limits and interpretation

• The proposed slice has at most five qubits and terminal Z-basis measurements; mid-circuit measurement, feed-forward and leakage are excluded.
• A valid Kraus channel does not establish an accurate model of a physical device.

Status and implementation boundary

The proposal records approval and implementation gates separately. A merged design alone is not evidence of executable support or production stability.