qCompute — Drift Engine

File: qc_Drift.md
Layer: RTT‑Inside Compute Harness
Status: Canonical (2026)

The Drift Engine computes and enforces structural drift for all qCompute sessions.

Drift is a measure of structural instability introduced by operators, backends, and environment transitions.
It is deterministic, rule‑driven, and replay‑verifiable.

qCompute computes structure, not amplitudes.

1. Purpose#

The Drift Engine:

• computes predicted drift
• computes measured drift
• accumulates drift within a frame
• enforces drift bounds
• triggers frame closure on overflow
• emits drift metadata for capture
• verifies drift during replay

Drift ensures that structural computation remains stable, bounded, and governed.

2. Drift Characteristics by Tier#

3. Environment Drift Bounds#

Drift bound is checked after every operator.

4. Drift Accumulation Model#

Each operator contributes:

drift_predicted: <float>
drift_measured: <float>
drift_total: drift_prev + drift_measured

Drift accumulation is:

• monotonic within a frame
• reset when a new frame opens
• environment‑bounded

5. Drift Overflow#

If:

drift_total > drift_bound

Then:

1. current frame closes
2. new frame opens
3. operator is routed into the new frame
4. drift resets to operator’s measured drift

Overflow is deterministic and replay‑verifiable.

6. Frame Interaction#

Drift interacts with frames as follows:

• r1: may reuse frame
• r2: may reuse frame or escalate tier → new frame
• r3: always opens new frame
• measurement: always opens new frame
• meta: always opens new frame

Drift overflow is an additional trigger for new frame creation.

7. Routing Interaction#

Router receives drift metadata:

routing:
  backend: "<backend>"
  reason: "<reason>"
  frame_action: "reuse" | "new"

Drift Engine may override Router’s frame decision if overflow occurs.

8. Capture Metadata#

Each operator produces drift metadata:

drift:
  predicted: <float>
  measured: <float>
  accumulated: <float>
  overflow: true | false

Each frame produces a drift summary:

frame_drift:
  total: <float>
  max: <float>
  environment_bound: <float>

All fields are required.

9. Replay Verification#

Replay recomputes drift:

• predicted drift
• measured drift
• accumulated drift
• overflow events
• frame boundaries
• environment bounds

Replay must confirm:

• drift never exceeds bound
• overflow events match capture
• frame boundaries match drift logic
• drift summaries match operator accumulation

Replay is deterministic and read‑only.

10. Invariants#

1. drift must not exceed environment bound
2. overflow must open a new frame
3. measurement resets drift
4. meta operators reset drift
5. r3 operators reset drift
6. drift accumulation must be monotonic within a frame
7. drift summaries must match operator accumulation
8. archive forbids all operators

11. Summary#

The Drift Engine ensures:

• structural stability
• deterministic accumulation
• governed overflow
• environment‑bounded execution
• replay‑verifiable correctness

Drift is a first‑class structural signal in qCompute.