qCompute — Backend Overview

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

Backends define the structural execution substrate for qCompute.

They do not simulate amplitudes.
They do not compute physics.
They provide structural legality, tier binding, and environment constraints.

qCompute computes structure, not amplitudes.

1. Backend List#

qCompute defines three canonical backends:

These backends are structural abstractions, not hardware interfaces.

2. Backend Identity#

Each backend has:

backend_id: "<id>"
tier_support: [r1 | r2 | r3]
environment_legal: ["sandbox" | "production"]
drift_profile: "low" | "medium" | "high"
frame_behavior: "reuse" | "new"
transition_behavior: "allowed" | "forbidden"

All fields are required.

3. Backend Descriptions#

3.1 local-sim#

backend_id: "local-sim"
tier_support: ["r1"]
environment_legal: ["sandbox", "production"]
drift_profile: "low"
frame_behavior: "reuse"
transition_behavior: "allowed"

Used for:

• primitive operators (r1)
• low‑drift structural steps
• stable frames

3.2 hybrid-sim#

backend_id: "hybrid-sim"
tier_support: ["r1", "r2"]
environment_legal: ["sandbox", "production"]
drift_profile: "medium"
frame_behavior: "reuse-or-new"
transition_behavior: "allowed"

Used for:

• composite operators (r2)
• tier escalation
• medium drift accumulation

Router may reuse or open a new frame depending on:

• tier escalation
• drift overflow
• measurement events

3.3 hardware-qpu-2#

backend_id: "hardware-qpu-2"
tier_support: ["r3"]
environment_legal: ["production"]
drift_profile: "high"
frame_behavior: "new"
transition_behavior: "allowed"

Used for:

• pulse operators (r3)
• high‑drift structural steps
• token‑gated operations

Always opens a new frame.

Forbidden in:

• sandbox
• archive

4. Environment Legality#

Archive is terminal and forbids all operators.

5. Tier Binding#

Tier binding is deterministic and replay‑verifiable.

6. Frame Interaction#

Backends influence frame behavior:

• local-sim: may reuse frame
• hybrid-sim: may reuse or open new frame
• hardware-qpu-2: always opens new frame

Frame boundaries are also influenced by:

• tier escalation
• tier decrease
• drift overflow
• meta operators
• environment transitions

7. Transition Interaction#

Backends must remain legal across transitions:

• sandbox → production: all backends legal
• production → archive: no backends legal
• archive → (forbidden)

Transitions close the current frame and open a new one.

8. Capture Metadata#

Each operator includes backend metadata:

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

Each frame includes backend summary:

frame_backend:
  id: "<backend-id>"
  tier_support: [...]
  drift_profile: "<profile>"

All fields are required.

9. Replay Verification#

Replay verifies:

• backend legality
• tier binding
• environment legality
• frame boundaries
• drift consistency
• transition correctness

Replay is deterministic and read‑only.

10. Summary#

Backends define:

• tier support
• environment legality
• drift profile
• frame behavior
• transition behavior

They are the structural substrate of qCompute.