# Quantum Simulation Engine: Domain-Driven Design - Strategic Design **Version**: 0.1 **Date**: 2026-02-06 **Status**: Draft --- ## Domain Vision The Quantum Simulation Engine provides **on-device quantum algorithm experimentation** within ruVector's always-on, agentic environment. It enables hybrid classical-quantum research on edge devices, allowing agents to leverage quantum algorithms (VQE, Grover, QAOA, QEC) without cloud services. > **This is not a cloud quantum API.** The engine answers: "What does this quantum circuit produce?" entirely on the local device, using classical state-vector simulation with SIMD acceleration. The engine follows ruVector's event-driven model: **inert when idle, activated on demand, resources released immediately**. A 20-qubit simulation allocates 16 MiB of state vector on activation and frees it the moment the circuit completes. No background threads, no persistent memory, no warm pools. ### The Universal Simulation Object The power lies in a **single underlying state-vector engine** inside ruqu-sim. Once the linear algebra is fixed, everything else becomes interpretation: | Domain | Qubits Become | Gates Become | Measurement Becomes | Circuit Becomes | |--------|---------------|--------------|---------------------|-----------------| | **Chemistry** | Molecular orbitals | Fermionic operators | Energy estimates | VQE ansatz | | **Optimization** | Decision variables | Mixing/cost ops | Cut values | QAOA circuit | | **Search** | Database indices | Oracle + diffusion | Found element | Grover iterations | | **Error Correction** | Data + ancilla qubits | Stabilizer checks | Syndrome bits | QEC cycle | | **Cryptography** | Key register bits | Quantum Fourier transform | Period estimate | Shor subroutine | | **Machine Learning** | Feature dimensions | Parameterized rotations | Classification | Quantum kernel | **Same linear algebra, different interpretations. Same state vector = superposition. Same measurement = probabilistic collapse with Born rule.** --- ## Strategic Design ### Core Domain **Quantum State Simulation** - The heart of the system, managing quantum state vectors, applying unitary gate operations, and performing projective measurements. This is where the primary complexity and innovation reside. **Most circuits run in a single fast pass; only large entangled states or iterative variational loops require sustained computation.** ### Supporting Domains 1. **Circuit Construction** - Building, validating, and optimizing quantum circuits 2. **State Management** - State vector lifecycle, entanglement tracking, memory gating 3. **Measurement & Observation** - Projective measurement, expectation values, syndrome extraction 4. **Algorithm Execution** - High-level quantum algorithm implementations (VQE, Grover, QAOA, QEC) 5. **Optimization & Backend** - SIMD acceleration, gate fusion, tensor network backends 6. **Deployment & Integration** - WASM compilation, agent bridge, coherence bridge to ruQu ### Generic Domains 1. **Linear Algebra** - Complex number math, matrix-vector products, Kronecker products (via `ruvector-math`) 2. **Random Sampling** - Measurement outcome sampling, noise injection (via `rand` crate) 3. **Logging/Tracing** - Event recording, performance metrics (via `tracing` crate + `ruvector-metrics`) ### Application Evolution | Timeline | Capabilities | Key Value | |----------|-------------|-----------| | **Phase 1 (Now)** | State vector sim, basic gates, VQE/Grover/QAOA | Local quantum experimentation without cloud | | **Phase 2 (6mo)** | Tensor networks, noise models, surface code cycles | Error correction research on edge devices | | **Phase 3 (12mo)** | GPU acceleration, OpenQASM 3.0 import, 30+ qubits | Production-grade quantum algorithm research | | **Phase 4 (24mo)** | Quantum hardware bridge, hybrid cloud-local execution | Real quantum device integration | > **Edge-First Quantum**: The system eventually enables agents to reason about quantum algorithms without any network dependency. --- ## Ecosystem Integration Map ``` +---------------------------------------------------------------------------+ | QUANTUM SIMULATION ENGINE | | | | +-------------------------------------------------------------------+ | | | CIRCUIT CONSTRUCTION DOMAIN | | | | QuantumCircuit | Gate | GateSchedule | CircuitOptimizer | | | | Parameterized templates (VQE ansatz, QAOA mixer, Grover oracle) | | | +-------------------------------------------------------------------+ | | | | | v | | +-----------------------------+ +-----------------------------+ | | | CORE: QUANTUM STATE | | STATE MANAGEMENT | | | | SIMULATION |<-| DOMAIN | | | | | | | | | | * State vector engine | | * Allocation / deallocation | | | | * Gate application (SIMD) | | * Entanglement tracking | | | | * Unitary evolution | | * Memory gating (zero-idle) | | | | * Tensor contraction | | * State checkpointing | | | +-----------------------------+ +-----------------------------+ | | | | | | | v v v | | +-----------------------------+ +-----------------------------+ | | | MEASUREMENT & | | ALGORITHM EXECUTION | | | | OBSERVATION DOMAIN | | DOMAIN | | | | | | | | | | * Projective measurement | | * VQE + classical optimizer | | | | * Expectation values | | * Grover auto-iteration | | | | * Shot-based sampling | | * QAOA graph-based circuits | | | | * Syndrome extraction | | * Surface code + decoder | | | +-----------------------------+ +-----------------------------+ | | | | | v | | +-----------------------------+ +-----------------------------+ | | | OPTIMIZATION & | | DEPLOYMENT & | | | | BACKEND DOMAIN | | INTEGRATION DOMAIN | | | | | | | | | | * SIMD dispatch | | * WASM bindings (ruqu-wasm) | | | | * Gate fusion | | * Agent bridge (activation) | | | | * Tensor network backend | | * Observability / metrics | | | | * Cache-local strategies | | * Coherence bridge (ruQu) | | | +-----------------------------+ +-----------------------------+ | | | +---------------------------------------------------------------------------+ | +--------------------+---------------------+ | | | v v v +--------------+ +-----------------+ +------------------+ | ruvector- | | ruvector- | | ruQu | | math (SIMD) | | metrics | | (decoder bridge) | +--------------+ +-----------------+ +------------------+ | | v v +--------------+ +-----------------+ +------------------+ | ruvector- | | ruvector- | | cognitum-gate- | | graph | | nervous-system | | kernel (tiles) | +--------------+ +-----------------+ +------------------+ | | v v +--------------+ +-----------------+ | ruvector- | | sona (adaptive | | mincut | | learning) | +--------------+ +-----------------+ ``` ### Crate-to-Context Mapping | Bounded Context | Primary Crate | Supporting Crates | |-----------------|---------------|-------------------| | Circuit Construction | `ruqu-sim` (new) | - | | Quantum State Simulation (Core) | `ruqu-sim` (new) | `ruvector-math` | | State Management | `ruqu-sim` (new) | - | | Measurement & Observation | `ruqu-sim` (new) | `rand` | | Algorithm Execution | `ruqu-sim` (new) | `ruvector-graph` (QAOA) | | Optimization & Backend | `ruqu-sim` (new) | `ruvector-math` (SIMD) | | Deployment & Integration | `ruqu-wasm` (new) | `ruqu`, `ruvector-metrics`, `ruvector-nervous-system` | --- ## Context Map ``` +-----------------------------------------------------------------------+ | QUANTUM ENGINE CONTEXT MAP | | | | [Published Language] | | OpenQASM 3.0 format | | | | | v | | +------------------+ +------------------+ | | | | Shared | | | | | CIRCUIT | Kernel | STATE | | | | CONSTRUCTION |<------->| MANAGEMENT | | | | | (Gate, | | | | | Builds circuits | QubitIdx| Allocates and | | | | Validates gates | types) | tracks state | | | +--------+---------+ +--------+---------+ | | | | | | | Customer | Customer | | | Supplier | Supplier | | v v | | +------------------+ +------------------+ | | | | | | | | | MEASUREMENT & |-------->| ALGORITHM | | | | OBSERVATION |Supplier | EXECUTION | | | | |Customer | | | | | Measures states | | Runs VQE/QAOA/ | | | | Extracts syndr. | | Grover/QEC | | | +--------+---------+ +--------+---------+ | | | | | | +------------+---------------+ | | | | | v | | +------------------+ +------------------+ | | | | | | | | | OPTIMIZATION & | | DEPLOYMENT & | | | | BACKEND | | INTEGRATION | | | | | | | | | | SIMD, fusion, | | WASM, agents, | | | | tensor networks | | ruQu bridge | | | +------------------+ +--------+---------+ | | | | | Conformist | Anti-Corruption | | (ruVector | Layer | | APIs) | (ruQu decoder) | | | | +--------------------------------------------------+---------------------+ | v [Existing ruVector Ecosystem] Context Relationships: <-------> Shared Kernel (shared types across boundary) -------> Customer-Supplier (downstream depends on upstream) Conformist: Deployment conforms to existing ruVector APIs ACL: CoherenceBridge wraps ruQu decoder behind anti-corruption layer Published Language: OpenQASM 3.0 for circuit interchange Open Host Service: ruqu-wasm exposes JS API ``` ### Relationship Summary | Upstream | Downstream | Pattern | Shared Types | |----------|------------|---------|-------------| | Circuit Construction | State Management | **Shared Kernel** | `Gate`, `QubitIndex`, `GateMatrix` | | Measurement & Observation | Algorithm Execution | **Customer-Supplier** | `MeasurementOutcome`, `ExpectationValue` | | State Management | Algorithm Execution | **Customer-Supplier** | `QuantumState`, `StateCheckpoint` | | State Management | Measurement & Observation | **Customer-Supplier** | `QuantumState`, `Amplitude` | | Optimization & Backend | Core Simulation | **Partnership** | `FusedGateMatrix`, `OptimizationHint` | | Existing ruVector APIs | Deployment & Integration | **Conformist** | ruVector event types, metric types | | ruQu decoder API | Deployment & Integration | **Anti-Corruption Layer** | Isolated behind `CoherenceBridge` | | Circuit Construction | External tools | **Published Language** | OpenQASM 3.0 circuit format | | Deployment & Integration | JS consumers | **Open Host Service** | `ruqu-wasm` JS API | --- ## Ubiquitous Language ### Quantum Fundamentals | Term | Definition | |------|------------| | **Qubit** | Fundamental unit of quantum information existing in superposition of |0> and |1> basis states | | **Amplitude** | Complex number representing probability amplitude of a basis state; measurement probability is its squared modulus | | **State Vector** | Array of 2^n complex amplitudes representing the full quantum state of an n-qubit register | | **Basis State** | One of 2^n classical bit-string configurations; each has an associated amplitude | | **Superposition** | State where multiple basis states have nonzero amplitude | | **Entanglement** | Quantum correlation preventing independent per-qubit factorization of the joint state | | **Born Rule** | Measurement probability equals squared modulus of amplitude: P(x) = |alpha_x|^2 | ### Circuit Model | Term | Definition | |------|------------| | **Gate** | Unitary matrix operation acting on 1 or 2 qubits; transforms state via matrix-vector multiply | | **Circuit** | Ordered sequence of gates applied to a qubit register; the program of a quantum computation | | **Gate Matrix** | Unitary matrix defining gate action; must satisfy U * U_dagger = I | | **Qubit Index** | Zero-based integer identifying a qubit; determines which amplitude pairs a gate addresses | | **Circuit Depth** | Maximum sequential gate layers; primary determinant of simulation time | | **Parameterized Gate** | Gate whose matrix depends on continuous real parameters (e.g., Ry(theta)) | | **Gate Fusion** | Combining adjacent gates on same qubits into a single matrix multiply | | **Gate Schedule** | Topologically sorted gate-to-timestep assignment respecting qubit-sharing constraints | ### Measurement & Algorithms | Term | Definition | |------|------------| | **Measurement** | Projective observation collapsing superposition to a basis state per the Born rule | | **Mid-Circuit Measurement** | Measurement during (not only at end of) circuit execution | | **Shot** | Single circuit execution + measurement; repeated shots build statistics | | **Expectation Value** | Observable average over quantum state: | | **Pauli String** | Tensor product of per-qubit Pauli operators (I/X/Y/Z) with coefficient | | **Hamiltonian** | Hermitian operator (weighted sum of Pauli strings) representing total energy | | **Syndrome** | Classical bits from ancilla measurements indicating error presence and location | | **Ansatz** | Parameterized circuit template encoding the variational search space | | **VQE** | Variational Quantum Eigensolver; iteratively minimizes Hamiltonian expectation | | **QAOA** | Quantum Approximate Optimization Algorithm; alternating cost/mixer unitaries | | **Grover Search** | Amplitude amplification finding marked items in O(sqrt(N)) queries | | **Oracle** | Black-box gate marking target states by phase flip | | **Surface Code** | 2D topological QEC code with stabilizer checks on lattice faces/vertices | | **Logical Error Rate** | Undetected logical error probability per QEC cycle | | **Decoder** | Classical algorithm mapping syndromes to corrections; bridge to ruQu | ### Simulation Infrastructure | Term | Definition | |------|------------| | **State Allocator** | On-demand allocation/deallocation enforcing zero-idle policy | | **Memory Estimate** | Predicted bytes: 2^n * 16; gating threshold for allocation | | **Entanglement Tracker** | Tracks qubit correlations enabling subsystem splitting | | **State Checkpoint** | Serialized state snapshot for mid-circuit save/restore | | **Tensor Network** | Alternative representation via contracted tensor factors; efficient for low entanglement | | **Contraction Path** | Tensor contraction order minimizing total FLOPs | --- ## Bounded Context Details ### Context 1: Circuit Construction Domain **Purpose**: Language for expressing quantum computations. Validation, scheduling, optimization, OpenQASM interchange. | Entity / Value Object | Type | Responsibility | |----------------------|------|---------------| | **QuantumCircuit** | Aggregate Root | Ordered gate collection with register metadata | | **Gate** | Entity | Single unitary with target qubits and optional parameters | | **GateSchedule** | Entity | Time-step assignment for parallel execution analysis | | **CircuitOptimizer** | Domain Service | Fusion, cancellation, and commutation rules | | GateId, QubitIndex, GateMatrix, ParameterBinding, GateType | Value Objects | Immutable circuit building blocks | **Events**: `CircuitCreated`, `GateAppended`, `CircuitOptimized`, `CircuitValidated`, `ParametersBound` **Invariants**: (1) Gate unitarity. (2) Qubit indices within bounds. (3) No duplicate targets per gate. (4) All parameters bound before execution. --- ### Context 2: State Management Domain **Purpose**: State vector lifecycle following zero-idle model. Entanglement tracking. Memory gating. | Entity / Value Object | Type | Responsibility | |----------------------|------|---------------| | **QuantumState** | Aggregate Root | Owns the 2^n complex amplitude array | | **EntanglementTracker** | Entity | Bipartite entanglement graph for subsystem analysis | | **StateAllocator** | Domain Service | On-demand allocation, immediate deallocation | | Amplitude, QubitCount, MemoryEstimate, StateCheckpoint | Value Objects | State representation primitives | **Events**: `StateAllocated`, `StateDeallocated`, `EntanglementDetected`, `SubsystemSplit`, `CheckpointCreated`, `MemoryLimitExceeded` **Invariants**: (1) Normalization preserved. (2) Zero-idle: no state persists beyond execution. (3) Allocation gated by device capacity. (4) Checkpoint restore reproduces exact amplitudes. --- ### Context 3: Measurement & Observation Domain **Purpose**: Projective measurement with collapse. Analytical expectation values. Syndrome extraction for QEC. | Entity / Value Object | Type | Responsibility | |----------------------|------|---------------| | **MeasurementEngine** | Aggregate Root | Born-rule sampling and state collapse | | **ExpectationCalculator** | Entity | Analytical from Pauli decomposition | | **SyndromeExtractor** | Entity | Ancilla measurement and classical bit extraction | | MeasurementOutcome, PauliString, Hamiltonian, SyndromeBits, ShotResult | Value Objects | Measurement data types | **Events**: `MeasurementPerformed`, `ExpectationComputed`, `SyndromeExtracted`, `ShotsCompleted` **Invariants**: (1) Born rule: probabilities sum to 1.0. (2) Post-measurement collapse to definite state. (3) Hamiltonian Hermiticity. (4) Syndrome bit count matches code. --- ### Context 4: Algorithm Execution Domain **Purpose**: High-level quantum algorithms as orchestrated loops over circuits, states, and measurements. | Entity / Value Object | Type | Responsibility | |----------------------|------|---------------| | **VQERunner** | Entity | Iterative ansatz parameter optimization to minimize energy | | **GroverSearch** | Entity | Oracle + diffusion with auto-computed iteration count | | **QAOASolver** | Entity | Graph-based cost/mixer circuit construction and angle optimization | | **SurfaceCodeSimulator** | Entity | Stabilizer cycles, syndrome extraction, decoder invocation | | AlgorithmResult, OptimizationTrace, CutValue, LogicalErrorRate, ConvergenceCriteria | Value Objects | Algorithm output types | **Events**: `VQEIterationCompleted`, `VQEConverged`, `GroverSearchCompleted`, `QAOARoundCompleted`, `SurfaceCodeCycleCompleted`, `LogicalErrorDetected` **Invariants**: (1) Grover iteration count = floor(pi/4 * sqrt(N/M)). (2) VQE energy is upper bound on ground state. (3) QAOA cost/mixer alternate with correct parameter count. (4) Surface code distance matches lattice. --- ### Context 5: Optimization & Backend Domain **Purpose**: Performance backends that accelerate simulation without altering semantics. SIMD, fusion, tensor networks. | Entity / Value Object | Type | Responsibility | |----------------------|------|---------------| | **SimulationBackend** | Aggregate Root | Selects optimal execution strategy | | **GateFuser** | Entity | Combines compatible gate sequences into single operations | | **TensorContractor** | Entity | Tensor network decomposition for low-entanglement states | | **SIMDDispatcher** | Entity | Platform detection and optimized kernel dispatch | | OptimizationHint, ContractionPath, FusedGateMatrix, PlatformCapabilities | Value Objects | Backend selection metadata | **Events**: `BackendSelected`, `GatesFused`, `TensorNetworkContracted`, `SIMDKernelDispatched` **Invariants**: (1) Fused gates produce identical results to sequential. (2) Tensor contraction matches state-vector. (3) SIMD falls back to scalar if unavailable. (4) Intermediates stay within memory budget. --- ### Context 6: Deployment & Integration Domain **Purpose**: WASM compilation, agent activation bridge, ruQu decoder anti-corruption layer, observability. | Entity / Value Object | Type | Responsibility | |----------------------|------|---------------| | **WASMBindings** | Entity | Open Host Service via wasm-bindgen JS API | | **AgentBridge** | Entity | ruvector-nervous-system integration for context-triggered activation | | **MetricsReporter** | Entity | Publishes SimulationMetrics to ruvector-metrics | | **CoherenceBridge** | Entity | ACL translating syndromes to ruQu's DetectorBitmap/SyndromeRound | | PlatformCapabilities, QubitLimit, SimulationMetrics, DecoderResult | Value Objects | Integration data types | **Events**: `SimulationRequested`, `SimulationCompleted`, `ResourcesReleased`, `DecoderInvoked`, `MetricsPublished` **Integration Patterns**: - **Anti-Corruption Layer**: CoherenceBridge isolates engine from ruQu's internal DDD model - **Conformist**: Deployment conforms to existing ruVector event types and metric schemas - **Open Host Service**: ruqu-wasm exposes clean JS/TS API for browser experimentation - **Published Language**: OpenQASM 3.0 for circuit interchange with external tools --- ## Cross-Cutting Concerns ### Zero-Idle Resource Model ``` IDLE (0 bytes) --> ACTIVATE (allocate 2^n * 16 bytes) --> COMPUTE --> RELEASE (0 bytes) ``` No warm pools, no pre-allocated buffers, no background threads. ### Memory Gating | Qubits | State Vector Size | Decision | |--------|-------------------|----------| | 10 | 16 KiB | Always permit | | 15 | 512 KiB | Always permit | | 20 | 16 MiB | Permit on most devices | | 25 | 512 MiB | Gate: check available RAM | | 30 | 16 GiB | Gate: likely refuse on edge | | 35+ | 512 GiB+ | Always refuse (state vector); consider tensor network | ### Error Model | Context | Error | Severity | Recovery | |---------|-------|----------|----------| | Circuit Construction | Non-unitary gate | Fatal | Reject circuit | | State Management | Memory limit exceeded | Recoverable | Try tensor network or refuse | | State Management | Normalization drift | Warning | Renormalize | | Measurement | Zero-probability outcome | Warning | Return uniform | | Algorithm Execution | VQE non-convergence | Recoverable | Return best-so-far | | Deployment | WASM memory limit | Fatal | Report to agent | | Deployment | ruQu decoder unavailable | Recoverable | Skip correction, log | ### Observability All simulation runs produce `SimulationMetrics` (circuit name, qubit count, gate count, depth, shots, backend type, wall time, peak memory, SIMD utilization) flowing through `ruvector-metrics` for unified dashboard integration. ### Security | Concern | Mitigation | |---------|------------| | Timing side channels in measurement | Constant-time sampling via rejection method | | Memory contents after deallocation | Zero-fill on deallocation (SecureAllocator mode) | | Denial-of-service via large qubit counts | Memory gating with hard upper bound per request | | Untrusted OpenQASM input | Parser validates unitarity and qubit bounds before execution | | WASM sandbox escape | No file I/O, no network; pure computation within WASM sandbox | --- ## Module Structure ``` crates/ruqu-sim/src/ +-- lib.rs # Public API +-- circuit/ # Circuit Construction context | +-- quantum_circuit.rs # QuantumCircuit aggregate | +-- gate.rs # Gate entity, GateType enum | +-- schedule.rs # GateSchedule | +-- optimizer.rs # CircuitOptimizer (fusion, cancel) | +-- openqasm.rs # OpenQASM 3.0 import/export +-- state/ # State Management context | +-- quantum_state.rs # QuantumState aggregate | +-- allocator.rs # StateAllocator (zero-idle) | +-- entanglement.rs # EntanglementTracker | +-- checkpoint.rs # StateCheckpoint +-- measurement/ # Measurement & Observation context | +-- engine.rs # MeasurementEngine | +-- expectation.rs # ExpectationCalculator | +-- syndrome.rs # SyndromeExtractor +-- algorithms/ # Algorithm Execution context | +-- vqe.rs, grover.rs # VQERunner, GroverSearch | +-- qaoa.rs # QAOASolver | +-- surface_code.rs # SurfaceCodeSimulator +-- backend/ # Optimization & Backend context | +-- simulation_backend.rs # SimulationBackend | +-- gate_fuser.rs # GateFuser | +-- tensor_network.rs # TensorContractor | +-- simd_dispatch.rs # SIMDDispatcher | +-- kernels/ # avx2.rs, avx512.rs, neon.rs, wasm_simd.rs, scalar.rs +-- types.rs, events.rs, error.rs crates/ruqu-wasm/src/ +-- lib.rs # wasm-bindgen entry +-- js_api.rs # JS-facing API +-- agent_bridge.rs # ruvector-nervous-system integration +-- coherence_bridge.rs # ACL for ruQu decoder +-- metrics.rs # ruvector-metrics export ``` ### Dependency Graph ``` ruqu-sim +-- ruvector-math (SIMD kernels, complex math) +-- rand (measurement sampling) +-- ruvector-graph (QAOA graph input) ruqu-wasm +-- ruqu-sim (core simulation) +-- ruqu (coherence bridge ACL) +-- ruvector-metrics (observability) +-- ruvector-nervous-system (agent activation) +-- wasm-bindgen (JS bindings) ``` --- ## Performance Targets | Metric | Target | |--------|--------| | Single-gate (1q, 20-qubit register) | < 50 us | | Full circuit (100 gates, 15 qubits) | < 10 ms | | Hamiltonian expectation (10q, 50 terms) | < 1 ms | | SIMD speedup over scalar | > 3x (AVX2), > 6x (AVX-512) | | Grover (20 qubits, 1 target) | < 500 ms | | VQE convergence (H2, 4 qubits) | < 5s, < 100 iterations | | State allocation/deallocation | < 10 us / < 1 us | | WASM circuit (10 qubits, 50 gates) | < 50 ms | --- ## References 1. Evans, E. (2003). "Domain-Driven Design: Tackling Complexity in the Heart of Software." 2. Vernon, V. (2013). "Implementing Domain-Driven Design." 3. Nielsen, M. A. & Chuang, I. L. (2010). "Quantum Computation and Quantum Information." 4. Peruzzo, A. et al. (2014). "A variational eigenvalue solver on a photonic quantum processor." 5. Farhi, E. et al. (2014). "A Quantum Approximate Optimization Algorithm." 6. Fowler, A. G. et al. (2012). "Surface codes: Towards practical large-scale quantum computation." 7. ruQu crate: Existing coherence assessment and syndrome processing in ruVector. 8. Coherence Engine DDD: `/docs/architecture/coherence-engine-ddd.md`