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README.md

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# QKD Formalization Project
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# Quantum Key Distribution: High Fidelity Implies Low Entropy
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Formalization of Peter Shor's certificate theorem for quantum key distribution in Lean 4.
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**Authors:** Ben Breen & Kfir Sulimany
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## Project Structure
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Lean 4 formalization of Lemma 1 from [*Unconditional Security Of Quantum Key Distribution Over Arbitrarily Long Distances*](https://arxiv.org/abs/quant-ph/9802025) by Lo and Chau (1998).
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```
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QKD/
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├── QKD/
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│ └── PeterShor.lean # Main formalization file
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├── lakefile.lean # Lake build configuration
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├── lean-toolchain # Lean version specification
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└── README.md # This file
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```
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## Main Results
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**Blueprint:** [BenKBreen.github.io/QKD](https://BenKBreen.github.io/QKD)
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This project formalizes:
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## Main Theorem
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1. **Schur Convexity**: The function `f(x) = ∑ᵢ xᵢ log(xᵢ)` is Schur-convex
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2. **High Fidelity Implies Low Entropy**: For a positive semidefinite density matrix ρ with trace 1, if there exists a unit vector achieving fidelity > 1-δ, then the von Neumann entropy is bounded
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### Key Theorem
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If a density matrix ρ has high fidelity (> 1-δ) with a pure state, then its von Neumann entropy is bounded:
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```lean
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theorem high_fidelity_implies_low_entropy_equivalent
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(R : ℕ)
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(δ : ℝ)
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(ρ : Matrix (Fin R) (Fin R) ℂ)
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(hR : 1 < R)
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(hδ_pos : 0 ≤ δ)
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(hδ_lt_one : δ < 1)
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(hδ_small : δ ≤ (R - 1 : ℝ) / R)
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(hρ_pos : ρ.PosSemidef)
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(trace_one : ∑ i, hρ_pos.isHermitian.eigenvalues i = 1)
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(hv : ∃ v : EuclideanSpace ℂ (Fin R), norm v = 1 ∧ (⟪v, ρ *ᵥ v⟫_ℂ).re > 1 - δ)
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:
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(vonNeumannEntropy ρ).re ≤ -(1 - δ) * Real.log (1 - δ) - δ * Real.log (δ / (R - 1))
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S(ρ) ≤ -(1-δ) log(1-δ) - δ log(δ/(R-1))
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```
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## Building the Project
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### Prerequisites
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- Lean 4.25.0-rc2
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- Lake (comes with Lean)
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### Installation
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1. Install [elan](https://github.com/leanprover/elan):
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```bash
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curl https://raw.githubusercontent.com/leanprover/elan/master/elan-init.sh -sSf | sh
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```
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2. Clone this repository:
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```bash
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git clone <repository-url>
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cd QKD
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```
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3. Build the project:
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```bash
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lake update
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lake exe cache get # Download mathlib cache
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lake build
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```
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### Working with the Code
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To work on the formalization:
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## Building
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```bash
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# Open in VS Code with Lean extension
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code .
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# Or build specific files
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lake build QKD.PeterShor
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lake update
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lake exe cache get
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lake build
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```
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## Key Definitions
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- **`vonNeumannEntropy`**: Von Neumann entropy S(ρ) = -tr(ρ log ρ)
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- **`SchurConvex`**: A function is Schur-convex if it preserves the majorization ordering
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- **`slope_comparison_tlogt`**: Key lemma showing t log t is a convex function
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## Dependencies
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- **Mathlib**: Lean's mathematical library
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- Matrix theory, linear algebra, analysis
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- Order theory (for Schur convexity)
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## References
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## Blueprint
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This formalization is based on Peter Shor's work on quantum key distribution security proofs.
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Generate the documentation:
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## License
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[Add appropriate license]
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## Contributing
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Contributions are welcome! Please feel free to submit pull requests or open issues.
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```bash
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cd blueprint
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leanblueprint web
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open web/index.html
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```
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