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No genuinely mixed two-qubit Hardy state

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QAlgorithms.Papers.Independent.Intakethird100.P47.hardy_no_mixed_state

by Gohan · Nov 1, 2026 · Mathlib 0df444a (Lean v4.33.1)

prove2quantum

For the noncommuting local projective bases (3), let ρ\rhoρ be a positive semidefinite two-qubit matrix of trace one satisfying all four Hardy conditions (1), including strictly positive success. Then there is a unit vector ψ\psiψ with ρ=∣ψ⟩⟨ψ∣\rho=|\psi\rangle\langle\psi|ρ=∣ψ⟩⟨ψ∣. Thus every such density is pure.

Preamble
import Definitions.Def_QAlgorithms_Papers_Independent_Intakethird100_P47
Formal statement
namespace QAlgorithms.Papers.Independent.Intakethird100.P47

open scoped ComplexConjugate

open scoped BigOperators

/-- II A, PDF p.3, citing [20]: every two-qubit density satisfying the positive-success
Hardy constraints for the fixed noncommuting bases (3) is pure. The normalized witness
is concluded, so a genuinely mixed state cannot satisfy these conditions. -/
theorem hardy_no_mixed_state (d : HardyParameters)
    (rho : Matrix (Bool × Bool) (Bool × Bool) ℂ)
    (hrho : IsDensityMatrix rho) (hHardy : HardyConditions (hardyBehavior d rho)) :
    ∃ psi : EuclideanSpace ℂ (Bool × Bool), IsState psi ∧ rho = pureDensity psi := by sorry

end QAlgorithms.Papers.Independent.Intakethird100.P47
Source
Device-independent quantum key distribution based on measurement inputs, arXiv:1308.6447v3, PDF p. 3, II A: no mixed two-qubit Hardy state [20]
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What the Lean code literally says, in plain math · gpt-6.1-sol

Identify the Boolean values false and true with 000 and 111, and let e0,e1e_0,e_1e0​,e1​ be the standard basis of C2\mathbb C^2C2. For every choice of complex numbers αP,βP\alpha_P,\beta_PαP​,βP​, indexed by P∈{0,1}P\in\{0,1\}P∈{0,1}, satisfying ∣αP∣2+∣βP∣2=1|\alpha_P|^2+|\beta_P|^2=1∣αP​∣2+∣βP​∣2=1 and 0<∣αP∣<10<|\alpha_P|<10<∣αP​∣<1 for each PPP, define uP=αPe0+βPe1u_P=\alpha_Pe_0+\beta_Pe_1uP​=αP​e0​+βP​e1​ and vP=βP‾e0−αP‾e1v_P=\overline{\beta_P}e_0-\overline{\alpha_P}e_1vP​=βP​​e0​−αP​​e1​. Here tensor products have coordinates (r⊗s)a,b=rasb(r\otimes s)_{a,b}=r_as_b(r⊗s)a,b​=ra​sb​. For every complex matrix ρ\rhoρ indexed by {0,1}2\{0,1\}^2{0,1}2 that is positive semidefinite and has trace 111, put qρ(w)=Re⁡tr⁡((ww∗)ρ)q_\rho(w)=\operatorname{Re}\operatorname{tr}((ww^*)\rho)qρ​(w)=Retr((ww∗)ρ), where (ww∗)z,z′=wzwz′‾(ww^*)_{z,z'}=w_z\overline{w_{z'}}(ww∗)z,z′​=wz​wz′​​. If qρ(e0⊗e0)>0q_\rho(e_0\otimes e_0)>0qρ​(e0​⊗e0​)>0, qρ(u0⊗e0)=0q_\rho(u_0\otimes e_0)=0qρ​(u0​⊗e0​)=0, qρ(e0⊗u1)=0q_\rho(e_0\otimes u_1)=0qρ​(e0​⊗u1​)=0, and qρ(v0⊗v1)=0q_\rho(v_0\otimes v_1)=0qρ​(v0​⊗v1​)=0, then there exists a vector ψ∈C{0,1}2\psi\in\mathbb C^{\{0,1\}^2}ψ∈C{0,1}2 with Euclidean norm 111 such that ρz,z′=ψzψz′‾\rho_{z,z'}=\psi_z\overline{\psi_{z'}}ρz,z′​=ψz​ψz′​​ for all z,z′∈{0,1}2z,z'\in\{0,1\}^2z,z′∈{0,1}2.

Human review
  • Endorsed by Shuze Chen · Nov 1, 2026

    Confirmed by the moderator at approval.

  • Endorsed by Gohan · Nov 1, 2026

    Confirmed by the mission captain (proposal self-audit).

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