Theorem 1 (part) — EQ-KL $\Leftrightarrow$ EQ-MEAS

Proved

ProjectiveMeasurementEquilibration.eqKL_iff_eqMeas

by Lucas · Oct 2, 2026 · Mathlib 0df444a (Lean v4.33.1)

projective-measurementquantum-mechanicsrelative-entropy

Throughout, $n$ is a finite index type, the Hilbert space is $\mathbb C^n$ , $X$ is a Hermitian $n\times n$ complex matrix with spectral decomposition $X=\sum_{x} x\,P_x$ ( $x$ ranging over the distinct eigenvalues, $P_x$ the orthogonal projection onto the $x$ -eigenspace, and $P_x=0$ for $x$ not an eigenvalue), and a density matrix is a positive semidefinite matrix of trace $1$ . For every map $\Phi$ on matrices, condition EQ-KL (for every density matrix $\rho$ , $\Phi(\rho)$ is a density matrix commuting with $X$ minimizing $D(\rho\|\sigma)$ among density matrices $\sigma$ commuting with $X$ ) holds if and only if condition EQ-MEAS holds. In finite dimension every density matrix has finite entropy, so Condition [B]FINENT is automatic.

Preamble

import Mathlib
import Definitions.Def_pme_quantum_basics
import Definitions.Def_pme_measurement_conditions

open Matrix Filter Topology

Formal statement

namespace ProjectiveMeasurementEquilibration

theorem eqKL_iff_eqMeas {n : Type} [Fintype n] [DecidableEq n] {X : Matrix n n ℂ} (hX : X.IsHermitian)
    (Φ : Matrix n n ℂ → Matrix n n ℂ) :
    EqKL X Φ ↔ EqMeas hX Φ := by sorry

end ProjectiveMeasurementEquilibration

Source

G. Carcassi (Assumptions of Physics collaboration), "Projective measurements are equilibration processes", AoP Technical Brief 017, https://assumptionsofphysics.org/resources/briefs, Theorem 1 and its proof (EQ-KL ⇔ EQ-MEAS under [B]FINENT), pp. 3–4

Read-back

What the Lean code literally says, in plain math · Aristotle (Harmonic) - same agent as the drafter; non-blind

Non-blind read-back. This read-back was written by the same agent (Aristotle, by Harmonic) that drafted the Lean statements, at the explicit request of the account owner. It is not blind and not independent testimony: the author knew the intended meaning while writing it. Reviewers should not treat it as an independent audit.

For every finite index type $n$ , Hermitian $X\in M_n(\mathbb C)$ and function $\Phi:M_n(\mathbb C)\to M_n(\mathbb C)$ , the following are equivalent:

(i) for every density matrix $\rho$ : $\Phi(\rho)$ is a density matrix, $X\Phi(\rho)=\Phi(\rho)X$ , and $D(\rho\|\Phi(\rho))\le D(\rho\|\sigma)$ (extended reals) for every density matrix $\sigma$ with $X\sigma=\sigma X$ ;

(ii) for every density matrix $\rho$ : $\Phi(\rho)=\sum_{x\in\Lambda}P_x\rho P_x$ .

$D$ is as in the basics file. No hypothesis on entropies appears in the statement.

Human review

Endorsed by Shuze Chen · Oct 8, 2026
Confirmed by the moderator at approval.
Endorsed by Lucas · Oct 8, 2026
Confirmed by the mission captain (proposal self-audit).

View graph

Theorem 1 (part) — EQ-KL ⇔\Leftrightarrow⇔ EQ-MEAS

Theorem 1 (part) — EQ-KL $\Leftrightarrow$ EQ-MEAS