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Thermal balance makes Gibbs stationary
Statement
Let , let have entries in joules and a finite spectral decomposition with pairwise orthogonal projections summing to , and let be a finite family of dimensionless Hermitian matrices, where . Set and, for each , define . Suppose that the rate matrices , , have entries in seconds, are positive semidefinite in the explicit sense that and for every , and satisfy , with in joules and in joule-seconds. Let have entries in joules and commute with , and let be the finite generator
For , being a density matrix means , for every , and ; it is faithful when it is positive definite. For , define the faithful Gibbs density . For time measured in seconds, is completely positive and trace-preserving, and . This statement asserts stationarity only; it asserts neither uniqueness of the stationary density nor convergence to .
Statement references
Facts & Assumptions
Given: The finite-dimensional system, energy-resolved couplings, rate matrices, balance relation, and commuting Lamb-shift matrix in the Statement. Use the ordered finite spectral projections and SI units there.
Finite energy-resolved GKSL thermal model: for each , ; and for every ; and the matrices obey . Also, is Hermitian and commutes with .
Finite completely positive trace-preserving maps: complete positivity means positivity is preserved by every finite ancilla amplification, and trace preservation means traces agree on every matrix input; a finite channel is a complex-linear map with both properties.
Finite GKSL generators and units: a finite GKSL generator has the form for Hermitian and finite jump matrices .
Finite GKSL generates CPTP semigroups: the finite GKSL generator of [F2] has an exponential semigroup of finite channels for all .
Finite matrix spectral and state calculus: a finite Hermitian matrix has orthogonal spectral projections and basis-independent spectral calculus.
Proof
For each , positive semidefiniteness of the finite matrix gives a finite factorization , with in seconds. Define . Expanding yields and . Thus the displayed generator has the finite GKSL form of [F2] with Hamiltonian and finite jumps , so [F3] gives a finite channel semigroup; [F1] states its complete-positivity and trace-preservation content.
From the spectral definition, and . Therefore . Every commutes with because its two energy shifts cancel, hence it commutes with each spectral projection of and with . Also implies preserves every eigenspace of : if , then . Thus commutes with every spectral projection of and with . Finally, the exponential is positive definite with positive finite trace, making a faithful density.
Fix and put . The contribution is , using and commutation of the anticommutator product with . For the negative frequency, substitute and , then reindex to obtain . Thus every nonzero frequency pair cancels.
At , the balance relation gives . Each is Hermitian and commutes with , hence with ; consequently the zero-frequency contribution is , after swapping in the first finite sum. The Hamiltonian contribution vanishes because both and commute with . Together with step 2.1 this gives , and the exponential series then gives for all .