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Finite GKSL generates CPTP semigroups

Statement

Let L be a finite GKSL generator on Md(C), d≥1, with the units and conventions of Finite GKSL generators and units. For a bounded operator A on a complex Banach space, write etA:=∑n=0∞tnAn/n! for the operator-norm convergent series of The exponential series of a bounded operator. Then Tt=etL is a finite channel for every t≥0. The family satisfies T0=id⁡, Tt+s=Tt∘Ts, and is norm-continuous. Time is measured in seconds; no negative-time channel or physical bath limit is asserted.

Facts & Assumptions

Given: d≥1, ℏ>0, H=H∗, a finite family La, and the resulting L of Finite GKSL generators and units.

[F1]

Finite GKSL generators and units: L(X)=−iℏ[H,X]+∑a(LaXLa∗−12{La∗La,X}).

[F2]

Finite completely positive trace-preserving maps: for d,e≥1, positivity sends every matrix X≥0 to a positive matrix. Complete positivity requires this after every finite ancilla extension; trace preservation separately requires equality of traces for every matrix input, without a positivity restriction on that input.

[F3]

Complete finite Choi-Kraus equivalence: finite Kraus maps are completely positive, and their Kraus adjoint-square sum characterizes trace preservation.

[F4]

Channel duals and norm closure: finite-dimensional completely positive maps are closed under induced-norm limits; finite channels are closed under such limits, composition, and finite convex combination.

[F5]

Finite matrix trace cyclicity and nondegeneracy: the finite matrix trace is cyclic.

[F6]

Every finite-dimensional normed space is Banach: every finite-dimensional real or complex normed space is complete.

[F7]

The exponential series of a bounded operator: on a complex Banach space the operator exponential series converges absolutely in operator norm, differentiates to AetA=etAA, and satisfies e(t+s)A=etAesA; it also has the stated quadratic remainder bound.

Proof

1.1givenF1F6F7algebra

Put D=∑aLa∗La, K=−iH/ℏ−D/2, N(X)=KX+XK∗, and J(X)=∑aLaXLa∗. Then L=N+J. Left multiplication by K and right multiplication by K∗ commute; expanding their norm-convergent exponential series and using the binomial formula gives etN(X)=etKXetK∗. Termwise adjunction gives etK∗=(etK)∗ for real t. For every ancilla dimension r, the amplification is conjugation by Ir⊗etK, hence preserves positivity. Thus etN is completely positive.

2.1step 1.1F3F4F6F7algebra

The map J is completely positive by [F3], and every iterate Jn is completely positive by composition. For t≥0, the series etJ=∑n=0∞tnJn/n! converges in finite-dimensional map norm; its partial sums are completely positive, so [F4] implies that etJ is completely positive.

3.1step 1.1step 2.1F4F7algebra

By [F7], for h≥0 and each bounded linear map A, ∥ehA−I−hA∥≤12h2∥A∥2eh∥A∥. Applying this to N,J,N+J shows that for fixed t, ∥ehNehJ−eh(N+J)∥≤Cth2 for 0≤h≤t, with finite Ct. Set h=t/n. The telescoping identity for the difference of the nth powers and the bound ∥esA∥≤es∥A∥ from [F7] show ∥(etN/netJ/n)n−et(N+J)∥≤nCt(t/n)2et(∥N∥+∥J∥)⟶0. Each product on the left before taking the limit is completely positive by step 1.1 and step 2.1; [F4] therefore proves that etL is completely positive.

4.1givenF2F5F7step 3.1algebra∎

Cyclicity of finite trace [F5] gives Tr⁡(L(X))=0 for every X: the commutator trace vanishes, and each Tr⁡(LaXLa∗) cancels 12Tr⁡(La∗LaX+XLa∗La). By [F7], the norm-convergent exponential series differentiates term-by-term on bounded time intervals, so ddtTr⁡(etLX)=Tr⁡(L(etLX))=0. Therefore Tr⁡(etLX)=Tr⁡X. The exponential law [F7] gives Tt+s=Tt∘Ts, and the series gives norm continuity. Together with complete positivity from step 3.1, [F2] makes every Tt a channel.

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