Chapter 4 Non-Commutative Random Walks
4.3 Quantum Markov Chains
In this section we recall the definition of quantum Markov chains (QMCs). The QMC approach to non-commutative dynamics was first introduced by Accardi in [1], was developed further for the Accardi-Ohya-Watanabe (AOW) entropy in [3], and can be thought of as a symbolic dynamics for QDSs which utilizes spin chains from quantum statistical mechanics. Another QMC approach was introduced by Tuyls in [61] for the study of the Alicki-Fannes (AF) entropy, which was introduced in [6]. Finally, a generalization of both QMC approaches was given in [38], where the authors introduced the Kossakowski-Ohya-Watanabe (KOW) entropy. Throughout this section, we will follow mainly the terminology and notations of [3] and [38], but we will follow the construction given in [61], which is most suitable for our purposes. Fix a stationary QDS (A,Θ, ρ). We will refer to any completely positive, unital map E :Md⊗ A → A as a transition expectation, for any d∈N. Let γ = (γi)di=1 be an operational partition of unity. Following [61, Page 413] (see also [38, Equation 3.14]), we will consider the transition expectation Eγ : Md⊗ A → A given by the equation
Eγ([ai,j]) = d
X
i,j=1
γi∗ai,jγj for all [ai,j]∈Md⊗ A. (4.14)
Remark 4.3.1. In the original paper, [3], the authors only considered γ that were partitions of unity and considered a simplified (as compared to Equation (4.14)) tran- sition expectation given by
Eγ([ai,j]) = d
X
i=1
γiai,iγi for all [ai,j]∈Md⊗ A.
Notice that there is no need for a γi∗ in the above equation since γi∗ = γi, for each
i, whenever γ is a partition of unity. We will consider this transition expectation further in Section 7.2.
expectation
Eγ,Θ = Θ◦ Eγ. (4.15)
A quantum Markov chain (QMC) is a pair {ρ,E} where ρ is an initial state and E is a transition expectation. We will be specifically interested in QMCs whose transition expectation is given by Equation (4.15). Given a QMC, we define the
quantum Markov stateψ ∈Σ(M⊗N
d ) by the equation
ψ(a1⊗ · · · ⊗an) = tr(ρE(a1⊗ E(a2⊗ E(· · · E(an⊗1)· · ·)))), (4.16) for all n ∈ N and a1, . . . , an ∈ Md. For notational convenience, we will often write
ψ ={ρ,E} whenever ψ is the quantum Markov state obtained from the QMC{ρ,E}
as defined in Equation (4.16).
Thejoint densities for ψare given by the density matricesρn ∈Md⊗n satisfying
ψ(a1⊗ · · · ⊗an) = tr(ρna1 ⊗ · · · ⊗an), (4.17) for all n ∈ N and a1, . . . , an ∈ Md. For any stationary QDS (A,Θ, ρ), operational partition of unity γ, and associated quantum Markov chain and state {ρ,Eγ,Θ} and
ψ, respectively, the joint densities of ψ given in Equation (4.17) are equal to the joint densities are equal to the joint densities of (A,Θ, ρ) with respect to γ given in Equation (4.10), as we will see in Section 7.
In a similar vein to the coupled classical system in Section 2.4, we can think of the coupled system Md⊗ A here. In this case we only have access to the measurements with values inMd; notice that when defining the joint correlations in Equation (4.16) we have assumed the state 1 on A. We can think of the output of the transition expectationEγ(a⊗1) as the most likely state ofAto have produced the measurement outcomea∈Mdwith respect to the partitionγ. In practice, we will usually apply the lifting E∗
γ,Θ : Σ(A)→Σ(Md⊗ A), in the sense of [2], to the initial stateρ iteratively to obtain the joint densities ρn. The iterative applications of the lifting Eγ,∗Θ can be
thought of in the Schrödinger Picture as providing, in the limit, the state ψ which contains all the correlations of a classical stochastic process!
We will finish this section by giving the QMC representation for a classical dy- namical system. Fix a DS (Ω,Σ, µ, f), a finite partition C ∈ Par(Ω) of size d
and let (L∞(Ω), Tf, µ) and γ be the associated QDS and partition of unity, respec- tively. For each k ∈ {1, . . . , d}, let ek = |kihk| in L∞({1, . . . , d}), where we identify
L∞({1, . . . , d}) with the diagonal matrices in Md, which we denote by diag(Md). The transition expectation Eγ :L∞({1, . . . , d})⊗L∞(Ω) →L∞(Ω) forγ, given in Equation (4.14), simplifies to Eγ( d X k=1 ek⊗fk) = d X k=1 1Ck·fk for any f1, . . . , fd∈L ∞ (Ω). (4.18)
Notice that since we have identified L∞({1, . . . , d}) with diag(Md), there are no off- diagonal entries to consider and Equation (4.18) is of the form introduced in Re- mark 4.3.1.
The QMC representing the DS (Ω,Σ, µ, f) with respect to C is then given by the pair {µ,Eγ} on the spin chain diag(Md)⊗N with quantum Markov state given by Equation (4.16). Recall that, in the symbolic dynamics picture for a classical DS, we define a measure ˆµon Ω∗ =⊕ NΩ (see Remark 2.3.3) by ˆ µ(CA1 ··· An 1 ··· n ) = µ(∩n k=1f −(k−1)(C)), (4.19) for any cylinder set in Σ∗, and extend uniquely to Σ∗. Again, we will denote ˆ
µ(CA1 ··· An
1 ··· n
) by µ(s,Cˆ)(A
1, . . . , An) as in Equation (2.9). On the other hand, the quantum Markov state ψ plays the role of µ(s,Cˆ) in the spin chain. Identifying each
Ak in Equation (4.19) with its representation in diag(Md); i.e. Ak=Pj:Cj⊆Akej, we
have
ψ(A1⊗ · · · ⊗An) = µ(Eγ,Tf(A1 ⊗ Eγ,Tf(A2⊗ Eγ,Tf(· · · Eγ,Tf(An⊗1)· · ·)))) (4.20)
= µ(f◦1A1 ◦f 2∩ · · · ∩fn◦1 An) = µ(∩n k=1f −k (Ak)).
Therefore, iff isµ-invariant; i.e. (Ω,Σ, µ, f) is stationary, we have that
ψ(A1⊗ · · · ⊗An) = µ(s, ˆ C)(A
1, . . . , An),
where µ(s,Cˆ)(A
1, . . . , An) is the notation given just beneath Equation (4.19). Hence
ψ can only take values depending on the partition γ (or C), similar to what we have seen before with classical symbolic dynamics.