In quantum information theory, quantum relative entropy is a measure of distinguishability between two quantum states. It is the quantum mechanical analog of relative entropy.
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For simplicity, it will be assumed that all objects in the article are finite dimensional.
We first discuss the classical case. Suppose the probabilities of a finite sequence of events is given by the probability distribution P = {p1...pn}, but somehow we mistakenly assumed it to be Q = {q1...qn}. For instance, we can mistake an unfair coin for a fair one. According to this erroneous assumption, our uncertainty about the j-th event, or equivalently, the amount of information provided after observing the j-th event, is
The (assumed) average uncertainty of all possible events is then
On the other hand, the Shannon entropy of the probability distribution p, defined by
is the real amount of uncertainty before observation. Therefore the difference between these two quantities
is a measure of the distinguishability of the two probability distributions p and q. This is precisely the classical relative entropy, or Kullback–Leibler divergence:
Note
As with many other objects in quantum information theory, quantum relative entropy is defined by extending the classical definition from probability distributions to density matrices. Let ρ be a density matrix. The von Neumann entropy of ρ, which is the quantum mechanical analaog of the Shannon entropy, is given by
For two density matrices ρ and σ, the quantum relative entropy of ρ with respect to σ is defined by
We see that, when the states are classical, i.e. ρσ = σρ, the definition coincides with the classical case.
In general, the support of a matrix M, denoted by supp(M), is the orthogonal complement of its kernel. When consider the quantum relative entropy, we assume the convention that - s· log 0 = ∞ for any s > 0. This leads to the definition that
when
This makes physical sense. Informally, the quantum relative entropy is a measure of our ability to distinguish two quantum states. But orthogonal quantum states can always be distinguished, via projective measurement. In the present context, this is reflected by non-finite quantum relative entropy.
In the interpretation given in the previous section, if we erroneously assume the state ρ has support in supp(ρ)⊥, this is an error impossible to recover from.
For the classical Kullback–Leibler divergence, it can be shown that
and equality holds if and only if P = Q. Colloquially, this means that the uncertainty calculated using erroneous assumptions is always greater than the real amount of uncertainty.
To show the inequality, we rewrite
Notice that log is a concave function. Therefore -log is convex. Applying Jensen's inequality to -log gives
Jensen's inequality also states that equality holds if and only if, for all i, qi = (∑qj) pi, i.e. p = q.
Klein's inequality states that the quantum relative entropy
is non-negative in general. It is zero if and only ρ = σ.
Proof
Let ρ and σ have spectral decompositions
So
Direct calculation gives
Since the matrix (Pi j)i j is a doubly stochastic matrix and -log is a convex function, the above expression is
Define ri = ∑jqj Pi j. Then {ri} is a probability distribution. From the non-negativity of classical relative entropy, we have
The second part of the claim follows from the fact that, since -log is strictly convex, equality is achieved in
if and only if (Pi j) is a permutation matrix, which implies ρ = σ, after a suitable labeling of the eigenvectors {vi} and {wi}.
Let a composite quantum system have state space
and ρ be a density matrix acting on H.
The relative entropy of entanglement of ρ is defined by
where the minimum is taken over the family of separable states. A physical interpretation of the quantity is the optimal distinguishability of the state ρ from separable states.
Clearly, when ρ is not entangled
by Klein's inequality.
One reason the quantum relative entropy is useful is that several other important quantum information quantities are special cases of it. Often, theorems are stated in terms of the quantum relative entropy, which lead to immediate corollaries concerning the other quantities. Below, we list some of these relations.
Let ρAB be the joint state of a bipartite system with subsystem A of dimension nA and B of dimension nB. Let ρA, ρB be the respective reduced states, and IA, IB the respective identities. The maximally mixed states are IA/nA and IB/nB. Then it is possible to show with direct computation that
where I(A:B) is the quantum mutual information and S(B|A) is the quantum conditional entropy.
Vedral V., 2002, Rep. Math. Phys. 74, 197, eprint quant-ph/0102094 * [1]