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Monotonicity of quantum relative entropy under nonlinear coarse graining

Jochen Rau

Published Apr 26, 2010• 0 citations
OpenAlex

Abstract

I show the monotonicity of quantum relative entropy under arbitrary coarse grainings, even nonlinear ones that cannot be represented by completely positive maps. The proof uses the quantum Stein lemma and properties of the Kawasaki-Gunton projector.

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Itis well known that in quantum information quantum relative is monotonically decreasing under the completely positive and trace-preserving maps. For a new proposed sandwiched R\'enyi quantum relative, a map that is linear trace-preserving and whose Hilber-Schmidt adjoint map satisfies Schwarz inequality, monotonicity still holds. We give a new proof of monotonicity of sandwiched R\'enyi relative entropy for α∈[12,1)\alpha\in[\frac{1}{2},1)α∈[21​,1). The proof is based on complex interplotation techniques, which already has been used to prove monotonicity under trace-preserving and positive map for α∈(1,∞)\alpha\in(1,\infty)α∈(1,∞).
We analyze the entropy behaviour of a discrete quantum channel defined by the composition of a unitary automorphism and a projective conditional expectation onto a finite-dimensional subalgebra. Let [Formula: see text], where [Formula: see text] is unitary and [Formula: see text] is the conditional expectation associated with an orthogonal projector decomposition of the identity. We note that the von Neumann entropy is monotone nondecreasing under repeated application of [Formula: see text]. Equality is characterized explicitly in terms of the invariant algebra of the channel. We further identify the fixed-point space as the intersection of the commutant of [Formula: see text] with the block-diagonal von Neumann algebra defined by the coarse-graining projectors. Finally, we describe the asymptotic structure under repeated application of the channel via spectral properties of completely positive trace-preserving maps. The results demonstrate that entropy growth in this setting arises solely from algebraic noncommutativity between unitary action and conditional expectation, without invoking thermodynamic limits, statistical ensembles, or environmental assumptions.