Quantum Markov chain
# Quantum Markov chain

In mathematics, the quantum Markov chain is a reformulation of the ideas of a classical Markov chain, replacing the classical definitions of probability with quantum probability.

## Introduction

Very roughly, the theory of a quantum Markov chain resembles that of a measure-many automaton, with some important substitutions: the initial state is to be replaced by a density matrix, and the projection operators are to be replaced by positive operator valued measures.

## Formal statement

More precisely, a quantum Markov chain is a pair $(E,\rho )$ with $\rho$ a density matrix and $E$ a quantum channel such that

    $E:{\mathcal {B}}\otimes {\mathcal {B}}\to {\mathcal {B}}$

is a completely positive trace-preserving map, and ${\mathcal {B}}$ a $C^*-algebra$ of bounded operators. The pair must obey the quantum Markov condition, that

    $\operatorname {Tr} \rho (b_{1}\otimes b_{2})=\operatorname {Tr} \rho E(b_{1},b_{2})$

for all $b_{1},b_{2}\in {\mathcal {B}}$.

