## Abstract

We describe a graphical calculus for completely positive maps and in doing so review the theory of open quantum systems and other fundamental primitives of quantum information theory using the language of tensor networks. In particular we demonstrate the construction of tensor networks to pictographically represent the Liouville-superoperator, Choi-matrix, process-matrix, Kraus, and system-environment representations for the evolution of quantum states, review how these representations interrelate, and illustrate how graphical manipulations of the tensor networks may be used to concisely transform between them. To further demonstrate the utility of the presented graphical calculus we include several examples where we provide arguably simpler graphical proofs of several useful quantities in quantum information theory including the composition and contraction of multipartite channels, a condition for whether an arbitrary bipartite state may be used for ancilla assisted process tomography, and the derivation of expressions for the average gate delity and entanglement delity of a channel in terms of each of the dierent representations of the channel.

Original language | English |
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Pages (from-to) | 759-811 |

Number of pages | 53 |

Journal | Quantum Information and Computation |

Volume | 15 |

Issue number | 9-10 |

Publication status | Published - 2015 |

Externally published | Yes |

## Keywords

- Choi-Jamiolkowski isomorphism
- Completely positive maps
- Graphical calculus
- Open quantum systems
- Quantum operations
- Tensor networks