Structure and Classification of Matrix Product Quantum Channels

TL;DR

This paper introduces a tensor-network framework for matrix product quantum channels, demonstrating their short-range correlation properties, phase classification, and methods for long-range entanglement generation with efficient implementation.

Contribution

It develops a comprehensive tensor-network framework for quantum channels, classifies their phases, and proposes efficient protocols for entanglement generation.

Findings

Purifying isometries can be implemented by constant-depth circuits.

All locally purified channels belong to a single phase.

Long-range entanglement can be generated with constant-depth measurement protocols.

Abstract

We develop a framework for Matrix Product Quantum Channels (MPQCs), a one-dimensional tensor-network description of completely positive, trace-preserving maps. We focus on translation-invariant channels, generated by a single repeated tensor, that admit a local purification. We show that their purifying isometry can always be implemented by a constant-depth brickwork quantum circuit, implying that such channels generate only short-range correlations. In contrast to the unitary setting, where one-dimensional quantum cellular automata (in one-to-one correspondence with matrix product unitaries) carry a nontrivial index, we prove that all locally purified channels belong to a single phase, that is, they can be continuously deformed into one another. We then extend the framework to a broader class of translation-invariant channels capable of generating long-range entanglement and show that these remain deterministically implementable in constant depth using two rounds of measurements and feedforward.