An operational continuum limit of quantum combs

TL;DR

This paper develops a continuous process tensor framework for quantum combs, connecting discrete multi-time quantum processes to a rigorous continuum model using bosonic Fock space.

Contribution

It introduces a fully continuous quantum process tensor model by representing discrete Choi matrices as vectors in bosonic Fock space, bridging a conceptual gap.

Findings

Established a mathematical foundation for continuous quantum process tensors.

Enabled analysis of multi-time correlations via continuous matrix product states.

Bridged the gap between process tensor theory and continuous quantum processes.

Abstract

Quantum combs are powerful conceptual tools for capturing multi-time processes in quantum information theory, constituting the most general quantum mechanical process. But, despite their causal nature, they lack a meaningful physical connection to time -- and are, by and large, arguably incompatible with it without extra structure. The subclass of quantum combs which assumes an underlying process is described by the so-called process tensor framework, which has been successfully used to study and characterise non-Markovian open quantum systems. But, although process tensors are motivated by an underlying dynamics, it is not a priori clear how to connect them to a continuous process tensor object mathematically -- leaving an uncomfortable conceptual gap. In this work, we take a decisive step toward remedying this situation. We introduce a fully continuous process tensor framework by showing how the discrete multi-partite Choi matrix becomes a vector in bosonic Fock space, which is intrinsically and rigorously defined in the continuum. With this equipped, we lay out the core structural elements of this framework and its properties. This translation allows for an information-theoretic treatment of multi-time correlations in the continuum via the analysis of their continuous matrix product state representatives. Our work closes a gap in the quantum information literature, and opens up the opportunity for the application of many-body physics insights to our understanding of quantum stochastic processes in the continuum.