Routing Quantum Control of Causal Order

TL;DR

This paper establishes a systematic method to decompose any quantum circuit with indefinite causal order into routed circuit representations, bridging two frameworks and enabling better analysis of quantum processes in fixed spacetime.

Contribution

It proves that any quantum circuit with indefinite causal order can be represented using a single routed graph, unifying two existing frameworks and facilitating analysis of complex quantum processes.

Findings

Systematic routed circuit decomposition for any QC-QC

Explicit construction of routed graphs for QC-QCs

Potential to address open problems in indefinite causal order

Abstract

In recent years, various frameworks have been proposed for the study of quantum processes with indefinite causal order. In particular, quantum circuits with quantum control of causal order (QC-QCs) form a broad class of physical supermaps obtained from a bottom-up construction and are believed to represent all quantum processes physically realisable in a fixed spacetime. Complementarily, the formalism of routed quantum circuits introduces quantum operations constrained by "routes" to represent processes in terms of a more fine-grained routed circuit decomposition. This decomposition, formalised using a so-called routed graph, represents the information flow within the respective process. However, the existence of routed circuit decompositions has only been established for a small set of processes so far, including both certain specific QC-QCs and more exotic processes as examples. In this work, we remedy this fact by connecting these two frameworks. We prove that for any given $N$, one can use a single routed graph to systematically obtain a routed circuit decomposition for any QC-QC with $N$ parties. We detail this construction explicitly and contrast it with other routed circuit decompositions of QC-QCs, which we obtain from alternative routed graphs. We conclude by pointing out how this connection can be useful to tackle various open problems in the field of indefinite causal order, particularly establishing circuit representations of subclasses of QC-QCs.