Indefinite causal order with output-signalling instruments
Abstract
Quantum processes can exhibit indefinite causal order, allowing coherent superpositions of distinct orderings of events. Whether such processes generate genuinely noncausal statistics, however, depends not only on their global causal structure but also on the local operations available to the parties. Here, we restrict communication to the classical outcomes of local operations, yielding a causal polytope with new causal inequalities that can be violated using measure-and-reprepare, entanglement-breaking instruments requiring no coherent transmission through the laboratories. Using a semidefinite-programming framework, we bound the achievable violations and reveal noncausal correlations that satisfy all conventional input-signalling inequalities, demonstrating that restricting the information available for communication can uncover forms of indefinite causal order inaccessible under less restrictive operational assumptions.
Related
- Superposition of dynamics, indefinite causal order, and quantum histories
- Measuring Geometric Phase based on Indefinite Causal Order in a Sagnac Interferometer
- Proof-of-Principle Implementation of Quantum Key Distribution in an Indefinite Causal Order
- Unbounded separation between definite and indefinite causal order in finite-dimensional quantum metrology
- What Can Be Computed with Indefinite Causal Structure?
BibTeX
@misc{silveira2026indefinite,
title = {{Indefinite causal order with output-signalling instruments}},
author = {Ricardo Silveira and Patrick Andriolo and Davide Poderini and Dmitry Melnikov and Rafael Chaves},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.31863},
eprint = {2609.31863},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.31863},
}
RIS
TY - UNPB
TI - Indefinite causal order with output-signalling instruments
AU - Ricardo Silveira
AU - Patrick Andriolo
AU - Davide Poderini
AU - Dmitry Melnikov
AU - Rafael Chaves
PY - 2026
DA - 2026/09/25
PB - arXiv
DO - 10.48550/arxiv.2609.31863
UR - https://arxiv.org/abs/2609.31863
AB - Quantum processes can exhibit indefinite causal order, allowing coherent superpositions of distinct orderings of events. Whether such processes generate genuinely noncausal statistics, however, depends not only on their global causal structure but also on the local operations available to the parties. Here, we restrict communication to the classical outcomes of local operations, yielding a causal polytope with new causal inequalities that can be violated using measure-and-reprepare, entanglement-breaking instruments requiring no coherent transmission through the laboratories. Using a semidefinite-programming framework, we bound the achievable violations and reveal noncausal correlations that satisfy all conventional input-signalling inequalities, demonstrating that restricting the information available for communication can uncover forms of indefinite causal order inaccessible under less restrictive operational assumptions.
ER -