The coupler Eve did not monitor: non-signalling reports from interacting cavities
Abstract
Eve receives genuine average photon counts from two opaque devices, finds input-independent single-party report distributions, and sees little activity in a specified leakage monitor. Do these observations establish physical isolation? We construct a cavity-based counterexample. Independently operated controls activate a shared interaction after the inputs arrive. Internal block measurements yield a CHSH score S=13/4 for four repetitions, approaching $4$ for large blocks. A fresh shared random bit symmetrizes the binary results without changing their parity; local preparation of vacuum or one-photon output states then produces truthful photon-number reports with exactly non-signalling marginals. A dispersive-bus model also permits weak monitored leakage; an illustrative dissipative calculation gives S≃3.1702 with ideal output encoding. These scores exceed 2√2 because the devices interact during their response, not because quantum mechanics is violated. The construction separates honest measurement, non-signalling reported statistics, and causal isolation, while making the additional randomness, preparation, and monitoring assumptions explicit.
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BibTeX
@misc{wu2026coupler,
title = {{The coupler Eve did not monitor: non-signalling reports from interacting cavities}},
author = {Yueying Wu},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.09103},
eprint = {2610.09103},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.09103},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The coupler Eve did not monitor: non-signalling reports from interacting cavities
AU - Yueying Wu
PY - 2026
DA - 2026/10/23
PB - arXiv
DO - 10.48550/arxiv.2610.09103
UR - https://arxiv.org/abs/2610.09103
AB - Eve receives genuine average photon counts from two opaque devices, finds input-independent single-party report distributions, and sees little activity in a specified leakage monitor. Do these observations establish physical isolation? We construct a cavity-based counterexample. Independently operated controls activate a shared interaction after the inputs arrive. Internal block measurements yield a CHSH score S=13/4 for four repetitions, approaching $4$ for large blocks. A fresh shared random bit symmetrizes the binary results without changing their parity; local preparation of vacuum or one-photon output states then produces truthful photon-number reports with exactly non-signalling marginals. A dispersive-bus model also permits weak monitored leakage; an illustrative dissipative calculation gives S≃3.1702 with ideal output encoding. These scores exceed 2√2 because the devices interact during their response, not because quantum mechanics is violated. The construction separates honest measurement, non-signalling reported statistics, and causal isolation, while making the additional randomness, preparation, and monitoring assumptions explicit.
ER -