Quantum

The coupler Eve did not monitor: a black-box cavity cheat

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Based on a feed excerpt; full article content has not been acquired. Source evidence retrieved 09/10/2026, 02:28:25 UTC.

Briefings

A new quantum protocol demonstrates that Eve can cheat a CHSH game by monitoring a shared beam-splitter interaction without detecting leakage. Eve receives two average photon counts while a monitor registers nothing, yet the devices still communicate. The protocol uses two independently operated controls activating a shared beam-splitter after inputs arrive. This allows Eve to convert reports into a CHSH game with an expected score of 13/4 for four repetitions per block, exceeding the quantum benchmark of 2√2. The example shows how exchange can coexist with a small probability of a click in a specified leakage monitor, without postselection.

Key points

• Eve receives two average photon counts while a monitor watching for external leakage may register nothing.
• The protocol uses two independently operated controls activating a shared beam-splitter interaction after the inputs arrive.

Context

This finding challenges assumptions about quantum device isolation and monitoring. It shows that even with a quiet monitor, Eve can detect signaling through the interaction of devices during their response phase.

Limitations

The example relies on a specific constructive counterexample rather than a general proof. The open-system calculation assumes a dispersive-bus model with specific parameters. The evidence does not specify the exact physical implementation or hardware used for the beam-splitter interaction.

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