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This study experimentally demonstrates a violation of Bell-like inequalities for causal order using photonic time-bin encoding and kilometer-scale fiber networks to simulate spacelike separation. The achievement provides robust evidence that physical processes can occur in an indefinite temporal sequence, advancing the foundational understanding of quantum mechanics beyond standard acyclic spacetimes. While current results rely on specific setup knowledge rather than being fully device-independent, they pave the way for future tests involving genuine quantum-spacetime interactions.
Experimental violation of a Bell-like inequality for causal order
arXiv:2506.20516v2 Announce Type: replace Abstract: Quantum mechanics is compatible with scenarios where physical processes happen in an indefinite order. In theory, this feature could be detected through violations of inequalities on the observed correlations, analogous to Bell inequalities. However, experimental demonstrations of such violations have been missing until recently due to the complexity of the required setup. Here we report an experimental violation of a Bell-like inequality involving the correlations of four parties, one of which is spacelike separated from the others. Our demonstration employs 3 km fiber spools to simulate spacelike separation, and achieves high-speed operations in photonic time-bin encoding, nanosecond synchronization, and accurate temperature stabilization. These experimental advances enable a violation by 5.7 standard deviations and open a path towards a certification of indefinite order in conditions that guarantee spacelike separation with existing state-of-the-art devices. However, the certification is not device-independent, as it relies on knowledge about the setup to exclude bidirectional signaling--a loophole inherent to implementations in classical acyclic spacetimes, which may be resolved in future quantum-spacetime tests.
Abstract:Quantum mechanics is compatible with scenarios where physical processes happen in an indefinite order. In theory, this feature could be detected through violations of inequalities on the observed correlations, analogous to Bell inequalities. However, experimental demonstrations of such violations have been missing until recently due to the complexity of the required setup. Here we report an experimental violation of a Bell-like inequality involving the correlations of four parties, one of which is spacelike separated from the others. Our demonstration employs 3 km fiber spools to simulate spacelike separation, and achieves high-speed operations in photonic time-bin encoding, nanosecond synchronization, and accurate temperature stabilization. These experimental advances enable a violation by 5.7 standard deviations and open a path towards a certification of indefinite order in conditions that guarantee spacelike separation with existing state-of-the-art devices. However, the certification is not device-independent, as it relies on knowledge about the setup to exclude bidirectional signaling--a loophole inherent to implementations in classical acyclic spacetimes, which may be resolved in future quantum-spacetime tests.
20 pages, 5 figures; comments are welcome!
Quantum Physics (quant-ph); Optics (physics.optics)
arXiv:2506.20516 [quant-ph]
(or arXiv:2506.20516v2 [quant-ph] for this version)
Sci. Adv.12,eaee2912(2026)
https://doi.org/10.1126/sciadv.aee2912 DOI(s) linking to related resources
Text integral sub licența deschisă a publicației · 194 cuvinte
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