In a landmark experiment on indefinite causal order, a team from the School of Physics, SEU, led by Prof. Xue Peng, has achieved a 24-standard-deviation violation of a Bell-like causal inequality. Using entangled photons and a photonic quantum switch with spacelike-separated measurements, the researchers provide compelling evidence for quantum causal structures beyond definite orders. Their findings appear in Science Advances under the title “Experimental violation of a Bell-like causal inequality in a photonic quantum switch.”
Indefinite causal order is a causal structure unique to quantum theory, in which the temporal order of different quantum operations can be placed in a coherent superposition. The quantum switch represents a paradigmatic platform for realizing indefinite causal order; however, existing experimental verifications have typically relied on specific descriptions of the experimental apparatus. By contrast, device-independent verification aims to determine causal structures solely from the input–output statistical correlations observed in experiments, thereby reducing dependence on device models. Nevertheless, the standard quantum switch itself does not violate causal inequalities in the usual sense. A recently proposed protocol—the VBC (Vilasini–Brukner–Colbeck) protocol—offers a new theoretical avenue for testing indefinite causal order in quantum switches by incorporating spacelike-separated measurement parties.
To address this challenge, the research team constructed a photonic quantum switch using a pair of high-quality polarization-entangled photons, and set up four participating parties: Alice 1, Alice 2, Bob, and Charlie. In the experiment, the measurement devices of Bob and Charlie were separated by approximately 20 meters, with the measurement settings on both sides generated by independent random number generators and switched in real time via high-speed electro-optic modulators, ensuring that the relevant measurement events satisfied spacelike separation conditions. On this basis, the team measured the complete conditional probability distributions under different input settings, and obtained an experimental value of 1.8090 ± 0.0024 for the Bell-like causal inequality, exceeding the upper bound of 1.75 allowed by definite causal order models—corresponding to a violation of 24 standard deviations.
Fig.: Schematic diagram of the spatiotemporal relationships among the measurement events of the participating parties in the photonic quantum switch experiment
This work simultaneously implements several key experimental elements required for device-independent tests of indefinite causal order. Under the fair-sampling assumption and the spatiotemporal conditions realized in the current experiment, it provides experimental evidence ruling out hidden-variable models with definite causal order. The significant violation of the Bell-like causal inequality offers further experimental support for indefinite causal order and paves the way toward fully loophole-free device-independent quantum causal tests in the future.
The first author of this paper is SEU Associate Prof. Qu Dengke, while the corresponding author is SEU Prof. Xue Peng. This research was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the China Postdoctoral Science Foundation, the Jiangsu Provincial Basic Research Program, and the Open Research Fund of the Beijing National Laboratory for Condensed Matter Physics.
Paper’s link: https://www.science.org/doi/full/10.1126/sciadv.aee9271
Source: School of Physics, SEU
Translated by: Melody Zhang
Edited by: Leah Li
