Counterintuitive causal structures in inter-chip photonic quantum computing.

Causal theory provides a consistent framework for describing relations among events or variables, and has recently been applied to quantum systems. Here, we apply causal analysis to the remote-controlled quantum computing model and uncover a bidirectional causal structure rooted in quantum entanglement, which inherently admits a counterintuitive causal component. Unlike the unidirectional causal structure in classical computing, this framework allows the quantum operation to precede-and causally influence-the parameters that nominally control it, thereby exhibiting counterintuitive causal behavior. We experimentally verify this phenomenon on two photonic chips connected by two 15-meter fibers, achieving an average fidelity of 90.43±2.42% over 100 single-qubit unitaries. Timing analysis further shows that, in our setup, the control parameter can theoretically lag behind the operation by 129.14 ns. To our knowledge, this is the first systematic use of causal modeling to analyze and experimentally reveal the non-classical causal features induced by quantum entanglement in a quantum computing setting, with implications for both quantum computing and causal inference theory.