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.
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.
Since Bell's theorem, it is known that local realism fails to explain quantum phenomena. Bell inequality violations manifestly show the incompatibility of quantum theory with classical notions of cause and effect. As recently found, however, the instrumental scenario-a pivotal tool in causal inference-allows for nonclassicality signatures going beyond this paradigm. If we are not limited to observational data and can intervene in our setup, then we can witness quantum violations of classical bounds on the causal influence among the involved variables even when no Bell-like violation is possible. That is, through interventions, the quantum behavior of a system that would seem classical can be demonstrated. Using a photonic setup-faithfully implementing the instrumental causal structure and switching between observation and intervention run by run-we experimentally witness such a nonclassicality. We also test quantum bounds for the causal influence, showing that they provide a reliable tool for quantum causal modeling.
The measurement of causal relationships within an optical field is a fundamental challenge that bridges quantum optics, photonics, and the theory of causality. This paper proposes a systematic framework for identifying and quantifying the causal structure between discrete optical events in a complex light network. Central to the approach is the construction of a causal matrix C_{ij} that represents the probability of event i causing event j, conditioned on the measurement of photon correlations, time‑stamping, and spatial mode overlap. By exploiting high‑resolution detectors and engineered photonic pathways, the framework can resolve causality in real time, even when multiple photons interfere within integrated photonic circuits. The potential for uncovering deep causal links in photonic systems suggests a pathway to Nobel‑level discoveries, unifying the physics of light with the emerging discipline of causal inference.
The framework of causal models is ideally suited to formalizing certain conceptual problems in quantum the-ory, and conversely, a variety of tools developed by physicists studying the foundations of quantum theory have applications for causal inference. This talk reviews some of the connections between the two fields. In partic-ular, it is shown that certain correlations predicted by quantum theory and observed experimentally cannot be explained by any causal model while respecting the core principles of causal discovery algorithms. Nonetheless, it is argued that by understanding quantum theory as an innovation to the theory of Bayesian inference, one can introduce a quantum generalization of the notion of a causal model and salvage a causal explanation of these cor-relations without fine-tuning. Furthermore, experiments exhibiting certain quantum features, namely, coherence and entanglement, enable solutions to causal inference problems that are intractable classically. In particular, while passive observation of a pair of variables cannot determine the causal relation that holds between them according to classical physics, this is not the case in quantum physics. In other words, according to quantum theory, certain kinds of correlation do imply causation. The results of a quantum-optical experiment confirming these predictions will be presented. This talk is based on the work described in Refs. [1] and [2].
Causal inference revealing causal dependencies between variables from empirical data has found applications in multiple subfields of scientific research. A quantum perspective of correlations holds the promise of overcoming the limitation of Reichenbach's principle and enabling causal inference with only observational data. However, it is still not clear how quantum causal inference can provide operational advantages in general cases. Here, we have devised a photonic setup and experimentally realized an algorithm capable of identifying any two-qubit statistical correlations generated by the two basic causal structures under an observational scenario, thus revealing a universal quantum advantage in causal inference over its classical counterpart. We further demonstrate the explainability and stability of our causal discovery method, which is widely sought in data processing algorithms. Employing a fully observational approach, our result paves the way for studying quantum causality in general settings.
A causal understanding of a physical theory is vital. They provide profound insights into the implications of the theory and contain the information required to manipulate, not only predict, our surroundings. Unfortunately, one of the most broadly used and successful theories, quantum theory, continues to evade a satisfactory causal description. The progress is hindered by the difficulty of faithfully testing causal explanations in an experimental setting. This thesis presents a novel causal discovery algorithm which allows a direct comparison of a wide variety of causal explanations for experimental data. They include causal influences both classical and quantum mechanical in nature. First we provide relevant background information, predominately on quantum mechanics, quantum optics and statistical inference. Next, we review the framework of classical causality and the connection between a causal assumption and statistical model. We then present a novel causal discovery algorithm for noisy experimental data. Finally, we perform two Bell experiments and apply the newly developed algorithm on the resulting data. \nThe causal discovery algorithm operates on observational data without any interven- tions required. It utilizes the concept of predictive accuracy to assign a score to each causal explanation. This allows the simultaneous consideration of classical and quantum causal theories. In addition, this approach allows the identification of overly complex explanations as these perform poorly with respect to this criterion. \nBoth experiments are implemented using quantum optics. The first Bell experiment has a near maximally entangled shared resource state while the second has a separable resource state. The results indicate that a quantum local causal explanation bests describes the first experiment, whereas a classical local causal explanation is preferred for the second. A super-luminal or super-deterministic theory are sub-optimal for both.
Quantum entanglement represents a fundamental resource for quantum information processing, yet the dynamics of multi-photon entangled systems remain inadequately characterized for practical quantum computing applications. This research investigates the generation, manipulation, and characterization of entangled states involving two to eight photons using spontaneous parametric down-conversion techniques. Through theoretical analysis and numerical simulations, we developed a comprehensive framework that quantifies entanglement fidelity, decoherence rates, and Bell inequality violations across various multi-photon configurations. Our findings reveal that entanglement fidelity decreases exponentially with increasing photon number, exhibiting a decay constant of 0.087±0.012 per additional qubit in optical systems. The research demonstrates significant variations in decoherence times across different physical implementations, with ion trap systems maintaining coherence 1.7 times longer than photonic platforms for equivalent qubit numbers. We established mathematical relationships between system parameters and entanglement quality metrics, identifying critical thresholds for fault-tolerant quantum gate operations. The theoretical framework incorporates realistic noise models, environmental coupling mechanisms, and measurement-induced perturbations that affect practical implementations. Our analysis indicates that five-photon entangled states maintain sufficient fidelity (>83.5%) for error-corrected quantum algorithms when decoherence times exceed 75 microseconds. These results provide essential guidelines for designing scalable quantum processors and establishing benchmarks for experimental validation of multi-qubit entanglement protocols.