Quantum causality violates classical intuitions of cause and effect and is a unique quantum feature different from other quantum phenomena such as entanglement and quantum nonlocality. In order to avoid the detection loophole in quantum causality, we initiate the study of the detection efficiency requirement for observing quantum causality. We first show that previous classical causal inequalities require detection efficiency at least $95.97\%$ ($89.44\%$) to show violation with quantum (nonsignaling) correlations. Next we derive a classical causal inequality $I_{222}$ and show that it requires lower detection efficiency to be violated, $92.39\%$ for quantum correlations and $81.65\%$ for nonsignaling correlations, hence substantially reducing the requirement on detection. Then we extend this causal inequality to the case of multiple measurement settings and analyze the corresponding detection efficiency. After that, we show that previous quantum causal inequalities require detection efficiency at least $94.29\%$ to violate with nonsignaling correlations. We subsequently derive a quantum causal bound $J_{222}$ that has a lower detection efficiency requirement of $91.02\%$ for violation with nonsignaling correlations. Our work paves the way towards an experimental demonstration of quantum causality and shows that causal inequalities significantly differ from Bell inequalities in terms of the detection efficiency requirement.

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• Nonlocality
• Quantum correlations in quantum information
• Quantum foundations
• Quantum networks