  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.
