Graph-theoretic approach to Bell experiments with low detection efficiency
Bell inequality tests where the detection efficiency is below a certain threshold $η_{\rm{crit}}$ can be simulated with local hidden-variable models. Here, we introduce a method to identify Bell tests requiring low $η_{\rm{crit}}$ and relatively low dimension $d$ of the local quantum systems. The method has two steps. First, we show a family of bipartite Bell inequalities for which, for correlations produced by maximally entangled states, $η_{\rm{crit}}$ can be upper bounded by a function of some invariants of graphs, and use it to identify correlations that require small $η_{\rm{crit}}$. We present examples in which, for maximally entangled states, $η_{\rm{crit}} \le 0.516$ for $d=16$, $η_{\rm{crit}} \le 0.407$ for $d=28$, and $η_{\rm{crit}} \le 0.326$ for $d=32$. We also show evidence that the upper bound for $η_{\rm{crit}}$ can be lowered down to $0.415$ for $d=16$ and present a method to make the upper bound of $η_{\rm{crit}}$ arbitrarily small by increasing the dimension and the number of settings. All these upper bounds for $η_{\rm{crit}}$ are valid (as it is the case in the literature) assuming no noise. The second step is based on the observation that, using the initial state and measurement settings identified in the first step, we can construct Bell inequalities with smaller $η_{\rm{crit}}$ and better noise robustness. For that, we use a modified version of Gilbert's algorithm that takes advantage of the automorphisms of the graphs used in the first step. We illustrate its power by explicitly developing an example in which $η_{\rm{crit}}$ is $12.38\%$ lower and the required visibility is $14.62\%$ lower than the upper bounds obtained in the first step. The tools presented here may allow for developing high-dimensional loophole-free Bell tests and loophole-free Bell nonlocality over long distances.