Negativity and steering: A stronger Peres conjecture

The violation of a Bell inequality certifies the presence of entanglement even if neither party trusts their measurement devices. Recently Moroder et al. [T. Moroder, J.-D. Bancal, Y.-C. Liang, M. Hofmann, and O. Gühne, Phys. Rev. Lett. 111, 030501 (2013)] showed how to make this statement quantitative, using semidefinite programming to calculate how much entanglement is certified by a given violation. Here I adapt their techniques to the case in which Bob's measurement devices are in fact trusted, the setting for Einstein-Podolsky-Rosen steering inequalities. Interestingly, all of the steering inequalities studied turn out to require negativity for their violations. This supports a significant strengthening of Peres's conjecture that negativity is required to violate a bipartite Bell inequality.

Figure 1

The results of Eq. (8) applied to a simple steering inequality. The lowest (red) curve is $l=1$, the next (blue) is $l=2$, and the highest (black) is $l=3$.

Figure 2

The results of Eq. (8) applied to another steering inequality. The lowest (red) curve is $l=1$, the next (blue) is $l=2$, and the highest (black) is $l=3$.