



01727nas a2200157   4500008004100000245004800041210004700089260001500136300001100151490000700162520129200169100001701461700001901478700001601497856005601513       2018                        eng d00aKeyring models: an approach to steerability0 aKeyring models an approach to steerability  c2018/01/02  a0221030 v593 a<p>If a measurement is made on one half of a bipartite system then, conditioned on the outcome, the other half has a new reduced state. If these reduced states defy classical explanation &mdash; that is, if shared randomness cannot produce these reduced states for all possible measurements &mdash; the bipartite state is said to be steerable. Determining which states are steerable is a challenging problem even for low dimensions. In the case of two-qubit systems a criterion is known for T-states (that is, those with maximally mixed marginals) under projective measurements. In the current work we introduce the concept of keyring models &mdash; a special class of local hidden state model. When the measurements made correspond to real projectors, these allow us to study steerability beyond T-states. Using keyring models, we completely solve the steering problem for real projective measurements when the state arises from mixing a pure two-qubit state with uniform noise. We also give a partial solution in the case when the uniform noise is replaced by independent depolarizing channels. Our results imply that Werner states, which are a special case of the previous states, are unsteerable under real projective measurements if and only if their efficiency is at most 2/π.</p>1 aMiller, Carl1 aColbeck, Roger1 aShi, Yaoyun  uhttp://aip.scitation.org/doi/full/10.1063/1.5006199