Quantum nonlocality has recently been classified into three distinct types: quantum entanglement, Einstein-Podolsky-Rosen (EPR) steering, and Bell's nonlocality. Experimentally Bell's nonlocality is usually tested by quantum violation of the Clause-Horne-Shimony-Holt (CHSH) inequality in the two-qubit system. Bell's nonlocality is the strongest type of nonlocality, also due this reason Bell-test experiments have encountered both the locality loophole and the detection loophole for a very long time. As a weaker nonlocality, EPR steering naturally escapes from the locality loophole and is correspondingly easier to be demonstrated without the detection loophole. In this work, we trigger an extraordinary approach to investigate Bell's nonlocality, which is strongly based on the EPR steering. We present a theorem, showing that for any two-qubit state
τ, if its mapped state
ρ is EPR steerable, then the state
τ must be Bell nonlocal. The result not only pinpoints a deep connection between EPR steering and Bell's nonlocality, but also sheds a new light to realize a loophole-free Bell-test experiment (without the CHSH inequality) through the violation of steering inequality.
Bell nonlocality and Einstein-Podolsky-Rosen (EPR) steering are every important quantum correlations of a composite quantum system. Bell nonlocality of a bipartite state is a quantum correlation demonstrated by some local quantum measurements, while EPR steering is another form of quantum correlations, observed firstly by Schrodinger in the context of famous EPR paradox. In this paper, we give some remarks on Bell nonlocality and EPR steering of bipartite states, including mathematical definitions and characterizations of these two quantum correlations, the convexity and closedness of the set of all Bell local states and the set of all EPR unsteerable states. We also derive a EPR-steering criteria, with which the EPR steerability of the maximally entangled states are checked.
Quantum mechanics has led to the biggest technological breakthroughs of last century. However, many paradoxes still remain in the foundations of quantum theory. In this research we study one effect that leads to such paradoxes; quantum correlations. Quantum nonlocality refers to the set of quantum correlations that are not explainable using local models. It is quantum nonlocality that has troubled physicists since the inception of quantum theory nearly 100 years ago. However, these nonlocal quantum correlations offer a range of advantages in quantum information tasks. For example, quantum key distribution and quantum computing are two tasks that are powered by quantum correlations. For this reason a wide range of quantum correlations are studied experimentally in this research. These include: entanglement, EPR-steering, Bell nonlocality and bilocality. We study these correlations using the framework of quantum information, using single photons as our chosen quantum information carriers. The first quantum correlation tested in this research is EPR-steering. EPR-steering is the application of the EPR-paradox to a quantum information task. We implement three different experiments on EPR-steering. By studying the properties of the quantum information task we find that: EPR-steering is more robust to depolarisation noise than Bell-nonlocality; EPR-steering is absolutely loss tolerant; and tests of EPR-steering are easier to carry out, than tests of Bell nonlocality, but harder to implement compared to tests of non-separability. As a result of these three findings we experimentally show that EPR-steering can be demonstrated on Belllocal states, we close the detection loophole in a photonic test of quantum nonlocality, and we discover and implement maximally parsimonious tests of non-separability, EPR-steering and Bell nonlocality.