Inequalities for nonideal correlation experiments

Louis Sica

Marco Minozzo

A. Bernal, M. Corgini, Дмитрий Петрович Санкович

Waldemar Kłobus, Andrzej Grudka, Andreas Baumgartner +3 more

We investigate theoretically the use of nonideal ferromagnetic contacts as a means to detect quantum entanglement of electron spins in transport experiments. We use a designated entanglement witness and find a minimal spin polarization of $\ensuremath{\eta}>1/\sqrt{3}\ensuremath{\approx}58%$ required to demonstrate spin entanglement. This is significantly less stringent than the ubiquitous tests of Bell's inequality with $\ensuremath{\eta}>1/\sqrt[4]{2}\ensuremath{\approx}84%$. In addition, we discuss the impact of decoherence and noise on entanglement detection and apply the presented framework to a simple quantum cryptography protocol. Our results are directly applicable to a large variety of experiments.

Matthias Lang, Anil Shaji, Carlton M. Caves

Sandu Popescu

Standard Bell inequalities apply to correlations arising when two or more macroscopically separated systems are each subjected to a single ideal measurement. While these inequalities demonstrate that quantum mechanics and local hidden variable theories yield incompatible predictions for pure states, they fail for mixed states: For mixed states, correlations arising from a single ideal measurement on each system may obey standard Bell inequalities, yet when each system is subjected to a sequence of ideal measurements the correlations are nonlocal. For some situations even this last procedure fails, and we must consider more complex (``nonideal'') measurements.