976 publications from this institution
We consider alternative models to quantum mechanics, that have been proposed in the recent years in order to explain the EPR correlations between two particles. Moving to the case of three particles, we show that these models violate the no-signaling condition. This shows that superluminal hidden communication (or influences) alone cannot explain the correlations between entangled quantum particles.
Erbium-doped solids are potential candidates for the realization of a quantum memory for photons at telecommunication wavelengths. The implementation of quantum memory proposals in rare-earth-ion-doped solids require spectral tailoring of the inhomogeneous absorption profile by efficient population transfer between ground-state levels (spin polarization) using optical pumping. In this paper we investigate the limiting factors of efficient optical pumping between ground-state Zeeman levels in an erbium-doped ${\mathrm{Y}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ crystal. We introduce two methods to overcome these limiting factors: stimulated emission using a second laser and spin mixing using radio frequency excitation. Both methods significantly improve the degree of spin polarization. Population transfer between two Zeeman levels with less than 10% of the total population in the initial ground state is achieved, corresponding to a spin polarization greater than 90%. In addition, we demonstrate spectral tailoring by isolating a narrow absorption peak within a large transparency window.
