Quantum interference and entanglement of photons that do not overlap in time We discuss the possibility of quantum interferences and entanglement of photons that exist at different intervals of time, i.e., one photon being recorded before the other has been created. The corresponding twophoton correlation function is shown to violate Bell’s inequalities. [1] [1] R. Wiegner, C. Thiel, J. von Zanthier, G. S. Agarwal, Opt. Lett. 36, 1512 (2011)
Quantum interference and nonlocality of independent photons from disparate sources We quantitatively investigate the nonclassicality and nonlocality of a whole new class of mixed disparate quantum and semiquantum photon sources at the quantum–classical boundary. The latter include photonadded thermal and photonadded coherent sources, experimentally investigated recently by Zavatta et al (2009 Phys. Rev. Lett. 103 140406). The key quantity in our investigations is the visibility of the corresponding photon–photon correlation function. We present explicit results on the violations of the Cauchy–Schwarz inequality—which is a measure of nonclassicality—as well as of Belltype inequalities. [1] [1] R. Wiegner, J. von Zanthier, G. S. Agarwal, J. Phys. B: At. Mol. Opt. Phys. 44, 055501 (2011)
Waveparticle duality revisited We investigate waveparticle duality in a symmetric twoway interferometer with a whichway detector. We find that it is important to state wether the interfering object or the whichway detector is read out first. In case that the interfering object is read out first, we discover that it is possible to use the information about its state to increase the distinguishability to a value which violates the widely accepted bounds set by an inequality introduced by Jaeger et al. (PRA 51, 54) and Englert (PRL 77, 2154). [1] [1] U. Schilling, J. von Zanthier quantph/1006.2037
Creating path entanglement and violating Bell inequalities by independent photon sourcess We demonstrate a novel approach of violating position dependent Bell inequalities by photons emitted via independent photon sources in free space. We trace this violation back to path entanglement created a posteriori by the selection of modes due to the process of detection. [1] [1] R. Wiegner, C. Thiel, J. von Zanthier, G. S. Agarwal, Phys. Lett. A 374, 3405 (2010)
Nonlocality from N > 2 Independent Single Photon Emitters We demonstrate that intensity correlations of second order in the fluorescence light of N > 2 singlephoton emitters may violate locality while the visibility of the signal remains below 71%. For this, we derive a homogeneous BellWignertype inequality, which can be applied to a broad class of experimental setups. We trace the violation of this inequality back to path entanglement created by the process of detection. [1] [1] C. Thiel, R. Wiegner, J. von Zanthier, G. S. Agarwal, Phys. Rev. A 82, 032116 (2010)
Entanglement and nonlocality of independent photons from mixed quantum and semiquantum sources Following the lead experiment by Pittman and Franson [Phys. Rev. Lett. 90, 240401 (2003)] on the violations of CHSH inequalities by mixed quantum and classical sources, we quantitatively investigate violations of CHSH inequalities and CauchySchwarz inequalities for a whole new class of mixed quantum and classical sources at the quantumclassical boundary. These include photon added thermal and coherent sources experimentally investigated recently by Zavatta et al. [Phys. Rev. Lett. 103, 140406 (2009)]. The key quantity in our investigation is the visibility of the corresponding photonphoton correlation function. We also resolve the question of the appropriate photonphoton correlation function to be used in the CHSH inequalities. This is crucial as in contrast to polarization degrees of freedom our signals do not involve dichotomic variables. [1] [1] R. Wiegner, J. von Zanthier, G. S. Agarwal, quantph/1005.4176

