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 two-photon 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 non-locality of independent photons from disparate sources
 

We quantitatively investigate the non-classicality and non-locality of a whole new class of mixed disparate quantum and semiquantum photon sources at the quantum–classical boundary. The latter include photon-added thermal and photon-added 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 non-classicality—as well as of Bell-type inequalities. [1]

 [1] R. Wiegner, J. von Zanthier, G. S. Agarwal, J. Phys. B: At. Mol. Opt. Phys. 44, 055501 (2011) 

 

Wave-particle duality revisited
 

We investigate wave-particle duality in a symmetric two-way interferometer with a which-way detector. We find that it is important to state wether the interfering object or the which-way 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 quant-ph/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) 

 

 Non-locality from N > 2 Independent Single Photon Emitters
 

We demonstrate that intensity correlations of second order in the fluorescence light of N > 2 single-photon emitters may violate locality while the visibility of the signal remains below 71%. For this, we derive a homogeneous Bell-Wigner-type 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 non-locality 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 Cauchy-Schwarz inequalities for a whole new class of mixed quantum and classical sources at the quantum-classical 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 photon-photon correlation function. We also resolve the question of the appropriate photon-photon 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, quant-ph/1005.4176