Entangled two-photon interference

Ayman F. Abouraddy, Magued B. Nasr, Bahaa E. A. Saleh +2 more

The visibilities of second-order (one-photon) and fourth-order (two-photon) interference have been observed in a Young's double-slit experiment using light generated by spontaneous parametric down-conversion and a photon-counting intensified charge-coupled device camera. Coherence and entanglement underlie one- and two-photon interference, respectively. As the effective source size is increased, coherence is diminished while entanglement is enhanced, so that the visibility of one-photon interference decreases while that of two-photon interference increases. This experiment demonstrates the complementarity between one- and two-photon interference (coherence and entanglement) in the spatial domain.

David S. Simon, Gregg S. Jaeger, Alexander V. Sergienko

Z. Y. Ou

Kaige Wang

A general theory is derived for the interference of a two-photon wavepacket in a beamsplitter. The theory is presented in the Schrödinger picture so that the quantum nature of the two-photon interference is explicitly revealed. We find that the topological symmetry of the probability-amplitude spectrum of the two-photon wavepacket dominates the nature of the two-photon interference, which may be distinguished by the increase or decrease of the coincidence probability in the absence of interference. However, two-photon entanglement can be identified by the nature of the interference. We demonstrate the necessary and sufficient conditions for perfect two-photon interference. It is shown that a two-photon entangled state with an anti-symmetric spectrum passes through a 50/50 beamsplitter with perfect transparency. The theory provides us with a unified understanding of the various two-photon interference effects.

Heonoh Kim, Sang Min Lee, Osung Kwon +1 more

We present two-photon interference experiments with polarization-entangled photon pairs in a polarization-based Franson-type interferometer. Although the two photons do not meet at a common beamsplitter, a phase-insensitive Hong-Ou-Mandel type two-photon interference peak and dip fringes are observed, resulting from the two-photon interference effect between two indistinguishable two-photon probability amplitudes leading to a coincidence detection. A spatial quantum beating fringe is also measured for nondegenerate photon pairs in the same interferometer, although the two-photon states have no frequency entanglement. When unentangled polarization-correlated photons are used as an input state, the polarization entanglement is successfully recovered through the interferometer via delayed compensation.

Paul G. Kwiat