The photon, its mode function and complementarity

TL;DR

This paper investigates the principle of complementarity using a double-slit experiment with entangled photons, demonstrating how mode function selection influences the visibility of interference and the availability of which-slit information.

Contribution

It reveals the critical role of photon mode functions and detection schemes in determining interference visibility and which-slit information in entangled photon experiments.

Findings

Interference visibility depends on the photon mode function.

Detection of entangled idler photons breaks symmetry and destroys interference.

Mode selection controls the access to which-slit information.

Abstract

We probe the principle of complementarity by performing a double-slit experiment based on entangled photons created by spontaneous parametric down-conversion from a pump mode in a TEM01-mode. Our setup brings out the need for a careful selection of the signal-idler photon pairs for our study of visibility and distinguishability. Indeed, when the signal photons interfering at the double-slit belong to this double-hump mode we obtain almost perfect visibility of the interference fringes and no "which-slit" information is available. However, when we break the symmetry between the two maxima of the mode by detecting the entangled idler photon, the paths through the slits become distinguishable and the visibility vanishes. It is the mode function of the photons selected by the detection system which decides if interference, or "which-slit" information is accessible in the experiment.