# Entanglement, complementarity, and vacuum fields in spontaneous parametric down-conversion

## TL;DR

This paper investigates the role of entanglement, vacuum fields, and complementarity in spontaneous parametric down-conversion, demonstrating high-visibility two-photon interference and explaining phenomena through quantum field correlations.

## Contribution

It provides a detailed analysis of how vacuum fields influence two-photon interference and clarifies the role of entanglement versus vacuum effects in SPDC.

## Key findings

- High-visibility two-photon interference observed.
- Vacuum fields play a crucial role in photon correlations.
- The Hong-Ou-Mandel dip can be explained via vacuum field effects.

## Abstract

Using two crystals for spontaneous parametric down-conversion in a parallel setup, we observe two-photon interference with high visibility. The high visibility is consistent with complementarity and the absence of which-path information. The observations are explained as effects of entanglement or equivalently in terms of interfering probability amplitudes, and also by the calculation of a second-order field correlation function in the Heisenberg picture. The latter approach brings out explicitly the role of the vacuum fields in the down-conversion at the crystals and in the photon coincidence counting. For comparison we show that the Hong-Ou-Mandel dip can be explained by the same approach in which the role of the vacuum signal and idler fields, as opposed to entanglement involving vacuum states, is emphasized.