Entanglement-preserving absorption of single SPDC photons by a single atom

Abstract

The emission and absorption of single photons by single atomic particles is a fundamental limit of light-matter interaction. As a controlled process, it is a tool for quantum optical information technology. A key task in quantum networks is to establish entanglement between distant atomic qubits that serve as nodes. Until now this has only been experimentally demonstrated [1] through an approach that uses a projective measurement of the photons emitted by the two distant atoms, to entangle them. Another strategy is to generate entangled photon pairs by spontaneous parametric down-conversion (SPDC), and make them interact with the distant atoms, transferring the entanglement from photon to atomic qubits [2,3]. We have recently demonstrated for the first time the heralded absorption of a single down-conversion photon by a single <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">40</sup> Ca <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ion [4]. Here we show that besides the time and frequency correlation that the photon pairs share, also their polarization entanglement is preserved during the absorption of one of the photons by a single atom.