**Quantum states prepared by real-world entanglement swapping**

Entanglement swapping between photon pairs is a fundamental building block in schemes using quantum relays or quantum repeaters to overcome the range limits of long distance quantum key distribution. We develop a theory to calculate the actual quantum states prepared by realistic entanglement swapping, which takes into account experimental deficiencies due to inefficient detectors, detector dark counts and multi-photon-pair contributions of parametric down conversion sources. We investigate how the entanglement present in the final state of the remaining modes is affected by the real-world imperfections. To test the predictions of our theory, comparison with experimental entanglement swapping is provided. Current efforts towards generalizing our results to iterations of several concatenated noisy entanglement swappings are of particular relevance to research on long distance quantum communication.