esting quantum correlations of three entangled photons generated in cascaded parametric down-conversion

Non-classical states of light are of fundamental importance for emerging quantum technologies. All optics experiments producing multi-qubit entangled states have until now relied on outcome post-selection, a procedure where only the measurement results corresponding to the desired state are considered. This method severely limits the usefulness of the resulting entangled states. fiere, we show the direct production of polarization- entangled photon triplets by cascading two entangled down-conversion processes[1]. First, a Sagnac source [2] produces non-degenerate entangled photon pairs at 776 nm and 842 nm in a Bell state. The 776 nm photon is sent through a polarizing Mach-Zehnder interferometer with a down-conversion crystal in each arm. If it is down-converted, a photon pair is created in modes 2 and 3 with a polarization state depending on the pump photon. The quantum stete describing the photon triplets is: