Fig. 1 | Nature Communications

Fig. 1

From: Simultaneous entanglement swapping of multiple orbital angular momentum states of light

Fig. 1

A simplified version of the experimental setup. Generation: the first β-barium borate crystal (BBO0) is pumped by a Ti:Sapphire laser to produce UV light via upconversion. BBO1 produces a downconverted pair A and B; BBO2 produces a downconverted pair C and D. Each is entangled in the state \(\left| {{{{\Psi }}^ + }} \right\rangle\). Hong–Ou–Mandel (HOM) filter: the path length of B is adjusted using a translation stage (TS) such that B and C interfere on a beamsplitter; they are projected onto the antisymmetric state when detected in coincidence in the multi-mode fibres (MMFs). Measurement: at this point, photons at A and D become entangled, which we measure using spatial light modulators (SLMs) in combination with single-mode fibres (SMFs). Detection: each photon is detected using a single-photon avalanche detector, and coincidences are determined using a four-way coincidence detection system. Inset: a conceptual diagram of entanglement swapping. Entanglement between A and B is transferred to A and D via interference at a beamsplitter and detection in coincidence

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