A high-dimensional spatially entangled two-photon state is generated via Type-II SPDC in a ppKTP crystal. The two photons are spatially separated by a polarizing beamsplitter (PBS) and sent to two parties, namely, Alice and Bob. Alice performs single-outcome projective measurements that measure whether a photon is carrying spatial mode a from modal basis . These are performed by a combination of an SLM (SLM 3 ), SMF and a single-photon avalanche photodiode (APD). Bob implements a top-down programmable circuit that is constructed from an MMF placed between two programmable SLMs (SLM 1,2 ). The circuit is used to program a variety of high-dimensional quantum gates and serves as a generalized multioutcome device. The circular inset shows a coincidence image depicting a five-outcome measurement in basis = 1 performed with the Fourier gate . The image is obtained by scanning a detector across the output of the circuit, conditioned on the measurement of at Alice, and shows a large intensity in mode 0 due to strong spatial-mode correlations. Coincidence detection events between Alice and Bob are registered by time-tagging electronics.
