Fig. 3: Sketch of the measurement setup.
From: Few-shot estimation of entanglement with Bell measurement assistance
Entangled biphoton states are transmitted to the measurement setup through uncharacterized fibers. Each photon within the setup undergoes a randomized unitary, selected from a Circular Unitary Ensemble (CUE) implemented using a set of quarter-half-quarter waveplates (depicted by the orange circle). Two optical switches, each featuring one input and two outputs, determine whether the pair of photons will be measured in computational basis (CM) along the outer path or in Bell basis (BM) along the inner path, with a probability of 1/2. Along the outer path, the CM setup consists of two polarizing beam splitters (PBSs). In contrast, the inner path includes an all-optical CNOT gate (depicted by the green circle), which consists of three Partial-Polarization-Beam-Splitter (PPBSs) with tH = 1 and tV = 1/3, following the setup in refs. 37,38,39. This setup, combined with an additional Hadamard gate (22.5circ half-wave plate) and CM, forms a complete BM. Finally, single-photon detectors (SPDs) register the pair of photons. Further details can be found in the Method.
