Figure 2: Experimental scheme.
From: Conclusive quantum steering with superconducting transition-edge sensors
Polarization-entangled two-photon states are generated in a periodically poled 10 mm KTiOPO4 (ppKTP) crystal inside a polarization Sagnac loop16,17. The continuous wave, grating-stabilized 410-nm pump laser (LD) is focussed into this crystal with an aspheric lens (L1, f=4.0 mm) and its polarization is set with a fibre polarization controller (PC) and a half-wave plate (HWP), controlling the entangled output state16. Bob filters his output photon with a long-pass glass filter (LP) and a 3-nm band-pass filter (BP), before collecting it with an aspheric lens (L2, f=18.4 mm) into a single-mode fibre. He performs his measurement in an external fibre bridge, with a combination of a quarter-wave plate (QWP), HWP, a polarizing beam displacer (BD) and multi-mode-fibre-coupled single-photon avalanche diodes (SPADs). To minimize loss, Alice performs her measurement directly at the source using a QWP, HWP and a polarizing beamsplitter (PBS), followed by a LP filter and fibre collection with focussing optics identical to Bob's, finally detecting her photons with highly efficient superconducting transition-edge sensors (TESs)18.
