Fig. 4: Experimental setup for measuring coherence of qubit with collective measurements.
From: Direct estimation of quantum coherence by collective measurements
The setup consists of three modules designed for single-photon source, two-copy state preparation (a, b) and collective measurement, respectively. In the single-photon source module, the photon pairs generated in spontaneous parametric down-conversion are coupled into single-mode fibers separately. One photon is detected by a single-photon detector (SPD) acting as a trigger. In the two-copy state preparation module, a prepares the first copy in the path degree of freedom of the photon; b prepares the second copy in the polarization degree of freedom of the photon. In the collective measurement module, combinations of beam displacers (BDs) and half-wave plates (HWPs) with certain angular settings are used to realize collective measurement, where H4 and H5 are 22.5∘. Four SPDs M1 to M4 correspond to the four outcomes of the collective measurement. Each SPD is a silicon avalanche photodiode (Si-APD), with detection efficiency of ~60%.
