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Research Article

Experimental Test of Tracking the King Problem

Figure 2

The experimental setup. Maximally entangled photon states are generated via spontaneous parametric down conversion on Alice’s station (see the upper left corner of the sketch). For a certain initial state (featured an illustration of here), the King chooses one of the MUBs to measure the qubit sent to him (in the black box) and returns it back to Alice through the single-mode fiber. In Alice’s station, she can retrodict the King’s choice of measurement with her control measurement (see the lower left corner of the sketch). The King’s choice of nonselective measurements in MUBs will lead to corresponding coincidences (processed by a FPGA), which are listed in the table at the bottom right corner of the figure (each coincidence click has the same probability of 0.5). The identical matrix means doing nothing to the qubit, of which only coincidence will click. BPF: bandpass filter; POL: polarizer; APD: avalanche single-photon detector.