Experimental Setup

We used an incandescent light bulb with a green light filter as our light source. The light bulb was embedded into a metal cylinder with a tiny aperture so the light intensity would be small enough for us to realize the single-photon event. Our alignment of the light path was conducted by using a laser. Specifically, we placed a laser in the light bulb position and adjusted the double slits until we could see an interference pattern in the position of the detector. Photons were detected using an H6240-01 photomultiplier tube (PMT). The PMT was connected to a 6602-countering card that consists of two counters, which were used to count the number of detected photons.

All the equipment was established inside a dark box so that the external light would not influence the measurement. As the computer screen was closed to the detector, we also attached some heat isolation sheet on the outside of box. The effect was obvious because the background counts of PMT decreased significantly compared with the situation without isolation sheets. The final dark counts (after we attached the isolation sheets) was 32.5 ± 3.1.

Figure 2. The setup for the double-slit experiment: the green light filter has bandwidth of 550 ± 5 nm; the width of single slit is 0.50 ± 0.05 mm; width of double slits is 0.04 ± 0.01 mm; the separation between double slits is 0.25 ± 0.01 mm; the distance between the detector and double slits is 445 ± 5 mm.

In which-path experiment, we attached two perpendicular polarizing films on the double slits. The laser was used for us to adjust the angle of the films. The angle was successfully adjusted when we saw the interference pattern completely disappear.

Figure 3. The setup for the which-path experiment: following the double slit experiment, two perpendicular polarizers were placed each slit and the detector.

In the quantum eraser experiment, the laser was used again for us to adjust the angle of the quantum eraser. We gradually rotated the polarizer until we saw the interference pattern was restored to the largest extent.

Figure 4. The setup for quantum eraser experiment: following the which-path experiment, a 45-degree polarizer was placed after between quantum markers and the detector.