An apparatus for a double-slit interference experiment in the single-photon regime is described. The apparatus includes a which-path marker that destroys the interference as well as a quantum eraser that restores it. We present data taken with several light sources, coherent and incoherent and discuss the efficacy of these as sources of single photons.

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Interference is not 100% complete at all other incident polarization orientations and non-existent near vertical and horizontal orientations. Thus, for unpolarized light, what interference there is near 45° gets swamped and washed out by the stronger single-slit pattern from all the other polarization orientations. For example, suppose the incident light is polarized 30° with respect to the vertical. Malus's law tells us that 1/4 of the incident intensity emanates from the slit with horizontal polarizer and 3/4 of the incident intensity from the other slit with vertical polarizer. One cannot hope to secure complete destructive interference with a factor of three difference in intensities.
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HN42HE was chosen for its combination of high transmittance and high extinction: 42% transmission for a single sheet and 0.002% transmission for two (crossed) sheets at 90°. A newer type (HN38S) is even better in this respect: 38% and 0.0004%. (One might guess that the S stands for “super.”)
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We found this to be the case using a low-pressure Na lamp and LED. These light sources are normally considered to be unpolarized but they are in fact very slightly polarized due to their construction geometry. Consequently, one can observe a very tiny hint of double-slit interference being restored by the quantum eraser, but it's really overwhelmed by the single-slit pattern.
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23.
For example, the second position where one obtains destructive interference for red light very closely coincides with the second maximum (constructive interference) for blue light.
24.
Four software packages are described at <http://andor.com/products/software>.
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