Experiment: Using a Mach Zehnder Interferometer to observe Quantum Interference using a Single Photon Beam

Below is a schematic of the Mach Zehnder Interferometer taken from the paper by B. J. Pearson and D. P. Jackson, "A hands-on introduction to single photons and quantum mechanics for undergraduates," Am. J. Phys. 78 471 (2010). Our setup differed due to space constraints: the interferometer was set up in reverse from this schematic, but in principle it is the same. See the picture further below. 

Below is a picture of our experimental setup of the interferometer. The blue laser line represents the 405nm blue pump laser, and the red laser lines represent the 810nm down-converted infrared laser beam.

Shown below are the interferometer results from Pearson and Jackson's paper. Untagged refers to when the HWP Polarizers are set equally and are uniform with the external Linear Polarizer (LP). This is sort of a 'default' setting supposed to resemble a simple, completely empty Mach Zehnder (MZ) cavity. Tagged refers to the same setup as Untagged but with the first HWP (HWP 1) offset by 45 degrees. Erased is a setup that is referred to as the Quantum Eraser: the two MZ HWPs are left in their Tagged positions and the external LP offsets the already-offset HWP 1. 

Below is our data. It uses the same labeling convention as explained in the above graph. Unlike the Pearson and Jackson's paper, a phase shift exists between the Tagged and the Erased results.