The quantum eraser & solving the half wave plate question

Published on: Wed Mar 23 2011

There were two things I wanted to try at the LTC. The first was to retry the quantum eraser experiment, using the steps as shown in the Scientific America article and my own thoughts about how to do it using a half wave plate. The second thing to do was confirm if the suspected half wave plate actually is a half wave plate.

Confirming the half wave plate

I had previously determined that this supposed half wave plate has an optical axis in the x and y direction which did not effect the transmitted light. At this point I was unsure if the plate was a half or quarter retarder. I know that a quarter wave plate would produce circularly polarized light if held at a 45 degree angle to in incoming polarization. To test for this I set up in this order:

Vertical Polarizer --> Horizontal --> No Dot (Shows I have vertical and horizontal polarizer’s in place)

Vertical Polarizer --> Plate 0 degrees --> Horizontal --> No Dot (Shows the Plate has no effect at 0 degrees)

Vertical Polarizer --> Plate 45 degrees --> Horizontal --> Dot (Still not a confirmed HWP though, it could be a QWP)

Vertical Polarizer --> Plate 45 degrees --> Horizontal --> Linear Polarizer Rotated at random --> Light blocked at times The fact that the light is blocked at times by the linear polarizer confirms the polarization is linear. Since the light is linearly polarized, but appears to have its axis rotated by 90 degrees, the plate is acting as a HWP.


The Quantum Eraser The general idea is to split a beam, have one half be horizontally polarized, the other vertically polarized, recombine the two and see no interference. This part is working, using the method described in Scientific America. Then, when I place a linear polarizer at 45 degrees in the recombined beam I should see a return of the interference pattern. This is not happening, and I am not sure why.

Jones Matrix for what I expected to happen.

I tried creating the V and H polarization using two different methods. The first one was to simply place a vertical polarizer in one beam, and a horizontal polarizer in the other beam. This did remove the interference, but it is not returning when I rotate a linear polarizer in the recombined beam. (Although the dot never completely vanishes.)

IDEA: Try rotating before the magnifying glass. Or is it that the 45 degree polarizer has no knowledge that previously the light was from two different split beams? It could have been any horizontal or vertical light?

The second method for creating V and H polarization was to vertically polarize the entire beam, then place the HWP at 45 degrees in one of the beams, shifting the vertical polarization to horizontal. This did create an interesting effect. The interference fringes became about 4x finer. I should find a way to secure the HWP in this position so I could inspect these fine interference fringes in closer detail. The expected result was for the interference pattern to vanish. I should try rotating a linear polarizer in this interference pattern and see what happens.

Another random question not relating to rest of article: Do two laser beams intersecting with each other cause any effects with each other? Is it possible to change polarization of light through magnetic fields?