Monthly Archives: March 2011

The quantum eraser & solving the half wave plate question

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?



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Trying to make a quantum eraser at the LTC

Today’s visit to the LTC began with reading various web pages about Quantum Eraser experiments. To help understand the quantum eraser experiment I watched a few videos about the famous “Young’s Double Slit” experiment. The videos demonstrated how an observer influences the pattern created by photons (or electrons) sent through two slits. When the slit choice of the photons is unobserved they create a double slit interference pattern, but when observed the photons create two bars. The quantum eraser experiment relies on the same idea of observing which way photons in two interfering laser beams are traveling (by measuring the polarization of the two laser beams).

I decided to try out a simple experiment in a Scientific America article about a DIY quantum eraser experiment, and thought I would improve it by using a fine strand to create the diffraction pattern instead of the suggested piece of wire. This turned out to be unwise, because despite creating a very nice diffraction pattern the initial spacing was so fine I was unable to complete the second part of the experiment, which is to polarize the two halves of the “split beam” in orthogonal directions. I left this project alone, but I think I could get it to work next time by using a lens to increase the size of my laser dot, a wire to split the beam and creating a spliced polarizer, ½ horizontal and ½ vertical. If this is placed directly after the wire the interference pattern should disappear and then reappear if I place another polarizer into the beam after the initial polarization.

I continued to try out this idea using the large interferometer, which has an interference pattern. I placed a horizontal polarizer in one beam and a vertical polarizer in the other. The interference pattern disappeared, because the two beams where no longer interacting with each other, but a reduced intensity light still shone through. Rotating a polarizer in this light just reduced the overall intensity. I do not recall if it completely darkened the light at any point and I forgot to record it. I’ll have to check again.

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Is it a half wave plate?

There is a small square of glass at the Laser Teaching Center, and Dr Noe and I thought it could be a wave plate (either ½ or ¼) but we were not sure. I investigated how it interacted with a laser and two linear polarizers to find out.

The laser was a red HeNe 5mw, initially non-polarized (Checked by rotating two linear polarizers in the beam, resulting in no dot). The beam traveled through a linear polarization filter (Dichroic plastic… FYI to self; Remember Dichroic glass) and emerged linearly polarized. A second polarizer, rotated 90 degrees, was placed in front of the viewing screen completely eliminating the dot.

***Note*** I did not have a tool for measuring intensity at the time, so I judged intensity on a scale of “no dot”, “medium dot”, “bright dot”. Next week I will see about exactly measuring the intensity of the dot.

I added the glass square in between the two polarization filters. For most of the rotation positions the dot became visible again. The only time it did not alter the state of the dot (no dot) was at four 90 rotations of itself.

The next question was if it was a ½ waveplate or quarter waveplate, to examine this I wanted to check for circular or elliptical polarization.


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Polarization & Photons

Polarization is the orientation of the oscillations of the particles composing a transverse wave.  The particles could be oscillating in an up down pattern solely along a single axis or they could be experiencing oscillations along both the y and z axis at the same time, while the wave propagates down the x axis. Because of the different oscillation patterns there is linear, circular or elliptical polarization of light.

Light may consist of many plane waves with different polarizations, making it appear that the light is non-polarized. Filters can be used to separate out plane waves with different orientations of polarization. The reason a filter works is that a horizontally polarized plane wave cannot pass through a vertical slit, and vice versa.

Interference is the phenomena where two light waves are at different phases and cancel/amplify each other, creating a pattern of stripes. Interference patterns appear when a single light beam is split into two beams, and one of the beams takes a slightly longer path before recombining with the original beam, causing a phase difference in the superposition beam.

A half wave plate reverses the direction of polarization of a light beam. For example, if light is filtered so that the beam in oscillating along a +45 degree angle (to the Normal), once it is passed through a half wave plate it will be oscillating along a -45 degree angle (to the normal).

A wave plate works by shifting the phase between the two polarization components of a light wave, so in a sense it is phase shifting the light beam as well.

If a single beam is linearly polarized, then split in two and one half of the beam is phase offset by taking a slightly longer path, there is an interference pattern.

Does adding a half wave plate remove the interference pattern?

Does then adding another phase offset bring it back?

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