Introduction and Motivation
The past semester at the Laser Teaching Center I studied an experiment called “The Quantum Eraser”. The idea to make the quantum eraser experiment came from my interest in quantum information. For some time now I have been talking with Dr. Noe about my interest in quantum computing and quantum information. I was reading many different books and articles on the subject and several of the books described optical quantum computers. Dr. Noe suggested the quantum eraser experiment, because many of the quantum computer descriptions I had read focused on the use of polarized photons as a type of qubit.
Background And History
The experiment is based on interference and polarization of light. Interference patterns happen when two rays of light from a single source are out of phase with each other and when they recombine they are superimposed, amplifying the intensity in some places while diminishing the intensity of the wave in others. Interference was demonstrated by Thomas Young and was one of the key indicators of the wave nature of light. “The Quantum Eraser” demonstration shows the wave nature of light.
Additional experiments by Robert Millikan confirmed Einstein’s suggestion that light consists of photons, discreet quantities of energy. “The Quantum Eraser” demonstration shows the dual nature of light, by demonstrating both the wave and particle aspects. The particle like aspect is the predictability of path. We are able to gain knowledge about the selected path of the light by using polarizing filters, which only let light of a selected orientation pass through. When we gain path knowledge the interference pattern disappears.
To get started on the experiment I began to read about quantum eraser demonstrations from the classical point of view. This experiment has been preformed many times before, and is often referred to as a “Which-way” or “Welcher Weg”, because the experiment is about “which way the light went”. The experiment is set up on an interferometer table with polarizing filters and a half-wave plate. The interferometer splits a laser beam in half and recombines the beams at a slight angle, producing interference, which is observed by seeing an intensity pattern in the projected light. The classical explanation is the electric field is linearly polarized in one direction, then split into two paths, and recombined to produce a fringe pattern. When the polarization of one of the paths is rotated the fringe pattern may be less visible, or not present at all.
In PHY 300 Waves and Optics, we were studying Jones Matrices for optical elements so I put together some Jones matrices for the quantum eraser layout, to see what arrangement would be ideal. One thing I had to do before setting up the experiment was determine if we had a half-wave plate on hand. There was a lens we thought was a half-wave plate, but we were not completely sure. Therefore the first portion of the set-up was to determine if the “supposed half-wave plate” was indeed a half wave plate. I wrote about this in two posts “Is it a half wave plate?” and “The quantum eraser & solving the half wave plate question”.
After determining we did have half-wave plate I set up one of the arrangements from the Jones matrices calculations. One arrangement, using a half-wave plate, was clearly superior to the other arrangement because the overall intensity was twice as great. This was because the half-wave plate was used to rotate the polarization along one beam, instead of using two polarizer’s to change the polarization along each path.
The measurements I wanted to study were the fringe visibility for a series of eraser [adjustable polarizer] orientations. To calculate fringe visibility I needed to record the fringe pattern many times at each eraser orientation. To do this an Arduino board and a Thor Labs DET 110 photodetector were used. The photodetector was attached to a translation stage and a power drill was used to move the photodetector through the fringes. Using this method I was able to record fringe patterns for six different rotations of the eraser and to record two sets of data at each rotation.
Understanding the Results
After setting up the experiment based on my classical understanding I was able to see how quantum information affected the outcome. One of the papers I had been reading to learn how to set up the experiment explained the results in terms of visibility and knowledge. This understanding of The Quantum Eraser relates to Launders Principle of Information, and how information is physical. The relationship between visibility and path knowledge is clearly present in my gathered data, but my results are less than ideal. Several factors contributed to noise within the data.
The most confusing effect is that when the half-wave plate was rotated 90 degrees it should have reduced the visibility of the fringes to a greater degree. This can be understood by seeing that the overall intensity of the fringes was significantly reduced, but the overall fringe visibility did not decrease as much as expected. I think this is due to uneven intensity through the two arms of the interferometer. The distribution is closer to 45/55 instead of 50/50. Also, the wide range in data points for each path knowledge measurement is due to the Gaussian distribution of the overall fringe pattern. Each fringe follows an intensity curve and the overall fringe pattern also follows an intensity curve. The third explanation is discrepancies between the wave-plate wavelength and the wavelength of the laser. Based on observations this third effect is minimal and should have contributed the least to unexpected results.
To measure the visibility I made a measurement of the fringe pattern at various rotations of the ‘Eraser’ polarizer. Then for each fringe pattern I calculated the visibility of each fringe. The reason I calculated the visibility of each fringe was because the overall intensity drifted upward as the measurement progressed.
Finally, there was one aspect of measurement I did not complete analytically. I wanted to see if the bright/dark lines would invert due to rotation of the quantum eraser. I believe this is the case based on observations but I have not made the necessary measurements to confirm this. To confirm this I would have positioned the photodiode in a single spot, then by rotating the eraser I could see this effect.
Studying the quantum eraser demonstration lead me to consider the wave-particle nature of light and I will often think of my observations in this experiment when thinking about the nature of light. During the course of this experiment I thought about the relationship between discreet and continuous. I was thinking about why we need to have the concept of zero in a discreet system and how waves are continuous while particles are discreet. In the practical sense I learned about polarization and Jones matrices, gained a better understanding of how an interferometer works and devised a way to record accurate measurements.
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