Final Report "The Quantum Eraser"
Carrie Segal
Spring 2011
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.
Experimental Set-up
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.
Conclusion
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.
Resources
The
Quantum Eraser Abstract [PDF]
Quantum
Eraser Presentation [PDF]
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