So what is a quantum eraser actually erasing? This question is addressed in some detail in two articles from 1999 and 2004 that one of us (Kwiat) coauthored. The conclusion of those investigations was that the phenomenon that everyone calls quantum erasure would perhaps more accurately be described as quantum editing.
The point is that the final analyzing element filters all of the events into two groups and one can choose to do that filtering in different ways. One can divvy up the observed photons into those that took the left path around the wire and those that took the right path or, by changing the orientation of the analyzer, one can instead divide the photons into those that produce fringes and those that produce anti-fringes. Thus we can "edit" reality in those two alternative ways. In the delayed-choice variant, we can do that editing long after the particles have finished doing whatever it is that they "did."
The two papers are available here as pdf files, or you can use the following references:
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"Quantitative Wave-Particle Duality and Non-Erasing Quantum Erasure," by P. D. D. Schwindt, P. G. Kwiat, and B.-G. Englert, Phys. Rev. A 60, 4285 (1999).
"Quantum-erasing the nature of reality, or perhaps, the reality of nature?" by P. G. Kwiat and B.-G. Englert, in Science and Ultimate Reality: Quantum Theory, Cosmology, and Complexity, John D. Barrow, Paul C. W. Davies, and Charles L. Harper, Jr., Editors, Cambridge University Press (2004).
The entire mechanism of gaining which-path information and subsequently erasing it is intimately connected with the quantum mechanical concept of entanglement, which Schrödinger described as the quintessential quantum mechanical feature. In the Schrödinger's cat experiment, the state of Schrödinger's cat (dead or alive) becomes entangled with the state of a radioactive atom (decayed or not). In the eraser experiment, the path of the quantum object (left or right) becomes entangled with the labeler (vertical or horizontal polarization) in exactly the same way. For a quantum physicist, keeping track of that entanglement is a key to making sense of what an eraser does.
According to our understanding of quantum mechanics, if you are treated as a quantum system, then your wavefunction should be continuously spreading out—according to the uncertainty principle, your momentum is slightly uncertain in all directions! So why don't we slowly "diffuse" out into the universe? One way to understand it is that we are constantly being measured. Our position is being "labeled" by our interactions with the environment around us, such as the air molecules bouncing off us, or the thermal infrared photons we constantly emit just because we are warm. Remember, even though no one is actively doing experiments to extract the potential position-labeling information from everything we interact with, its mere existence serves to prevent us from being in quantum superpositions of different locations. A quantum physicist would say that we are "entangled" with the other quantum systems in our local environment.
We don't actually know that what we described in the previous paragraph is what happens with big objects such as people—it's possible that the rules of quantum mechanics don't even apply to big objects. For this very reason it is interesting to see if there is a limit to the size or kind of things that can be made to display quantum interference. So far as we know, quantum mechanics should apply at these larger scales, but a good scientist waits for experimental proof!
More to Explore:
• View the slideshow of quantum erasure in action
• Discuss the experiment in the blog
• What You Will Need For the Experiment
• What Polarizers Do To Photons
• How A Quantum Eraser Works
• Notes on Polarizing Film
• Troubleshooting the Experiment
• More Experiments
• Answer to the 3-Polarizer Puzzle Featured in the Print Edition
• Whither Waves? More About Interference
• Cutting-Edge Experiments: Interfering Soccer Balls
• Delayed-Choice Experiments
• What Do the Quantum Particles Really Do?
• What is Being Erased?
