SciAm's DIY Quantum Eraser: Not Real?

[Karl Coryat]

I recently tried a "DIY Quantum Eraser" experiment that was in the May 2007 issue of Scientific American (http://www.scientificamerican.com/article.cfm?id=slide-show-do-it-yourself-diy-quantum-eraser"). The basic idea is it's a double-slit experiment where photons going through each slit are "marked" by linear-polarizing the light at a right angle relative to the other slit, then "erasing" the marking by introducing a 45-degree polarization. It worked well, but I had trouble believing that I was witnessing a true "quantum eraser" effect. It struck me that two light waves, linear-polarized at right angles to each other, perhaps would not interfere regardless of the circumstances.

A little digging online and I found the http://encyclopedia2.thefreedictionary.com/Fresnel-Arago+laws" , which state that light polarized at right angles does not interfere.

I looked all over for someone pointing out SciAm's error, but found nothing. A few people have written about performing the experiment, but none protested that the observed results aren't what they are purported to be.

Am I right in assuming that Scientific American got it wrong -- that this experiment confirms not quantum erasure, but the Fresnel-Arago laws? Or, is it that the Fresnel-Arago laws predate quantum mechanics, and quantum erasure actually explains why two orthogonally polarized beams will not interfere?

 

[khkwang]

The way I see it (and take my opinion with a grain of salt because I'm just learning about the quantum eraser now and my background on quantum mechanics is pretty lacking), two waves purely polarized at right angles to each other don't interfere because the polarization itself gives the observer the information required to deduce which "slit" the photon went through. This information thus collapses the interference pattern.

In other words I agree with your second option: that the quantum eraser experiment explains why right angle polarized rays don't interfere.

Btw, I'm really glad I found this thread. I'm doing a short presentation on the quantum eraser in class soon, and it'd be great if I can get this little home made experiment to work.

 

[eaglelake]

You are correct! With only the x- and y-linear polarizers in place, there is no interference. But, if you then repeat the experiment with the 45 degree polarizer added, there is interference. Actually, you should do three experiments to "see" the "eraser" effect.
1 - Do the experiment without any polarizers. There is interference.
2 - Do the experiment with x-polarizer covering slit A and y-polarizer covering slit B. There is no interference.
3 - Do the experiment with all three polarizers in place. Again, there is interference, as in the first experiment. It is as if the 45 degree polarizer has "erased" the "markers" that told us which slit the photon went through.
Best wishes

 

[eaglelake]

I just read the Scientific American article and it is correct as written. However, it is true that long before quantum mechanics it was known that only parallel components of E can interfere.
Best wishes

 

[JimJohnston]

I recently tried a "DIY Quantum Eraser" experiment that was in the May 2007 issue of Scientific American (explained and illustrated here). The basic idea is it's a double-slit experiment where photons going through each slit are "marked" by linear-polarizing the light at a right angle relative to the other slit, then "erasing" the marking by introducing a 45-degree polarization. It worked well, but I had trouble believing that I was witnessing a true "quantum eraser" effect. It struck me that two light waves, linear-polarized at right angles to each other, perhaps would not interfere regardless of the circumstances.

A little digging online and I found the Fresnel-Arago laws, which state that light polarized at right angles does not interfere.

I looked all over for someone pointing out SciAm's error, but found nothing. A few people have written about performing the experiment, but none protested that the observed results aren't what they are purported to be.

Am I right in assuming that Scientific American got it wrong -- that this experiment confirms not quantum erasure, but the Fresnel-Arago laws? Or, is it that the Fresnel-Arago laws predate quantum mechanics, and quantum erasure actually explains why two orthogonally polarized beams will not interfere?

-------------------------
You are correct. You can use classical theory of the electromagnetic field, except for coincidence counting rates. But not at all, at low photon number in the field. Weak fields do not indicate low photon number; they can be overlapping very long photon wave-packets in weak fields, and it is a lot easier to just think in terms of the field.
(John Archibald Wheeler comments that one can use classical field (with minor adjustment to the energy in some cases), in his delayed choice article [J. A. Wheeler, In Mathematical Foundations of Quantum Mechanics, edited by A.R. Marlow (Academic Press, NY, 1978). pp. 9-48.])
There are many stories about behavior of photons that miss-the-mark.
Further, the superposition principle in quantum theory is mathematically the same as Huygen's in classical optics, in terms of adding wave sources.