We're planning the science fair for my wife's high school later this year (if the quarantine allows us...). We are looking into doing this simple quantum eraser experimenter from Scientific American, but I was wondering if there was a way to put together a slightly delayed-choice version of the same.

I believe the simple quantum eraser experiment with polarizers from Scientific American, despite the explanations, tend to make kids deduce that the polarizer path labeler destroys the interference pattern "because you messed with the beam", and not because which-way information is encoded. In other words, takes some of the awe away.

Building a true delayed-choice quantum eraser (Kim et al., 1999) experiment would be overkill and beyond our amateur capabilities. So we devised an augmented experiment that would be somewhere in between:

  • add a BBO crystal after the slit to get entangled photons.
  • add a splitting prism (Glen-Thompson prism) to split the beam into signal and idler photons.
  • send the signal photons to the screen (d0 in the Kim et al. experiment).
  • put the polarizing film on the idler photon beam to manually turn on/off the which-path detection (d3 and d4detectors).

Augmented quantum eraser experiment

When the polarizing film pair is manually added into the idler beams we would expect it to destroy the signal photon interference on the screen. This would be unintuitive ("spooky") to the teens naked eyes as interacting on a second beam would destroy the interference pattern seen on the screen. Rather then delayed-choice, this is more of an action-at-a-distance situation as we're not going to be delaying the choice, just seeing that acting on one beam will change the behavior of another.

Now, I'm afraid my flaw here is that after adding a BBO crystal and/or the prism, noise is introduced that the interference pattern will be destroyed at the screen. Is my assumption correct? Is there any additional set up that could fix this?

Or... is there a way to split entangled beams in a way that the experiment will work for the naked eye? Actually, just being able to alter patterns, regardless of shape, on a screen at a distance would be enough to make this result in a mind-bending experience for the kids!


The quantum eraser is the strict application of a theory to a complex experimental setup .... it has led to controversy and I would say no definite conclusions. If other theories are applied the experimental behaviour can be explained and "erasing" is not required.

BBO crystals are used in 2 ways (your experiment uses it incorrectly), 1) depending on the incoming beam polarization the beam will take one path or another ... i.e. a beam can be split it 2 paths especially if you have a laser (all photons polarized in one direction) and as you rotate it you will see one path then the other .... at an ambiguous angle 2 beams are produced 2) if you excite with a higher energy beam (UV? - dangerous) the crystal will produce entangled IR (invisible to the eye) photons, again at slight different angles.

The entangled photons are way beyond your budget and you need special equipment to observe .... a lot of work for an experiment that is likely not showing any erasing at all.

A good experiment may be to simply split your beam with a 50/50 mirror ... set up an interference in one path (path 1) that is adjustable .... the intensity of the second path will vary as the "interference" path length is varied in path 1.

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