Quantum Eraser Experiment: Fact or Fiction?

[g.lemaitre]

How accurate is this statement:

After the photon passes through the detector, it then passes through the eraser. The eraser is an electronic device that alters the wave-function of the photon, just like the detector is an electronic device that alters the wave-function of the photon.

This guy is claiming that after the photon goes through the detector it then goes through an eraser and the eraser returns it to an interference pattern.

This is from Greene's Fabric of the Cosmos

A simple version of the quantum eraser experiment makes use of the double-slit setup, modified in the following way. A tagging device is placed in front of each slit; it marks any passing photon so that when the photon is examined later, you can tell through which slit it passed. The question of how you can place a mark on a photon—how you can do the equivalent of placing an “L” on a photon that passes through the left slit and an “R” on a photon that passes through the right slit—is a good one, but the details are not particularly important. Roughly, the process relies on using a device that allows a photon to pass freely through a slit but forces its spin axis to point in a particular direction. If the devices in front of the left and right slits manipulate the photon spins in specific but distinct ways, then a more refined detector screen that not only registers a dot at the photon’s impact location, but also keeps a record of the photon’s spin orientation, will reveal through which slit a given photon passed on its way to the detector.

When this double-slit-with-tagging experiment is run, the photons do not build up an interference pattern, as in Figure 7.4a. By now the explanation should be familiar: the new tagging devices allow which-path information to be gleaned, and which-path information singles out one history or another; the data show that any given photon passed through either the left slit or the right slit. And without the combination of left-slit and right-slit trajectories, there are no overlapping probability waves, so no interference pattern is generated.

It seems to me that they are erasing the tagging device above the slits after the photon passes through it, they are not sending a photon through an eraser.

 

[Cthugha]

How accurate is this statement:

After the photon passes through the detector, it then passes through the eraser. The eraser is an electronic device that alters the wave-function of the photon, just like the detector is an electronic device that alters the wave-function of the photon.

Not very accurate. It is also not close to what Greene is claiming.

First of all, it is important to note that the 'tagging device' is NOT a detector. It just leaves some mark on the beam using a reversible interaction (like polarization rotation), but does not perform any measurement. The eraser is not necessarily an electronic device. In most cases, it is just a piece of optics. For example, if one used orthogonal linear polarizations for tagging beforehand, a polarizer oriented at 45° to these polarizations will do as it let's both polarizations pass with equal probability and the resulting beam will have the same polarization irrespective of which of the two initial polarizations entered the eraser.

 

[g.lemaitre]

I want to get a second opinion on what this guy is saying, the words in bold. A third and fourth opinion would be nice.

117938Aa wrote:
Actually, I am interested to see your reaction to the following: you have asserted that the detector and the quantum eraser destroys the interference pattern. Why?

The detector destroys the interference pattern. The eraser restores it.

117938Aa wrote:
There must be some natural law that does this.

Natural laws don't do anything. The detector is made up of charged particles. Charged particles interact with photons. The natural laws that describe how the detector interacts with the photon are the same natural laws that describe how charged particles interact with photons. Specifically, the law in question is quantum electrodynamics.

117938Aa wrote:
At first it was thought that the interference pattern was the result of particles interfering with other particles but it turned out that the interference pattern shows up even when one shoots the photons through the slit one by one.

Right. We now understand that the wave function of the photon interferes with itself. Specifically, when the photon passes through plate with the two slits, the wave-function of the photon interacts with the plate such that you have two wave fronts coming out, one through each slit. Those two wave fronts interfere with each other.

117938Aa wrote:
So why should the removal of the detector and the photon going through the quantum eraser restore the interference pattern? What natural law is causing this?

Again, quantum electrodynamics describes this. No natural law is causing it, because natural laws do not cause anything to happen. They are descriptions, not causes.

What causes the interference pattern to be destroyed when a detector is put into the system is the detector itself. The detector interacts with the photon in such a way that the wave function is no longer two wave fronts coming out, one through each slit, but instead a single wave front coming out from just one of the slits.

And likewise, what causes the interference patter to reappear when the eraser is used is the eraser itself. The eraser interacts with the photon, resulting in two wave fronts that interfere with each other. As a result, the interference pattern reappears.

All of this is accurately described by quantum electrodynamics, which is part of the standard model.

 

[g.lemaitre]

Still hoping for an answer to this.