The double slit experiment

alan white

Pardon me if I am jumping in to an established thread. I wonder if anyone might like to analyze the following thought experiment:


The double slit test is performed using a right/left particle detector. Photons are projected in quantities sufficient to ensure each particle can be allocated to either right or left slits by the detector.
The phosphor screen is hidden for the time being. It is expected based on prior tests that reading all right/left info prior to viewing the phosphor screen will cause 100% of the particle tracks on the phosphor screen to be without an interference pattern. One should see two vertical "blob" patterns beyond each of the two slits.
Another experiment is done. This time the information of left/right is destroyed before being read by burning the CD on which the detector has stored the info.
Now, opening the door to the screen room and revealing the phosphor screen shows an interference pattern as expected from prior tests.
Next, another experiment is carried out, but this time a cat is placed in a sealed room stocked with enough food for a year.
A device is also put in the sealed room. It is a canister of poison activated by a camera mounted in the screen room that reads the difference between an interference pattern and a simple twin "blob" pattern.
The poison is released if and when an interference pattern is cast (but not yet seen) on the phosphor screen. Since the screen is hidden until a choice has been made at some time within a year's period to either destroy or read right/left info, it is assumed that only at that time would a cat locked in the room actually be either killed or not. However, in spite of the time of choice being randomly selected to fall sometime within the one year period, it should become obvious that if the cat dies, it occurs as soon as an interference pattern was cast on the phosphor screen; in other words, on the day of the test, up to one year in the past. The cat, then, if dead, will show signs of having died at the very outset.
Of course, any decision to view or destroy right/left info is the causal precedent to the cat's destiny.
The question is, what is the state of the cat in the interim between the beginning of the experiment and the choice to destroy the right/left info or not?
What if this same experiment is done using a human subject?

thank you, Alan White

DrChinese

I hadn't provided references previously because they are quite complicated, very difficult to discuss in the forum. So please read this very closely, and you may need to read it several times. It is easy to get lost in the mechanics and miss the fact that this is an ACTUAL experiment and has been accepted by the science community. It is fully in keeping with ordinary QM.

The "summary":

http://grad.physics.sunysb.edu/~amarch/

The actual experiment, in PDF:

Double-slit quantum eraser, Walborn et al, 2002

Keep in mind that this experiment actually shows you both techniques in a single experiment if you will read it fully. Again, it is complicated. The point being, as I said in an earlier post, that the physical mechanism of a polarizer (or any other device by which you learn which-slit information) is NOT in and of itself responsible for causing the double slit pattern interference to disappear. If it were, then you could not have interference in the presence of 2 polarizers and you can - depending solely on their *relative* angle. And also, the pattern is can be dependent on the nature of a measurement on a remote particle. If you measure it such that which-slit information is obtained (or could be in principle) then there is no interference - even when they are sufficiently separated in space or time. In fact, the erasure can occur AFTER the pattern on a screen is created. (Please note that in such case - i.e. erasure - coincidence counting is required to discern the pattern.)

The loss of the DS interference is a consequence of the Heisenberg Uncertainty Principle, the physical mechanism for which is not understood. However, the mathematical mechanism is well understood and has been for over 80 years. The referenced experiment shows many interesting facets of QM in one paper. Good luck!

DarioC

Once more... into the breach, Chuckle.

That summary is really well written. Those (students?) put Brian Greene to shame.

But then they say:

"In case you might be suspicious of the quarter wave plates, it is worth noting that given a beam of light incident on a double slit, changing the polarization of the light has no effect whatsoever on the interference pattern. The pattern will remain the same for an x polarized beam, a y polarized beam, a left or a right circularly polarized beam."

But that isn't quite the same as being polarized x and y at the "same" time, is it?

Further on it is explained the the slit photons are either (x or y) and then they find it peculiar that placing a polarizer that results in (x and y) in the other path returns the interference pattern. Even in electronics that combination will give you an output.

Guess I need to spend more time reading the experiment paper. It is very similar to the other (2007) experiment that I found.

About the same level of "spin your head around" as quadrature modulation, with a little woo-e-woo thrown it.

Thanks for the direct reference, appreciate it.

DC

DrChinese

A plate works for superpositions (x and y) too.

DrChinese

Welcome to PhysicsForums, Alan!

As noted in another thread going on now, the cat example is not always the best to analyze. There is nothing special about the rules having to do with living or macroscopic objects OTHER than the fact that such objects decohere rapidly. In other words, they are almost impossible to maintain in a state of superposition. That is because there are many clues as to what is going on. So your example makes so many assumptions of one kind or another which cannot be considered realistic. So that is why these thought experiments don't give you very much useful to work with.

alan white

Thanks, but in what way would there be a problem maintaining the cat in a superposition?
If no erasure is done, an interference pattern kills the cat, and if erasure is done, the cat lives.
Doing nothing (not erasing) kills the cat by ordinary means---- immediately. A camera reads the interference pattern which allows poison canister to open. i could do that experiment at home with some low tech equipment.
Adding a detector, there becomes a choice to erase or not. It seems to me that the experiment is air tight, but I must be missing something.

yuiop

In another thread ( see post #2 of http://www.physicsforums.com/showthread.php?t=469014), it seems to be demonstrated in a thought experiment, that a vertically polarised entangled photon passing through a vertically orientated polariser near the source, has its entanglement destroyed. This would seem to indicate that some sort of "change" happens to a photon passing through a polariser, even if it is ideally aligned with the polariser, so it seems unrealistic to say photon passing through a polariser is completely undisturbed by the transition. Secondly, the well known fact that a photon has almost zero chance of passing through two polarisers at 90 degrees to each other and 25% chance of passing through if an additional polariser at 45 degrees is place between the first two polarisers, strongly suggests that the middle polariser disturbs the photon in some way.

thenewmans

Rawtrax! Welcome!

I think I see what you’re looking for. The double slit experiment is so easy to do. I bought a laser off Amazon for 1 penny! (+$5 shipping). One way is to put it on the table and stand a pin up in front of it. Another is to poke 2 pinholes in aluminum foil just as close together as you can and stand that up in front of the laser. These are like the slits. Stand it up with a paper clip or a bag clip because you can’t hold it steady enough with your hand. It should shine on a wall that’s a few feet away. You will see little lines in the light spot. That’s the interference pattern or diffraction pattern. The wavelength is roughly .0005mm and the 2 holes are .5mm apart. But even that is close enough to make it work.

This was first done about 200 years ago to demonstrate that light is made up of waves just like splashing in the bath tub. About 100 years ago, they started thinking that light is actually made up of particles. So they figured out how to shoot 1 photon at a time and then put photodiodes in front of the slits. They convert light into electricity. If photons are waves then both should detect the photon. Instead only 1 at a time detects the photon. So when you remove the detectors, light looks like waves. But when you put them back, they look like particles.

The explanation for this is beyond me. But basically there are some things in this world you cannot measure if you’re trying to measure something related at the same time. This is Bohr’s principle of complementarity.