In double-slit experiment, if one directs coherent light from two sources into each slit separetely, is ridges pattern expected to be seen?

Question

I've read several QA here: Is coherent light required for interference in Young's double slit experiment?

Why can't we duplicate the double slit experiment with two parrallel sources of light.. why must the light enter the two slits externally?

Why don't two identical monochromatic lamps form interference patterns?

Why can't we duplicate the double slit experiment with two parrallel sources of light.. why must the light enter the two slits externally?

From answers and comments I could not see definite answer if photon "interfere" only with itself, somewhere it was claimed but mostly discussion was around coherence requirement, I do not recall experiments where slits were separated by a wall. Were such experiments done with a wall and synchronized lasers something? TIA

Web search for "double slits experiment slits isolated by a wall" found no relevant top results, "separated slits" finds separation by distance variations. Making a "wall" at home is not complex, making two lasers coherent is not trivial to test for myself.

I expect photons to be out-of-phase with each other from a laser and, in one replaces them with classical waves there will be no clear ridges pattern, but I do not know exactly how lasers work, maybe lasers can create in-phase radiation. Or maybe there are more factors in play.

Answer 1

Forget photons: interference is a wave phenomenon. It predicts where you will detect photons, but it does not involve photons.

You can do this experiment by tuning in to weak stations at night (to get better range) using an AM radio. The sound will generally "flutter". That's two or more stations on the channel producing a moving interference pattern. The "carrier waves" are coherent, but they are not locked in phase, and have slightly different frequencies.

The same thing happens (in principle) with two lasers, but they must either be extremely coherent and tuned almost to the same frequency, or they must be extremely bright so you can do the experiment very quickly. The problem is that in a practical experiment, while the interference is present, the fringes move so rapidly that they are difficult to detect.

Answer 2

Let's say we have a red laser and a green laser, pointed at the same area on a color film.

Now a red dot on the film is produced by a red photon, which is produced partly (99.9999%) by the red laser, and partly (0.0001%) by the green laser.

This photon's probability of making a dot on a film varies in space, because at some points the photon is coherent (the two parts have the same phase) and at some points the photon is anti-coherent (the two parts have the opposite phase)

Now we say that red and green lasers do not produce an interference pattern, because it is very hard to detect the 0.00001% variation on density of dots. And also because we consider it an "error" when a green laser produces a red photon.

Now maybe a green laser produces a red photon only once a day, and a red laser produces million red photons a day. So then that is the reason why each red photon produced by these two lasers is partly (0.00001%) produced by the green laser, and partly (0.99999%) produced by the red laser.

The above story is an answer to the question, if you think about it carefully enough. I just left out all unnecessary stuff, like the wall and the slits.

Of yes, interference pattern of two lasers pointed at a screen has been observed. Those lasers were made as identical as possible.

Answer 3

The saying “interferes with itself” is historical and misleading … the modern way is to say “each photon determines its own path”.

The photon is actually coherent with the apparatus and that it why it can show up on the screen. Similar photons of similar coherence are also present on the screen. For a non laser source many photons are rejected and are actually reflected at the slit.

It can even be said that the apparatus setup/ geometry influences the production of the photon … i.e. photons will generate from a close source that fit the geometry, similar to a laser cavity.

The best theory is the Feynman path integral. By summing phases of each possible path we get it’s probability. It turns out that paths with lengths of integer wavelengths are most possible.

The excited electron in the atom is already working with the em field even before the photon is emitted …. This a possible reason why the photons path is determined.