Newton’s corpuscular theory of light was the accepted model of light propagation in the 17th and 18th centuries. The double-slit experiment, first performed by Thomas Young in 1801, seemed to revive the wave theory of light. In the modern version of this experiment, a coherent light source such as a laser is aimed at an opaque plate in which either one or two parallel slits have been cut. The rectangular beams illuminate a screen placed a short distance beyond the plate, and the images reveal some unanticipated results that caused theoreticians to revise accepted notions of the nature of light and space.
Interestingly, the double-slit experiment continues to be a source of confusion even for some experienced physicists, as certain YouTube videos point out. So it is worth reviewing the experiment and its somewhat weird results.
The double-slit experiment is sometimes called the simplest way of understanding the ambiguity of quantum mechanics. In the experiment, light shined on two slits passes through them to produce a pattern on a distant screen. Today, lasers often serve as the light sources. The light going through each slit interferes with one another and the result is a series of bright and dark lines on the screen. Where the peak of the wave of light going through one slit coincides with the trough of light going through the other slit, the result is a dark spot. If peaks or troughs from both slits line up, a bright spot appears.
Physics students reproduce this experiment each year in introductory laboratories. If there is only one slit, not two, there is no interference because the light has only one source. The interference patterns seen in the experiment illustrates the wave nature of light.
Other experiments show light is also a particle, and particles of light are called photons. The question to ask is what would be expected if light acted like a particle when it went through the double slits. The answer: The particle would go through one slit or the other, but not both. The resulting pattern on a distant screen would be two patches where the particles hit. The rest of the screen would have no hits.
Now consider adding a detector around the two slits to see if the photon goes through one slit or the other. That shouldn’t happen with a wave, but it does with a photon. Interestingly, the pattern on the distant screen in this case looks like light is composed of particles, resembling spots as from an ink jet printer nozzle. In the past, some people have interpreted this ambiguous result as saying that photons act like waves when you’re not looking at them but like particles when you are.
Now consider shooting one photon at a time at the two slits. If light acted like a wave, the expected result would be a very faint interference pattern. Surprisingly, that’s not what happens. Instead, the photon is detected at a single spot on the screen as would be expected from a particle. Now further suppose a second photon gets sent through the slits. Like the first one, it appears at a single spot. But when the third, fourth, hundredth, millionth and zillionth photon goes through the slits, they start building up a traditional interference pattern.
Thus it seems that individual photons travel through both slits, yet they are detected like particles and also seem to be governed by the mechanics of waves.
Now suppose we add a crystal to the experiment which turns the incoming photon into two photons each having half the energy of the first one. These two photons are said to be entangled, meaning if you measure one, you know things about the other. To the double-slit apparatus we now add two detectors which measure not the photons hitting the screen, but rather which slit each photon goes through. We do this by letting the detectors measure the cousin photon which never hits the screen.
Thus the ambiguity: If the two detectors are turned off, observers see a wave pattern. But if the detectors are on and observers look to see through which slit the photon passed, you get a particle pattern.
Now further suppose there are cables going to the two detectors that are quite long so the photon hits the screen before the detectors can measure its cousin. The result is unchanged. The interpretation is that detecting the cousin photon affects what the photon hitting the screen does in the past. The particle or wave pattern is created by whether a detector in the future sees which slit the photon goes through.
In a nutshell, if the original two detectors are used in the original experiment, a particle pattern appears. However, if two new detectors are added, a wave pattern appears. And detection in all four detectors happen after the interference pattern is created.
In actuality, photons going through only one slit generates a one-slit diffraction pattern. This is a pattern that resembles a blurry blob. And if you add the blobs from the two individual slits, they’ll overlap and still pretty much look like one blob rather than like two cleanly separated blobs that are often depicted in descriptions of this experiment. A point to note is the sum of the images from both separate slits is not the image obtained from both slits together.
If you create these entangled pairs after the double slit, the wave-function of the photon depends on through which slit it passes. This information comes from the location where the photon and cousin pairs were created and is usually called the “which way information.” Because of this which-way information, the photons on the screen can’t create an interference pattern.
On the other side of the entangled particles, particles get measured in two different ways as depicted in the nearby drawing. In the first case, you measure the which-way information directly via two detectors D1 and D2. The first detector is on the path of the photons from the left slit, the second detector on the path of the photons from the right slit. Measuring photons with these two detectors yields no interference pattern. But alternatively you can turn off the first two detectors and instead combine the two beams in two different ways. In the drawing, two mirrors just redirect the beam. There’s a beam splitter in the diagram that lets half of the photons go through while the other half is reflected.
The point of the setup is to combine the two beams so you no longer know from which way the photon came. This effect is sometimes called the “erasure” of the “which way information.” The combined beams get measured in detectors D3 and D4. A measurement on one of those two detectors does not tell you through which slit the photon went.
Finally, you measure the distribution of photons on the screen that are the entangled partners of those photons that went to D3. These photons create an interference pattern. You can alternatively measure the distribution of photons on the screen that are partner particles of those photons that went to D4. Those also create an interference pattern. This is the “quantum erasure.” It seems you’ve managed to get rid of the which way information by combining those paths, and that restores the interference pattern. In the delayed-choice quantum eraser experiment, the erasure happens well after the entangled partner particle hit the screen. This is fairly easy to do just by making the paths of those photons long enough.
It’s sometimes said that the choice of what you measure on the one side of the experiment decides what happened on the other side before you made that choice. But physicists point out this is nonsense. Because of which-way information, the photons on the screen can’t create an interference pattern. It doesn’t matter what takes place on the other side of the experiment. The photons on the screen will always create the same pattern. And it’ll never be an interference pattern.
A point to note is that if you use detectors D3 and D4 those interference patterns are not the same. And if you add them you get exactly the same patterns as you get from detectors D1 and D2, namely, two overlapping blurry blobs. This is why it matters that you know the combined pattern of two single slits doesn’t give you two separate blobs, as is often shown on depictions of the experiment.
What actually happens in the eraser experiment is a sampling of the photon pairs in two groups. And that’s done in two different ways. When using detectors D1 and D2 the entangled partners on the screen do not create an interference pattern for each detector separately. When using detectors D3 and D4, they each separately create an interference pattern but together they don’t. This means the interference pattern really comes from selectively disregarding some of the particles. In a nutshell, these interference patterns will always combine to give the original non-interference pattern.
Nevertheless, the strange result of the double-slit experiment is that if you look at the wave-function of a single particle, its properties are distributed in space. Yet when you measure it, the particle is suddenly in one particular place.
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