Quantum Computing: Implications of The Double Slit and Quantum Eraser Experiments.

Preface: 

Before I start talking about the Double Slit and Quantum Eraser Experiments I'd like to preface this post by saying that this may make more sense after watching some videos on the Double Slit Experiment or reading my previous article. I'll embed two videos from PBS Spacetime that I think do a really good job of explaining these topics(A lot of what I'm explaining here comes from these videos). There's also a lot of other good videos on the topic by Youtubers like Vertaisum.

Implications of The Double Slit Experiment:

In my previous article about the Double Slit experiment it seemed as though the Double Slit Experiment confirmed that light acts as a wave. However, it has also been proven that light can also be represented by indivisibly small packets of electromagnetic waves called photons. The word 'indivisibly' is important here. It means that these photons cannot be split in half and enter through both slits and recombine afterwards. Yet despite the fact that these photons must enter a singular slit, we still get an interference pattern conducting this experiment(even if you fire them one at a time!).

But hold on, if you fire individual photons, how are they interfering with each other? Each photon somehow hits the screen behind the two slits as though they know their most probable landing spots. This also holds true for other particles like electrons(and even whole atoms and molecules in some cases). This suggests that rather than the physical particles(photons, electrons, etc.) interacting with each other as a wave, the probabilities of their future and current positions are interacting with each other as some sort of wave. A wave of uncertain possible positions that at some point, for some reason, collapse into a singular position. This wave-like nature of position also applies to other properties of objects in quantum states, like the spin of an electron. The mathematical description of this wave-like distribution of properties is called a wave function. 

(A visualization of a wave-function containing all possible positions for a molecule called a Bucky Ball. Images from PBS Space Time on YouTube.) 

The strangest part about all of this is that we really don't know what this probability wave is made of-- though there are various interpretations of what it could be. For example, the Copenhagen interpretation states that the wave function doesn't have a physical nature and is simply made of pure possibility. It also states that, prior to measurement, it's meaningless to define a particle's properties.  

The Quantum Eraser Experiment:

When conducting a Double Slit experiment, measurement of which slits the individual particles pass through collapse the wave-function. The Quantum Eraser Experiment is an attempt to try and measure which slits the particles pass through after the particles have already hit an interference screen(where we get to see the interference pattern). 

(A diagram of the Quantum Eraser Experiment. Images from PBS Space Time on YouTube.)

The experiment makes use of a special crystal(seen in front of the slit screen) that absorbs an incoming photon and creates two new identical photons that have half the energy of the original. One of these halves goes toward the interference screen while the other heads toward detectors(A and B) that correspond to the slit they passed through. Despite the fact that photons heading towards the interference screen reach their destination before their other halves, if detectors A or B light up, the interference pattern never appears and you are left with a clump of dots on the screen. It's as though the measurement retroactively affects the pattern shown on the screen.

You may have noticed that there are also detectors C and D. Those detectors and the clump of mirrors near them are called the Quantum Eraser; their job is to erase any information we had about which slits the photons passed through. The two mirrors next to detectors A and B and the left-most mirror near C and D are special mirrors called beam splitters. They allow 50 percent of photons to pass and reflect the other 50 percent. This arrangement means that if C or D light up, we have no idea which slits the photons went through. If we look only at the photons on the interference screen whose pairs went to detectors C and D, we see an interference pattern again.

It's almost as though the universe goes back in time to change the interference pattern after it figures out whether or not we made a measurement. This is pretty odd to think about.

This experiment also presents an example of entanglement: an important concept in Quantum Physics. When particles are entangled, knowing a property of one particle instantly tells us the property of it's partner, no matter the distance! For example, if a pair of particles are created in such a way that their total spin is 0 and if one particle's spin is clockwise around an axis, then the spin of the other particle on the same axis will be counterclockwise. The photon pairs created by special crystal in this experiment are entangled particles. 

With that being said, I've reached the limits of my knowledge when it comes to the Quantum Eraser Experiment. Hopefully you found the topic as strange and interesting as I did. I'll continue to post more interesting concepts in Quantum Physics(like how this relates to Quantum Computing) as I learn more.