Hey guys!
I'm Jon and WELCOME, to Respect Your Intellect!
We've already covered the original Double-Slit experiment as well as two of the main variations,
the "Which-Way" and the "Delayed Choice" variations.
Today, we'll be covering the Delayed Choice Quantum Eraser experiment that builds on top
of the previous knowledge.
Don't forget to hit the like and subscribe buttons and lllet's get started!
[Intro]
In order for this video to make any sense, you should definitely check out my previous
video on the original Double-Slit experiment and the "Which-Way" and "Delayed Choice" variations.
For the rest of this video, I'll assume that you've already watched it and can follow along.
So as you should already know at this point, every detection that we try to make about
which path the particle took results in the destruction of the interference pattern.
At this point, we also know that it doesn't matter if the detection is made before the
slits or after the slits, it still ends up destroying the interference pattern, as we
saw in the Delayed Choice variation.
One thing that we need to talk about though is that the observation of a particle is never
just an observer "looking" at the particle from a distance.
What's actually required is a device that can detect it and it always ends up acting
like an interaction with the real world; in other words, outside of the quantum system.
With the light source being coherent, all particles have the same coherent waveforms
as they travel.
When they hit detectors, it messes with this coherence and affects the particle or wave.
So the real challenge is in performing a measurement without collapsing the wave function.
Since physicists are extremely resourceful, they came up with a way of doing exactly that.
What they did was to use a very specific crystal that absorbs incoming photons and converts
them into two new photons that each have half of the energy of the original photon.
This pair is also quantum-entangled which is another matter entirely that would require
its own video to explain.
For now, the only thing that's important to know is that particles that are entangled
are able to pass information instantaneously across any distance; and yes, this means it
transfers that information faster than the speed of light.
So to get back to the experiment, when the pair is created by the crystal, the goal is
to then send one photon at a time very normally to the end screen to see the resulting pattern,
which is the "signal" photon, while we perform measurements on its twin, which is the "idler"
photon.
This should avoid the problem of messing with the coherence of the signal photon because
we "never" interact with it.
So let's take a look at a very simplified example of the elements we'll see so that
we can familiarize ourselves with how the actual experiment will look like.
We'll start with a light source here on the left that will shoot photons towards the right.
At this point, it will encounter a beam splitter that will let half of the photons through
while reflecting the other half, essentially separating the photons into two different
paths.
Next, we'll reflect them once more with a simple mirror so that they can eventually
intersect and that's going to be the portion we'll examine.
So at this point, we can easily conclude that if a photon is moving right, it will have
been reflected at the beam splitter and taken the red path.
Similarly, we can also easily conclude that a photon moving up will have taken the blue
path and will have went through the beam splitter.
So at this point, we have the "which-way" information because we can easily conclude
which path the photons took.
The quantum eraser part of this experiment happens when we scramble or obscure this information,
of which path the photons took, by using another beam splitter at that intersection.
Now, we can no longer conclude which path the photons took because they could all either
be reflected or have went through the last beam splitter; essentially "erasing" the which-way
information.
So now that you have a grasp of how we can erase the information of which path the photons
took, let's take a look at how the actual quantum eraser experiment is setup.
To begin with, the light source shoots out photons in the same way as the original double-slit
experiment and it goes through the slits also in the same way as the original.
Where this new variation starts is at the crystal on the other side of the slits.
This crystal is the one that will convert photons into an entangled pair and since it's
after the slits, it makes it a delayed choice variation; hence the full name, Delayed Choice
Quantum Eraser Experiment.
After the crystal is a prism that causes the entangled photons to diverge from each other,
essentially splitting them so that we can treat them as completely separate entities.
So now that we have the photons separated into an entangled pair and diverged from each
other, we can put the first detector which will act as the end screen that we're familiar
with; we'll call it detector "S" and it will catch the photons that get diverged upwards,
regardless of which slit it went through.
This is the detector that we'll be using to see whether we're destroying the interference
pattern or not by performing observations on the idler twin.
The only thing we use on the way to the end screen is a lens that will re-target the photons
to the detector that catches the signal photons so that at the detector, they land in the
same place.
Now for the photons that are diverged downwards, we'll use another prism to further diverge
the photons into two non-parallel paths.
At this point, we can put mirrors that will reflect the photons into detectors A and B
and we'll talk about what happens when the experiment is setup this way.
It's also important to note that the A and B detectors are setup along a longer path
so the signal twin will always hit detector S well before the idler twin hits detectors
A or B.
As you can see at this point, when a photon hits detector A, we can easily conclude that
it went through slit A since it's the only possible path.
The same thing with detector B that can only trigger when the photon went through slit
B. Even though we can tell where the "idler" photon came from, we still never interacted
with their signal twin and it reached detector S "before" we performed an observation.
So the coherence that we already talked about should have been preserved and there shouldn't
be any reason that it would stop acting like a wave, which would give us an interference
pattern.
So at this point, I'll encourage you to pause the video and think about what you think you
might see on detector S when we perform observations with detectors A and B. Will you see an interference
pattern by just having observed the idler twin?
Or not?
If you guessed that the interference pattern would be destroyed, then you are correct!
So the signal photon reached detector S and landed somewhere according to its wave function,
which should yield an interference pattern.
Later, the idler photon reaches detector A or B and somehow retroactively influences
the previous landing position of its entangled partner.
This is already awesome and you might already be impressed but wait, there's even more to
this experiment.
So now, instead of the mirrors that reflect the beams to detectors A and B, we'll use
beam splitters to reflect half the photons to detectors A and B and let the other half
through to detectors C and D. These new detectors are the quantum eraser and their job is to
destroy information about the path the photons took; essentially erasing the "which-way"
part of this experiment and as we also saw in the simpler example.
So the only thing that we'll add here for the photons that are let through are two mirrors
that will reflect them back to another beam splitter.
This makes it so that when photons reach detectors C and D, we can no longer know which slit
the photons originally came through because now they could have come from either slit
and ended up in either the C or D detectors.
So when we were only using detectors A and B, we ended up destroying the interference
pattern.
What do you think will happen now if we only use detectors C and D that no longer have
the which-way information about the photon's path?
Will we see and interference pattern emerge or will it still be destroyed, like before?
Feel free to pause the video here and think about it.
If you guessed that the interference pattern would be restored, then you are correct!
So simply by obscuring the information about which path the photon took, it's able to go
back to behaving like a wave and go through both slits at the same time, restoring the
interference pattern.
It's as if the universe says, alright, the observer lost the information about which
slit we went through so it's safe to have gone through both of them again.
It's really as if every single time we have the which-way information about the path of
a photon, the universe doesn't allow it to have behaved like a wave at all.
It's also clearly able to retroactively change the result of its entangled partner even if
the measurement is performed "after" the signal photon should already have ended its journey
and landed in one spot, as a wave.
This gives birth to a very interesting discussion about cause and effect and how we perceive
it.
So let's recap what happened in this experiment.
First, we produce the interference pattern like in the original double-slit experiment
by sending out single photons.
Second, we add a "which-way" detection that marks through which slit the photon passed
through and demonstrate that the interference pattern is destroyed.
In other words, the existence of the "which-way" information causes the destruction of the
interference pattern.
And third, we add another step that erases the "which-way" information and demonstrate
that the interference pattern is restored due to no longer having the "which-way" information
of which slit the photon went through.
What we can take away from this experiment is that it doesn't matter whether the information
is erased before or after the signal photon already hit the detection screen.
As long as the "which-way" information exists somewhere, the interference pattern is destroyed.
This also includes retroactively rewriting the past if the detection of the "which-way"
information happens after the signal photon already hit the detection screen.
It also doesn't matter how far apart they are or how long an observation is made after
one landed.
We could have one entangled photon land somewhere while an observation is made on the other
billions of years later on the other side of the universe and the first photon will
still have been collapsed.
This is absolutely amazing and part of why science is so stimulating!
On top of raising questions about causality, or even retrocausality, this also brings up
other questions about the interpretations of quantum mechanics.
In total, there are more than a dozen different ways of interpreting how the quantum world
works and none of them have been proven false as of yet.
They each differ fundamentally on questions such as if quantum mechanics is deterministic
or random, which elements are considered "real", and what is the nature of measurement, among
others.
The solution to why we get these results from the delayed choice quantum eraser experiment
may lie in the fact that the photons are entangled.
Since entangled particles can affect each other instantaneously across any distance,
even faster than light, it could explain why the idler photon is able to change the behavior
of the signal photon.
There's also the fact that photons don't experience time while traveling at the speed of light
so that might also be part of why the results are the way they are.
Perhaps future variations of this experiment will help us establish which interpretation
of quantum mechanics is correct, or which we need to review or change to better reflect
reality.
So what do you think about this variation of the double-slit experiment?
Does it make you appreciate science any more than you already did?
Let me know in the comments because I'd love to hear what you guys think about this one!
By the way, my Discord server is also currently open to the public and I'd like to grow the
chat into a nice, active, community of science lovers so if you're interested, check the
description for the invite link because I'd love to have more direct contact with you
guys.
We're about 40 people right now and growing slowly so come join us!
If you have any suggestions about science topics or recent science news that you'd like
me to cover, don't hesitate to write it down in the comments!
Also, if you have any science questions for the series where I answer your science questions,
ask me that too in the comments.
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