Hey guys!
I'm Jon and WELCOME, to Respect Your Intellect!
Quantum Entanglement is perhaps the most mysterious of the quantum effects and in this video,
we'll talk about what it is, how it works, and what we know and don't know about it.
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[Intro]
If you've been following some of my earlier videos that brush on the topic of quantum
effects, you should already be aware of the wave-particle duality of particles in the
quantum world; where particles travel like waves and collapse to particles when they
interact with the outside world.
If you're not sure how this all works yet, I recommend you check out my Double Slit Experiment
playlist where I have a video on the Double Slit Experiment and the Quantum Eraser Experiment
that show how particles behave.
To start off, let's talk about what the difference is between simple particles and quantum entangled
particles.
Simple particles, as you should already be aware, all travel as waves; just like waves
in a pond.
This wave contains all the probabilities of all the possible locations that this particle
could be found on the wave.
It's when we make a measurement to observe the particle that the wave will collapse and
the particle will choose a single location and path.
To put it simply, particles never resolve their definite locations unless it's made
absolutely necessary by an interaction.
Entangled particles are essentially two simple particles that spatially overlapped while
they were travelling as waves and somehow become connected to each other.
Since their waves affected each other during the overlap, they can no longer be described
independently of each other and therefore become entangled.
At this point, Quantum Mechanics requires us to use a combined wave function that encompasses
all the states of both particles at the same time in order to accurately describe the entangled
system.
Once particles are entangled, at any moment and regardless of the distance between them,
any interaction of one of the entangled particles will instantly collapse the other.
This happens faster than the speed of light and even back in time, as was shown in my
Quantum Eraser video.
So if we entangle particles and make sure not to measure them or interact with them
in any way, to maintain their uncertainty and superposition of all their states, we
can then separate them however we like and only later collapse them.
Another thing that's important to know is that there is a clear correlation between
the particles.
When we generate an entangled pair in such a way that the total spin is known to be zero,
then a measurement that yields a clockwise spin on one particle guarantees that the other
is spinning counter-clockwise.
To this date, quantum entanglement and this correlation has been demonstrated literally
countless times with photons, electrons, molecules, and even small diamonds.
There's a principle in Classical Physics that states that the universe keeps existing independently
of the mind of the observer.
In other words, the universe continues to exist and function regardless if we're looking
at it or not.
This is called Realism and Einstein was a big believer in this idea.
The other idea that Einstein really believed in was Locality.
This is the idea that things in the universe can only affect their direct neighbours and
propagate outwards that way at a speed limited to the speed of light.
However, even if this is something that we often take for granted, Quantum Mechanics
makes us question whether we should reject even this basic premise.
Quantum Entanglement demonstrates violations of locality because it's an effect that happens
faster than light and also even violates causality because the effect can precede the cause and
seemingly rewrite the past.
One good example of this is when we use gravitational lensing to look at distant galaxies that sent
out light billions of years ago.
Gravitational lensing happens when the photons that are on a straight path toward us are
bent around a massive object between us and the object we're looking at.
This makes the photons bend before they resume their straight path toward us.
If we use a single telescope to look at this, it's not known which path the photon took
around the massive object and we get an interference pattern; showing that the photons travelled
as waves.
However, when an astronomer on Earth today uses two telescopes to determine exactly which
path the photons took, the interference pattern is destroyed; showing that the particles did
not travel as waves.
So how we choose to act today influences particles that left billions of years ago; showing that
our present decisions can affect the past and that time plays no role in the results
of this experiment.
Neils Bohr and Albert Einstein had a series of public disputes about quantum mechanics
which are remembered because of their incredible importance to the philosophy of science.
These disputes called attention to an element of quantum theory called quantum non-locality
and it's central to our modern understanding of the physical world.
This quantum non-locality contradicts the assumptions of local realism that's found
in classical physics and that we're all already naturally familiar with.
So for the disputes, on the side of local realism was Einstein and on the side of quantum
non-locality was Bohr.
Einstein called quantum entanglement "spooky action at a distance" and was defending local
realism by using a theory that states that everything needed to be determined at the
moment of creation since information can't travel between the particles faster than the
speed of light.
This is the Hidden Variables Theory and it simply states that there are hidden variables
in quantum mechanics that are encoded at the moment of creation that we're simply not aware
of.
An example of this is having 2 boxes with gloves in it.
One box would have the left glove and the other would have the right glove.
Even if you don't know which one contains which, it wouldn't matter how much distance
there is between the boxes because as soon as you would open one, you'd also know what
the other box contains.
Originally, Einstein was not the only one to think that this Hidden Variables Theory
was the only way out of this paradox of faster-than-light communication between the particles; there
were many scientists that believed the same thing.
Bohr, on the other hand, was simply saying that it's futile to try and apply realism
to unobserved parts of the universe and that without interactions, everything was existing
as a fuzzy mixture of all their possible states simultaneously.
In order to settle the dispute of whether there were hidden variables encoded at the
moment of creation, we needed to test the predictions made by both Einstein and Bohr.
There's a theorem called Bell's Theorem that was devised specifically to determine whether
or not entangled particles communicate information faster than the speed of light.
Bell came up with a way of testing this by creating Bell's Inequalities that should be
satisfied if the local hidden variables theory was correct; but many different experiments
show that it's "not" satisfied.
We do this by using our measuring devices that affect the particles at the time of measurement.
What happens is that whenever we measure the spin vertically, we get an up or down result
and whenever we measure the spin horizontally, we get a left or right result.
Any other orientation in between is a fraction of probability that depends on the angle between
the vertical and horizontal positions.
I won't go into too much mathematical details here but what we would expect to see if hidden
variables were true is that we'd get a correlation between the particles at the very least more
than 55% of the time.
What we actually see is that the correlation is exactly 50% of the time so the hidden variables
theory can't be true.
The theorem goes on to state that there is no theory of local hidden variables that can
account for "all" the predictions of quantum mechanics.
So the Hidden Variables Theory doesn't work and the idea of particles having local hidden
variables is shown to be false.
In order to explain what's happening with entanglement, the property that usually has
to take the fall is the idea of locality that Einstein was defending; this idea that no
physical effect can travel faster than light.
Because of these results, the consensus today is that Bohr was victorious in this debate
and the spin information is "not" encoded in the particle at the moment of creation.
Now as to "how" that information is communicated to the other particle of the entangled pair
is still the mystery we're trying to figure out today.
Different interpretations are attempting to explain this in different ways but we don't
have a clear answer right now.
Since the measurement always yields a 50% possibility of measuring the spin in one direction
or the other, it can't really be used to transmit information faster than light.
For example, if Alice has a particle and Bob has the other particle of the entangled pair,
there is nothing that Alice can do that could send a definite and chosen value of say 0
or 1 to Bob without breaking the entanglement.
Anything you do to force a 0 or a 1 will break the entanglement and Bob's value will have
no correlation with Alice.
And without forcing a value, you're always sending random data that Bob can't make sense
of.
It's only when you can physically compare notes that you realize that you have exact
opposite values and this physical comparison is still within the constraints of the speed
of light.
So as it stands today, we would still, at the very least, be limited to the speed of
light to transfer the information about particles and compare notes to realize the correlation
of opposite values on both sides.
Another thing that's worth mentioning at this point is that we can't really guarantee that
the transmission of the spin direction from one entangled partner to the other is instantaneous.
It certainly "looks" instantaneous as far as we can tell up to now but all we can say
with absolute certainty is that it's at least 10,000 times faster than the speed of light.
As we continue to test this over greater and greater distances, we might become more and
more certain that the transmission is essentially instantaneous.
So is entanglement something we find in nature or is it something we have to always make
in a lab?
Well in nature, we do find entanglement in many different places.
One example is the electrons of every atom that has more than one electron in their shell;
due to their proximity to each other.
It's also suggested that photosynthesis, which is the chemical reaction that transforms light
into energy for plants, involves entanglement for the transfer between the light-harvesting
complexes and the photosynthetic reaction centers, where the kinetic energy is harvested
in the form of chemical energy.
Without such a process involving entanglement, we can't explain the efficient conversion
of optical energy into chemical energy.
There were experiments performed with femtosecond spectroscopy, which is an area of chemistry
that studies chemical reactions on extremely short timescales, like atoms in molecules
re-arranging themselves, that were able to show results that support this theory of photosynthesis
using entanglement.
As for methods for creating a pair of entangled particles, it's not always the easiest thing
to do but we "can" do it.
With photons, we can create an entangled pair by using a crystal that splits photons into
two identical photons of half the energy of the original.
This method is called Spontaneous Parametric Down-Conversion, or SPDC for short.
The best example of this being used is in the Quantum Eraser experiment; of which I
have a video that you can check out.
We can also create entanglement for other particles that never directly interacted by
using entanglement swapping, which is actually a form of Quantum Teleportation that we'll
cover soon in another video.
Another thing we can do is to entangle independently-prepared particles by spatially overlapping their wave
functions at least partially.
Once they overlap, they affect each other's possible states and become entangled because
they can no longer be described as independent particles.
Alright but now, how can we use entanglement to our advantage?
Well, even though pretty much everyone agrees that entanglement can't be used to transmit
information faster than the speed of light, we're not getting discouraged and entanglement
in communication and computing are very active areas of research today.
In 2014, we took the first pictures of objects using photons that never interacted with the
subject but were instead entangled with other particles that did interact with the subject.
This new Quantum Imaging technique could prove useful where low-light imaging is important,
like biological or medical imaging.
Also, in 2016, many companies successfully created quantum computers and started allowing
its users to openly experiment with concepts of quantum mechanics, including quantum entanglement.
So with all this research happening and all these possibilities at our disposal, there's
good reason to be hopeful for very significant advancements in the future.
It may be that we never find a way to use entanglement to transfer information faster
than light but it's also impossible to tell what discoveries might happen in the future
that might put this back on the table as a possibility.
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