Hey guys! Jade here. So teleportation is
such a fascinating topic but it hasn't
actually been done yet and we don't even
know if it's possible. Something that is
possible is quantum teleportation and
that's pretty cool as well. So today I'm
here with my friend Toby from Australia
and we're going to tell you all about it.
Jade is
currently in the US and i'm in
Australia, but I want to send her a
present. A cute and fluffy present.
There's just one problem, and that's that
cats hate water, and there's a lot of
water between here and Pittsburgh. A boat is a
no and a plane would be too uncomfortable. I
think I'll simply have to teleport
her. Classically all I would need to do
is record the exact structure of her. Send
that data to Jade and have her printed out
on the other side using a sophisticated
3d printer. Although, even if the resolution of
that printer was extremely high, the task
of measuring the exact structure of something
is not so simple when you consider the
quantum nature of reality. To be able to
record the exact structure of the cat I
would need to record the momentum and
position of every particle. However this is
impossible due to the Heisenberg
uncertainty principle which says that the
more I know about one of these
quantities the less I know about the
other. I'll never be able to gain enough
information to create an exactly blue print.
And even if I could the process of
measuring individual particles would
probably kill the cat before I could make
an exact copy.
There is however an alternative form of
teleportation and that is quantum
teleportation. Quantum teleportation
would not be able to send a cat but at
least Jade would get something from me. I'm
going to attempt to send the contents of
this box, box A, to her box, box B on the
other side of the world. The contents
of this box is quantum information and
that is something that can be
communicated over the phone or internet.
Quantum information tells us something
about the state of a quantum system
such as whether a particle is spin up
or spin down. Spin relates to the fact
that when you put particles through a
magnetic field they act like little
magnets and can get deflected up or down.
One way that objects can get magnetic
properties is by spinning like a planet
which is why we use the word spin,
although what is important is that it's
something about the particles that makes
them get deflected, some up some down,
when they put through a magnetic field. It's
just two distinct orientations.
So how does quantum teleportation work?
Well the information needs to be sent through
two entangled particles. When a particle
with spin zero decays into two particles
with spin half, the total spin after the
decay still needs to be zero. This is
because of the conservation of angular
momentum.
This means that the two decayed particles
need to have opposite spin. One spin up
and the other spin down. But remember
that quantum physics is a bit weird and
particles only choose a state when they're being
measured. Until then, they exist in a superposition of all
possible states. So say I go back to
Australia and Toby and I go and
collect these two decayed particles. She takes one to
Canberra and I take one to Pittsburgh, but
the whole time
neither of us have looked to see which one we
have. So both particles are in this weird
quantum state, with a 50/50 chance of
collapsing into spin up or spin down when measured. As
soon as one of us looks into our box, our
particle collapses into spin up or spin
down. But the crazy thing is at the exact
same time the other particle on the other
side of the world collapses into the opposite
state. This happens instantaneously.
There was no time or means for one particle to
communicate to the other. These particles
are said to be entangled as they seem
to somehow magically know the state
of the other. Einstein called this "spooky
action at a distance" as it went against
everything he thought he knew about
physics. And turns out we use this spooky action
to do quantum teleportation. So quantum
teleportation should really be called
quantum communication because we're
not actually teleporting anything
physical. We're teleporting information
which is pretty much communication.
I mean think of a telegram. You're not actually
sending the physical message, you're
sending the information through two
telegraphs. This is how quantum
teleportation works but with two big
differences. Instead of sending
information between two telegraphs we're
sending information between two entangled
particles. Also we're sending quantum
information
which is a bit different from
classical information. For one quantum
information needs to be embedded in a
quantum particle and two, quantum
information can't be copied or destroyed.
Fun facts! A bit of quantum information
is called a qubit. If Toby in Camberra
wants to send me a qubit in Pittsburgh, it needs to be
embedded in the third particle, which we'll call
particle X. So now Toby has particle X
and the first part of the entangled pair
of particles ,which we'll call particle A.
She wants to send a qubit from particle
X to my entangled particle, which we'll call
particle B. She can do this by performing a
quantum operation on her particles which
due to the entanglement, makes the qubit from
particle X appear on particle B. But
because quantum information can't be
copied or destroyed the original qubit
from Toby's particle X is now transformed into
something else. Physicists were probably
tempted to call this teleportation because
now my particle B is effectively
Toby's particle X. Its kind of like when
you're faxing something. The piece of paper
on the receiving end isn't the same as
the one that was sent but all the
information is the same. The difference
between this and quantum teleportation
is the original information is
transformed into something else in the
process. But this isn't the whole picture.
Some of the quantum information that we need
from particle X to get to particle B can't be sent
through the two entangled particles. We
need a classical means of communication
like the internet or a phone. A lot of
people think that quantum teleportation
means that we can communicate faster
than the speed of light, but it's not true.
Remember that we still need this
classical channel to transport some of the
information. This is all beautifully
encapsulated in something called the no-go theorem.
It says that information can't be
communicated by entanglement
alone. You need the classical channel
so that relativity remain safe. So
faster-than-light communication is a
no-go.
So will we ever be able to teleport cats?
Well - Hey Jade I sent you a present,
go check your box!
Hey cool! Quantum information! Thanks for watching
guys, make sure to check out Toby's channel
Tibees. You can click here or in the
description below, and let me help you
this. If you could teleport anywhere in
the world right now where would it be?
Let me know in the comments below and
see you next time. Bye!