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U2L1a.txt
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#
# File: content-mit-8370x-subtitles/U2L1a.txt
#
# Captions for course module
#
# This file has 61 caption lines.
#
# Do not add or delete any lines. If there is text missing at the end, please add it to the last line.
#
#----------------------------------------
So today, I will be talking about two simple quantum
protocols.
So this is the first lecture in the protocols section,
which is, I guess, the second in this section of the course.
And today, we're talking about teleportation and superdense
coding.
And I will try to go through these fairly slowly,
because, you know, the material I'm
presenting it's incredibly easy to make a sign mistake or--
and I have a tendency to do that.
And I will try very hard not to.
So first I want to tell a little bit
about the history of teleportation and superdense
coding.
And this is more or less irrelevant to the later part.
And it's just an interesting historical--
you know, historical anecdote about how they were discovered.
So Asher Peres and Bill Wootters--
one and two--
Peres, Wootters-- were considering
the following scenario.
You had these two qubits that were either in this state,
or both in this state, or both in the state, where these three
states are 120 degrees apart.
And they wanted to--
they were asking how well can you--
how well can you predict or test which of these three states
they're in if you have these three things, qubits.
They're not completely distinguishable.
And if they're both in the same lab, the answer is very easy.
But if they're in two different labs,
the best that Peres and Wootters could do
was come up with a very complicated protocol, which
didn't get very close to the answer if they were in one lab.
So more distinguishable in one lab than two.
So then we started wondering about how could you
better distinguish them.
So here we have Alice with one of our qubits
and Bob with the other.
And you know, the labs are far apart.
So they're not allowed to bring these two qubits together.
So what additional resources could they
use to distinguish them?
And they discovered that an EPR pair 0, 0 minus 1 plus 1, 1--
actually it doesn't matter which--
it doesn't matter whether this is 0, 1 minus 1, 0 or 0,
0 plus 1, 1.
An EPR pair helps.
And then they started figuring out why an EPR here helps.
And an EPR pair helps, because it lets you teleport
Alice's qubit to Bob's lab.
And then Bob can perform the measurement
with both particles in the same lab.
So that was a much more surprising discovery
than just the fact that an EPR helps distinguish, you know,
this probability distribution of correlated qubits in two
different labs.