U2L1h.txt

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So I wanted to just say that superdense coding now--
and I'm not going to do the circuit
diagram for superdense coding.
What I will do is the basic idea.
Alice and Bob-- oh, wait, that's small chalk.
Let's use nice big chalk.
Alice and Bob share 0, 0 plus 1, 1.

And let's say Alice takes 0, 0, and she does her--
OK, 0, 1; 1, 0, and 1, 1.
Identity 0, 1, she can apply sigma x.
1, 0, she can apply sigma z.
And 1, 1, she can apply sigma z sigma x.

What happens to the state if Alice applies that
to the first qubit?
Well, here she gets--
well, she applies the identity, she gets 1 of root 2, 0, 0
plus 1, 1.
If she applies sigma x, she gets 1 of root 2, 1 0 plus 0, 1.
If she applies sigma z, she gets 1 over 2, 0, 0 minus 1, 1.
And if she apply sigma x, first, and sigma z second,
she gets 1 over root 2, minus 1, 0 plus 0, 1.

So after she does--
so she encodes the message by applying one of these four
operators to the first qubit--
to her qubit.
She then sends her qubit to Bob.
Bob has all four of these states.
And he can do a measurement and tell which of these states
he has, because these are all--

yeah, these are all four distinguishable states.
So there is a measurement, which will--
whose outcome will tell you which state we'll have.

So that's superdense coding.

Alice sends her qubit to Bob.

B, et cetera.