M1L3c_CQ1.txt

#
# File:   content-mit-8-421-1x-subtitles/M1L3c_CQ1.txt
#
# Captions for 8.421x module
#
# This file has 20 caption lines.
#
# Do not add or delete any lines.
#
#----------------------------------------

Let's now talk about features of, yes,
the most important spin for us, namely the two-level system,
which is spin 1/2.

The most genetic system is an electron in a magnetic field, B
times e z.
And why don't we start with a clicker question.
So the question is, what is the level structure of the electron
in the magnetic field?
It's a two-level system, and you have two options,
A and B. One option is, the upper state is spin up,
the ground state is spin down.
Or, the opposite.
So in other words, tell me whether an electron
is in the lower state when the spin is up
or when the spin is down.
You would say it's a stupid definition,
but we talk all the time about an electron is in the spin
down state, or the spin up state. Which is the lowest energy state of the electron?