[6.3] Fixed operation of annihilation operator

[dryicecube]

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Changes made

$\sigma ^ + \ket{1}=\ket{0}$ was incorrect.
Changed the operation of and creation operator in section 6.

Justification

The $\sigma^+$ is a creation operator, but for a two-level system, $\sigma ^ + \ket{1}=\text{NULL}$. The actual operation is $\sigma ^ - \ket{1}=\ket{0}$ i.e. annihilation operator takes the system from $\ket{1}$ to $\ket{0}$

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[nbronn]

Thanks for fixing the error... LGTM!

[frankharkins]

Thank you!