Outer Product

operates on two vectors and produces a matrix
notation is `\ket{x}\bra{y}`
- `\ket{x}` is a ket
- `\bra{y}` is a bra
matrix-multiplication, associates with inner-product
a sum `\ket{x}\bra{0} + \ket{y}\bra{1}` where `\ket{x}` and `\ket{y}` are orthonormal produces a unitary-operator sending `\ket{0}` to `\ket{x}` and `\ket{1}` to `\ket{y}`