ket1 ^^ ket2
raise the value of ket1 to the power of the value of ket2
we do our best to preserve categories
if one ket has a category, and the other doesn't then the result has that category
if ket1 and ket2 have different categories then return the empty ket |>
if ket1 and ket2 are 2D, then apply power component-wise