Dirac notation, also called bra-ket notation, is a notation invented by Dirac which is very useful in quantum mechanics. The notation defines the "ket"
vector, denoted 
,
and its conjugate transpose, called the "bra" vector and denoted 
. The "bracket" is then defined by 
.
Dirac notation satisfies the identities
 |
 |
where 
is the complex conjugate.
