\Ket{\psi}The ket notation \Ket{\psi} is used in quantum mechanics to denote a vector in a complex Hilbert space, often representing the state of a quantum system. It is part of the bra-ket notation introduced by Paul Dirac.
Quantum state as a superposition of basis states
\Ket{\psi} = \alpha \Ket{0} + \beta \Ket{1}Eigenvalue equation in quantum mechanics, where H is the Hamiltonian operator
H \Ket{\psi} = E \Ket{\psi}Transformation of a quantum state by a unitary operator U
\Ket{\phi} = U \Ket{\psi}