In quantum mechanics, Bra-ket notation is a standard notation for describing quantum states, composed of angle brackets and vertical bars. It can also be used to denote abstract vectors and linear functionals in mathematics. It is so called because the inner product (or dot product) of two states is denoted by a ⟨bra|c|ket⟩;
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consisting of a left part, called the bra ( /brɑː/), and a right part, called the ket ( /kɛt/). The notation was introduced in 1939 by Paul Dirac and is also known as Dirac notation, though the notation has precursors in Grassmann's use of the notation for his inner products nearly 100 years previously.
Bra-ket notation is widespread in quantum mechanics: almost every phenomenon that is explained using quantum mechanics—including a large portion of modern physics — is usually explained with the help of bra-ket notation. The expression is typically interpreted as the probability amplitude for the state ψ to collapse into the state ϕ.
Read more about Bra-ket Notation: Usage in Quantum Mechanics, Properties, Composite Bras and Kets, The Unit Operator, Notation Used By Mathematicians