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Dirac Notation

Dirac Notation. Each state is denoted by a ket |>. Individual kets are distinguished by the labels placed inside the ket symbol |A>, |B>, etc. States can be added to yield a new state  Superposition To describe STATES, we use vectors. VECTORS represent STATES

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Dirac Notation

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  1. Dirac Notation Each state is denoted by a ket |>. Individual kets are distinguished by the labels placed inside the ket symbol |A>, |B>, etc States can be added to yield a new state  Superposition To describe STATES, we use vectors. VECTORS represent STATES Each vector can have finite or infinite number of elements A vector has direction and length, and so do the kets A state of a dynamical system = direction of ket Length and sign are irrelevant

  2. kets Complex number If a state is the superposition of 2 states, then the corresponding ket is the linear combination of 2 other kets

  3. Cont. CM: addition of 2 identical statesnew state QM: addition of 2 identical states same state CM: state can have 0 amplitude (no motion) QM: |ket> CANNOT have 0 amplitude, STATE  direction of vector , and if there is a vector, there is a length.

  4. bras To each ket |A>, there corresponds a dual or adjoint quantity called by Dirac a bra; it is not a ket-- rather it exists in a totally different space

  5. Cont.

  6. Length and phase The direction of the vector defines the dynamical state, and the length is not important  We can always use normalized vectors

  7. Operators An operator is a rule that transforms a ket (or bra) in another ket (or bra) Every observable is associated with an operator Notation: I use ^ (as most other authors do), Fayer’s book uses underline

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