Bra–ket notation

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In quantum mechanics, bra–ket notation is a standard notation for describing quantum states. It can also be used to denote abstract vectors and linear functionals in mathematics. The notation begins with using angle brackets, ⟨ and ⟩, and a vertical bar, |, to denote the scalar product of vectors or the action of a linear functional on a vector in a complex vector space. The scalar product or action is written as

${\displaystyle \langle \phi {\mid }\psi \rangle .}$

The right part is called the ket /kɛt/; it is a vector, typically represented as a column vector and written

${\displaystyle |\psi \rangle .}$

The left part is called the bra, /brɑː/; it is the Hermitian conjugate of the ket with the same label, typically represented as a row vector and is written

${\displaystyle \langle \phi |.}$

A combination of bras, kets, and operators is interpreted using matrix multiplication. A bra and a ket with the same label are Hermitian conjugates of each other.

Bra-ket notation was introduced in 1939 by Paul Dirac^[1][2] and is also known as the Dirac notation.

The bra-ket notation has a precursor in Hermann Grassmann's use of the notation ${\displaystyle [\phi {\mid }\psi ]}$ for his inner products nearly 100 years earlier.^[3]

Introduction

Bra–ket notation is a notation for linear algebra, particularly focused on vectors, inner products, linear operators, Hermitian conjugation, and the dual space, for both finite-dimensional and infinite-dimensional complex vector spaces. It is specifically designed to ease the types of calculations that frequently come up in quantum mechanics.

Its use in quantum mechanics is quite widespread. Many phenomena that are explained using quantum mechanics are usually explained using bra–ket notation.

In simple cases, a ket |m⟩ can be described as a column vector, a bra with the same label ⟨m| is its conjugate transpose (which is a row vector), and writing bras, kets, and linear operators next to each other implies matrix multiplication.^[4] However, kets may also exist in uncountably-infinite-dimensional vector spaces, such that they cannot be literally written as a column vector. Also, writing a column vector as a list of numbers requires picking a basis, whereas one can write "|m⟩" without committing to any particular basis. This is helpful because quantum mechanics calculations involve frequently switching between different bases (e.g. position basis, momentum basis, energy eigenbasis, etc.), so it is better to have the basis vectors (if any) written out explicitly. In some situations involving two important basis vectors they will be referred to simply as "|-⟩" and "|+⟩".

The standard mathematical notation for the inner product, preferred as well by some physicists, expresses exactly the same thing as the bra–ket notation,

${\displaystyle (\phi ,\psi )=\langle \phi {\mid }\psi \rangle ={\bigl (}\langle \phi |{\bigr )}\,{\bigl (}|\psi \rangle {\bigr )},}$

Bras and kets can also be configured in other ways, such as the outer product

${\displaystyle |\psi \rangle \langle \phi |}$

which can also be represented as a matrix multiplication (i.e., a column vector times a row vector equals a matrix).

If the ket is an element of a vector space, the bra is technically an element of its dual space—see Riesz representation theorem.

Vector spaces

Vectors vs kets

In mathematics, the term "vector" is used to refer generally to any element of any vector space. In physics, however, the term "vector" is much more specific: "Vector" refers almost exclusively to quantities like displacement or velocity, which have three components that relate directly to the three dimensions of the real world. Such vectors are typically denoted with over arrows (r→) or boldface (r).

In quantum mechanics, a quantum state is typically represented as an element of an abstract complex vector space—for example the infinite-dimensional vector space of all possible wavefunctions (functions mapping each point of 3D space to a complex number). Since the term "vector" is already used for something else (see previous paragraph), it is very common to refer to these elements of abstract complex vector spaces as "kets", and to write them using ket notation.

Ket notation

Ket notation, invented by Dirac, uses vertical bars and angular brackets: |A⟩. When this notation is used, these quantities are called "kets", and |A⟩ is read as "ket-A".^[5] These kets can be manipulated using the usual rules of linear algebra, for example:

${\displaystyle {\begin{aligned}|A\rangle &=|B\rangle +|C\rangle \\|C\rangle &=(-1+2i)|D\rangle \\|D\rangle &=\int _{-\infty }^{\infty }e^{-x^{2}}|x\rangle \,\mathrm {d} x\,.\end{aligned}}}$

Note how any symbols, letters, numbers, or even words—whatever serves as a convenient label—can be used as the label inside a ket. For example, the last line above involves infinitely many different kets, one for each real number x. In other words, the symbol "|A⟩" has a specific and universal mathematical meaning, while just the "A" by itself does not. For example, |1⟩ + |2⟩ might or might not be equal to |3⟩. Nevertheless, for convenience, there is usually some logical scheme behind the labels inside kets, such as the common practice of labeling energy eigenkets in quantum mechanics through a listing of their quantum numbers.

Inner products and bras

An inner product is a generalization of the dot product. The inner product of two vectors is a scalar. In neutral notation (notation dedicated to the inner product only), this might be written (A, B), where A and B are elements of the abstract vector space, i.e. both are kets.

Bra–ket notation uses a specific notation for inner products:

${\displaystyle (A,B)=\langle A|B\rangle ={\text{the inner product of ket }}|A\rangle {\text{ with ket }}|B\rangle }$

Bra–ket notation splits this inner product (also called a "bracket") into two pieces, the "bra" and the "ket":

${\displaystyle \langle A|B\rangle ={\bigl (}\langle A|{\bigr )}\,{\bigl (}|B\rangle {\bigr )}}$

where ⟨A| is called a bra, read as "bra-A", and |B⟩ is a ket as above.

The purpose of "splitting" the inner product into a bra and a ket is that both the bra ⟨A| and the ket |B⟩ are meaningful on their own, and can be used in other contexts besides within an inner product. There are two main ways to think about the meanings of separate bras and kets. Accordingly, the interpretation of the expression ⟨A|B⟩ has a second interpretation, namely that of the action of a linear functional per below.

Bras and kets as row and column vectors

For a finite-dimensional vector space, using a fixed orthonormal basis, the inner product can be written as a matrix multiplication of a row vector with a column vector:

${\displaystyle \langle A|B\rangle \doteq A_{1}^{*}B_{1}+A_{2}^{*}B_{2}+\cdots +A_{N}^{*}B_{N}={\begin{pmatrix}A_{1}^{*}&A_{2}^{*}&\cdots &A_{N}^{*}\end{pmatrix}}{\begin{pmatrix}B_{1}\\B_{2}\\\vdots \\B_{N}\end{pmatrix}}}$

Based on this, the bras and kets can be defined as:

${\displaystyle {\begin{aligned}\langle A|&\doteq {\begin{pmatrix}A_{1}^{*}&A_{2}^{*}&\cdots &A_{N}^{*}\end{pmatrix}}\\|B\rangle &\doteq {\begin{pmatrix}B_{1}\\B_{2}\\\vdots \\B_{N}\end{pmatrix}}\end{aligned}}}$

and then it is understood that a bra next to a ket implies matrix multiplication.

The conjugate transpose (also called Hermitian conjugate) of a bra is the corresponding ket and vice versa:

${\displaystyle \langle A|^{\dagger }=|A\rangle ,\quad |A\rangle ^{\dagger }=\langle A|}$

because if one starts with the bra

${\displaystyle {\begin{pmatrix}A_{1}^{*}&A_{2}^{*}&\cdots &A_{N}^{*}\end{pmatrix}}\,,}$

then performs a complex conjugation, and then a matrix transpose, one ends up with the ket

${\displaystyle {\begin{pmatrix}A_{1}\\A_{2}\\\vdots \\A_{N}\end{pmatrix}}}$

Bras as linear functionals

A more abstract definition, which is equivalent but more easily generalized to infinite-dimensional spaces, is to say that bras are linear functionals on the space of kets, i.e. linear transformations that input a ket and output a complex number. The bra linear functionals are defined to be consistent with the inner product. Thus, if ⟨B| is the linear functional corresponding to |B⟩ under the Riesz representation theorem, then

${\displaystyle \langle A|B\rangle =\langle B|{\bigl (}|A\rangle {\bigr )}\,,}$

i.e. it produces the same complex number as the inner product does. The terminology for the right hand side is though not inner product, which always involves two kets. Confusing this is harmless, since the same number is produced in the end.

In mathematics terminology, the vector space of bras is the dual space to the vector space of kets, and corresponding bras and kets are related by the Riesz representation theorem.

Non-normalizable states and non-Hilbert spaces

Bra–ket notation can be used even if the vector space is not a Hilbert space.

In quantum mechanics, it is common practice to write down kets which have infinite norm, i.e. non-normalizable wavefunctions. Examples include states whose wavefunctions are Dirac delta functions or infinite plane waves. These do not, technically, belong to the Hilbert space itself. However, the definition of "Hilbert space" can be broadened to accommodate these states (see the Gelfand–Naimark–Segal construction or rigged Hilbert spaces). The bra–ket notation continues to work in an analogous way in this broader context.

Banach spaces are a different generalization of Hilbert spaces. In a Banach space B, the vectors may be notated by kets and the continuous linear functionals by bras. Over any vector space without topology, we may also notate the vectors by kets and the linear functionals by bras. In these more general contexts, the bracket does not have the meaning of an inner product, because the Riesz representation theorem does not apply.

Usage in quantum mechanics

The mathematical structure of quantum mechanics is based in large part on linear algebra:

• Wave functions and other quantum states can be represented as vectors in a complex Hilbert space. (The exact structure of this Hilbert space depends on the situation.) In bra–ket notation, for example, an electron might be in the "state" |ψ⟩. (Technically, the quantum states are rays of vectors in the Hilbert space, as c|ψ⟩ corresponds to the same state for any nonzero complex number c.)
• Quantum superpositions can be described as vector sums of the constituent states. For example, an electron in the state |1⟩ + i |2⟩ is in a quantum superposition of the states |1⟩ and |2⟩.
• Measurements are associated with linear operators (called observables) on the Hilbert space of quantum states.
• Dynamics are also described by linear operators on the Hilbert space. For example, in the Schrödinger picture, there is a linear time evolution operator U with the property that if an electron is in state |ψ⟩ right now, at a later time it will be in the state U|ψ⟩, the same U for every possible |ψ⟩.
• Wave function normalization is scaling a wave function so that its norm is 1. Since virtually every calculation in quantum mechanics involves vectors and linear operators, it can involve, and often does involve, bra–ket notation. A few examples follow:

Spinless position–space wave function

Discrete components A_k of a complex vector |A⟩ = ∑_k A_k |e_k⟩, which belongs to a countably infinite-dimensional Hilbert space; there are countably infinitely many k values and basis vectors |e_k⟩.

Continuous components ψ(x) of a complex vector |ψ⟩ = ∫ dx ψ(x)|x⟩, which belongs to an uncountably infinite-dimensional Hilbert space; there are infinitely many x values and basis vectors |x⟩.

 

Components of complex vectors plotted against index number; discrete k and continuous x. Two particular components out of infinitely many are highlighted.

The Hilbert space of a spin-0 point particle is spanned by a "position basis" { |r⟩ }, where the label r extends over the set of all points in position space. This label is the eigenvalue of the position operator acting on such a basis state, ${\displaystyle {\hat {\mathbf {r} }}|\mathbf {r} \rangle =\mathbf {r} |\mathbf {r} \rangle }$. Since there are an uncountably infinite number of vector components in the basis, this is an uncountably infinite-dimensional Hilbert space. The dimensions of the Hilbert space (usually infinite) and position space (usually 1, 2 or 3) are not to be conflated.

Starting from any ket |Ψ⟩ in this Hilbert space, we can define a complex scalar function of r, known as a wavefunction:

${\displaystyle \Psi (\mathbf {r} )\ {\stackrel {\text{def}}{=}}\ \langle \mathbf {r} |\Psi \rangle \,.}$

On the left side, Ψ(r) is a function mapping any point in space to a complex number; on the right side, |Ψ⟩ = ∫ d³r Ψ(r) |r⟩ is a ket.

It is then customary to define linear operators acting on wavefunctions in terms of linear operators acting on kets, by

${\displaystyle A\Psi (\mathbf {r} )\ {\stackrel {\text{def}}{=}}\ \langle \mathbf {r} |A|\Psi \rangle \,.}$

For instance, the momentum operator p has the following form,

${\displaystyle \mathbf {p} \Psi (\mathbf {r} )\ {\stackrel {\text{def}}{=}}\ \langle \mathbf {r} |\mathbf {p} |\Psi \rangle =-i\hbar \nabla \Psi (\mathbf {r} )\,.}$

One occasionally encounters an expression such as

${\displaystyle \nabla |\Psi \rangle \,,}$

though this is something of an abuse of notation. The differential operator must be understood to be an abstract operator, acting on kets, that has the effect of differentiating wavefunctions once the expression is projected into the position basis,

${\displaystyle \nabla \langle \mathbf {r} |\Psi \rangle \,,}$

even though, in the momentum basis, the operator amounts to a mere multiplication operator (by iħp).

Overlap of states

In quantum mechanics the expression ⟨φ|ψ⟩ is typically interpreted as the probability amplitude for the state ψ to collapse into the state φ. Mathematically, this means the coefficient for the projection of ψ onto φ. It is also described as the projection of state ψ onto state φ.

Changing basis for a spin-1/2 particle

A stationary spin-1/2 particle has a two-dimensional Hilbert space. One orthonormal basis is:

${\displaystyle |{\uparrow }_{z}\rangle \,,\;|{\downarrow }_{z}\rangle }$

where |↑_z⟩ is the state with a definite value of the spin operator S_z equal to +1/2 and |↓_z⟩ is the state with a definite value of the spin operator S_z equal to −1/2.

Since these are a basis, any quantum state of the particle can be expressed as a linear combination (i.e., quantum superposition) of these two states:

${\displaystyle |\psi \rangle =a_{\psi }|{\uparrow }_{z}\rangle +b_{\psi }|{\downarrow }_{z}\rangle }$

where a_ψ and b_ψ are complex numbers.

A different basis for the same Hilbert space is:

${\displaystyle |{\uparrow }_{x}\rangle \,,\;|{\downarrow }_{x}\rangle }$

defined in terms of S_x rather than S_z.

Again, any state of the particle can be expressed as a linear combination of these two:

${\displaystyle |\psi \rangle =c_{\psi }|{\uparrow }_{x}\rangle +d_{\psi }|{\downarrow }_{x}\rangle }$

In vector form, you might write

${\displaystyle |\psi \rangle \doteq {\begin{pmatrix}a_{\psi }\\b_{\psi }\end{pmatrix}}\quad {\text{or}}\quad |\psi \rangle \doteq {\begin{pmatrix}c_{\psi }\\d_{\psi }\end{pmatrix}}}$

depending on which basis you are using. In other words, the "coordinates" of a vector depend on the basis used.

There is a mathematical relationship between a_ψ, b_ψ, c_ψ and d_ψ; see change of basis.

Misleading uses

There are a few conventions and abuses of notation that are generally accepted by the physics community, but which might confuse the non-initiated.

It is common to use the same symbol for labels and constants in the same equation. For example, α̂ |α⟩ = α |α⟩, where the symbol α is used simultaneously as the name of the operator α̂, its eigenvector |α⟩ and the associated eigenvalue α.

Something similar occurs in component notation of vectors. While Ψ (uppercase) is traditionally associated with wavefunctions, ψ (lowercase) may be used to denote a label, a wave function or complex constant in the same context, usually differentiated only by a subscript.

The main abuses are including operations inside the vector labels. This is done for a fast notation of scaling vectors. E.g. if the vector |α⟩ is scaled by √2, it might be denoted by |α/√2⟩, which makes no sense since α is a label, not a function or a number, so you can't perform operations on it.

This is especially common when denoting vectors as tensor products, where part of the labels are moved outside the designed slot, e.g. |α⟩ = |α/√2₁⟩ ⊗ |α/√2₂⟩. Here part of the labeling that should state that all three vectors are different was moved outside the kets, as subscripts 1 and 2. And a further abuse occurs, since α is meant to refer to the norm of the first vector—which is a label denoting a value.

Linear operators

Linear operators acting on kets

A linear operator is a map that inputs a ket and outputs a ket. (In order to be called "linear", it is required to have certain properties.) In other words, if A is a linear operator and |ψ⟩ is a ket, then A|ψ⟩ is another ket.

In an N-dimensional Hilbert space, |ψ⟩ can be written as an N × 1 column vector, and then A is an N × N matrix with complex entries. The ket A|ψ⟩ can be computed by normal matrix multiplication.

Linear operators are ubiquitous in the theory of quantum mechanics. For example, observable physical quantities are represented by self-adjoint operators, such as energy or momentum, whereas transformative processes are represented by unitary linear operators such as rotation or the progression of time.

Linear operators acting on bras

Operators can also be viewed as acting on bras from the right hand side. Specifically, if A is a linear operator and ⟨φ| is a bra, then ⟨φ|A is another bra defined by the rule

${\displaystyle {\bigl (}\langle \phi |{\boldsymbol {A}}{\bigr )}|\psi \rangle =\langle \phi |{\bigl (}{\boldsymbol {A}}|\psi \rangle {\bigr )}\,,}$

(in other words, a function composition). This expression is commonly written as (cf. energy inner product)

${\displaystyle \langle \phi |{\boldsymbol {A}}|\psi \rangle \,.}$

In an N-dimensional Hilbert space, ⟨φ| can be written as a 1 × N row vector, and A (as in the previous section) is an N × N matrix. Then the bra ⟨φ|A can be computed by normal matrix multiplication.

If the same state vector appears on both bra and ket side,

${\displaystyle \langle \psi |{\boldsymbol {A}}|\psi \rangle \,,}$

then this expression gives the expectation value, or mean or average value, of the observable represented by operator A for the physical system in the state |ψ⟩.

Outer products

A convenient way to define linear operators on a Hilbert space H is given by the outer product: if ⟨φ| is a bra and |ψ⟩ is a ket, the outer product

${\displaystyle |\phi \rangle \,\langle \psi |}$

denotes the rank-one operator with the rule

${\displaystyle {\bigl (}|\phi \rangle \langle \psi |{\bigr )}(x)=\langle \psi |x\rangle |\phi \rangle }$.

For a finite-dimensional vector space, the outer product can be understood as simple matrix multiplication:

${\displaystyle |\phi \rangle \,\langle \psi |\doteq {\begin{pmatrix}\phi _{1}\\\phi _{2}\\\vdots \\\phi _{N}\end{pmatrix}}{\begin{pmatrix}\psi _{1}^{*}&\psi _{2}^{*}&\cdots &\psi _{N}^{*}\end{pmatrix}}={\begin{pmatrix}\phi _{1}\psi _{1}^{*}&\phi _{1}\psi _{2}^{*}&\cdots &\phi _{1}\psi _{N}^{*}\\\phi _{2}\psi _{1}^{*}&\phi _{2}\psi _{2}^{*}&\cdots &\phi _{2}\psi _{N}^{*}\\\vdots &\vdots &\ddots &\vdots \\\phi _{N}\psi _{1}^{*}&\phi _{N}\psi _{2}^{*}&\cdots &\phi _{N}\psi _{N}^{*}\end{pmatrix}}}$

The outer product is an N × N matrix, as expected for a linear operator.

One of the uses of the outer product is to construct projection operators. Given a ket |ψ⟩ of norm 1, the orthogonal projection onto the subspace spanned by |ψ⟩ is

${\displaystyle |\psi \rangle \,\langle \psi |\,.}$

Hermitian conjugate operator

Just as kets and bras can be transformed into each other (making |ψ⟩ into ⟨ψ|), the element from the dual space corresponding to A|ψ⟩ is ⟨ψ|A^†, where A^† denotes the Hermitian conjugate (or adjoint) of the operator A. In other words,

${\displaystyle |\phi \rangle =A|\psi \rangle \quad {\text{if and only if}}\quad \langle \phi |=\langle \psi |A^{\dagger }\,.}$

If A is expressed as an N × N matrix, then A^† is its conjugate transpose.

Self-adjoint operators, where A = A^†, play an important role in quantum mechanics; for example, an observable is always described by a self-adjoint operator. If A is a self-adjoint operator, then ⟨ψ|A|ψ⟩ is always a real number (not complex). This implies that expectation values of observables are real.

Properties

Bra–ket notation was designed to facilitate the formal manipulation of linear-algebraic expressions. Some of the properties that allow this manipulation are listed herein. In what follows, c₁ and c₂ denote arbitrary complex numbers, c* denotes the complex conjugate of c, A and B denote arbitrary linear operators, and these properties are to hold for any choice of bras and kets.

Linearity

• Since bras are linear functionals,

${\displaystyle \langle \phi |{\bigl (}c_{1}|\psi _{1}\rangle +c_{2}|\psi _{2}\rangle {\bigr )}=c_{1}\langle \phi |\psi _{1}\rangle +c_{2}\langle \phi |\psi _{2}\rangle \,.}$

• By the definition of addition and scalar multiplication of linear functionals in the dual space,^[6]

${\displaystyle {\bigl (}c_{1}\langle \phi _{1}|+c_{2}\langle \phi _{2}|{\bigr )}|\psi \rangle =c_{1}\langle \phi _{1}|\psi \rangle +c_{2}\langle \phi _{2}|\psi \rangle \,.}$

Associativity

Given any expression involving complex numbers, bras, kets, inner products, outer products, and/or linear operators (but not addition), written in bra–ket notation, the parenthetical groupings do not matter (i.e., the associative property holds). For example:

${\displaystyle {\begin{aligned}\langle \psi |{\bigl (}A|\phi \rangle {\bigr )}={\bigl (}\langle \psi |A{\bigr )}|\phi \rangle \,&{\stackrel {\text{def}}{=}}\,\langle \psi |A|\phi \rangle \\{\bigl (}A|\psi \rangle {\bigr )}\langle \phi |=A{\bigl (}|\psi \rangle \langle \phi |{\bigr )}\,&{\stackrel {\text{def}}{=}}\,A|\psi \rangle \langle \phi |\end{aligned}}}$

and so forth. The expressions on the right (with no parentheses whatsoever) are allowed to be written unambiguously because of the equalities on the left. Note that the associative property does not hold for expressions that include nonlinear operators, such as the antilinear time reversal operator in physics.

Hermitian conjugation

Bra–ket notation makes it particularly easy to compute the Hermitian conjugate (also called dagger, and denoted †) of expressions. The formal rules are:

• The Hermitian conjugate of a bra is the corresponding ket, and vice versa.
• The Hermitian conjugate of a complex number is its complex conjugate.
• The Hermitian conjugate of the Hermitian conjugate of anything (linear operators, bras, kets, numbers) is itself—i.e.,

${\displaystyle \left(x^{\dagger }\right)^{\dagger }=x\,.}$

• Given any combination of complex numbers, bras, kets, inner products, outer products, and/or linear operators, written in bra–ket notation, its Hermitian conjugate can be computed by reversing the order of the components, and taking the Hermitian conjugate of each.

These rules are sufficient to formally write the Hermitian conjugate of any such expression; some examples are as follows:

• Kets:

${\displaystyle {\bigl (}c_{1}|\psi _{1}\rangle +c_{2}|\psi _{2}\rangle {\bigr )}^{\dagger }=c_{1}^{*}\langle \psi _{1}|+c_{2}^{*}\langle \psi _{2}|\,.}$

• Inner products:

${\displaystyle \langle \phi |\psi \rangle ^{*}=\langle \psi |\phi \rangle \,.}$

Note that ⟨φ|ψ⟩ is a scalar, so the Hermitian conjugate is just the complex conjugate, i.e.

${\displaystyle {\bigl (}\langle \phi |\psi \rangle {\bigr )}^{\dagger }=\langle \phi |\psi \rangle ^{*}}$

• Matrix elements:

${\displaystyle {\begin{aligned}\langle \phi |A|\psi \rangle ^{*}&=\left\langle \psi \left|A^{\dagger }\right|\phi \right\rangle \\\left\langle \phi \left|A^{\dagger }B^{\dagger }\right|\psi \right\rangle ^{*}&=\langle \psi |BA|\phi \rangle \,.\end{aligned}}}$

• Outer products:

${\displaystyle {\Big (}{\bigl (}c_{1}|\phi _{1}\rangle \langle \psi _{1}|{\bigr )}+{\bigl (}c_{2}|\phi _{2}\rangle \langle \psi _{2}|{\bigr )}{\Big )}^{\dagger }={\bigl (}c_{1}^{*}|\psi _{1}\rangle \langle \phi _{1}|{\bigr )}+{\bigl (}c_{2}^{*}|\psi _{2}\rangle \langle \phi _{2}|{\bigr )}\,.}$

Composite bras and kets

Two Hilbert spaces V and W may form a third space V ⊗ W by a tensor product. In quantum mechanics, this is used for describing composite systems. If a system is composed of two subsystems described in V and W respectively, then the Hilbert space of the entire system is the tensor product of the two spaces. (The exception to this is if the subsystems are actually identical particles. In that case, the situation is a little more complicated.)

If |ψ⟩ is a ket in V and |φ⟩ is a ket in W, the direct product of the two kets is a ket in V ⊗ W. This is written in various notations:

${\displaystyle |\psi \rangle |\phi \rangle \,,\quad |\psi \rangle \otimes |\phi \rangle \,,\quad |\psi \phi \rangle \,,\quad |\psi ,\phi \rangle \,.}$

The unit operator

Consider a complete orthonormal system (basis),

${\displaystyle \{e_{i}\ |\ i\in \mathbb {N} \}\,,}$

for a Hilbert space H, with respect to the norm from an inner product ⟨·,·⟩.

From basic functional analysis, it is known that any ket |ψ⟩ can also be written as

${\displaystyle |\psi \rangle =\sum _{i\in \mathbb {N} }\langle e_{i}|\psi \rangle |e_{i}\rangle ,}$

with ⟨·|·⟩ the inner product on the Hilbert space.

From the commutativity of kets with (complex) scalars, it follows that

${\displaystyle \sum _{i\in \mathbb {N} }|e_{i}\rangle \langle e_{i}|=1\!\!1}$

must be the identity operator, which sends each vector to itself.

This, then, can be inserted in any expression without affecting its value; for example

${\displaystyle {\begin{aligned}\langle v|w\rangle &=\left\langle v\left|\sum _{i\in \mathbb {N} }\right|e_{i}\right\rangle \langle e_{i}|w\rangle \\&=\left\langle v\left|\sum _{i\in \mathbb {N} }\right|e_{i}\right\rangle \left\langle e_{i}\left|\sum _{j\in \mathbb {N} }\right|e_{j}\right\rangle \langle e_{j}|w\rangle \\&=\langle v|e_{i}\rangle \langle e_{i}|e_{j}\rangle \langle e_{j}|w\rangle \,,\end{aligned}}}$

where, in the last identity, the Einstein summation convention has been used.

In quantum mechanics, it often occurs that little or no information about the inner product ⟨ψ|φ⟩ of two arbitrary (state) kets is present, while it is still possible to say something about the expansion coefficients ⟨ψ|e_i⟩ = ⟨e_i|ψ⟩* and ⟨e_i|φ⟩ of those vectors with respect to a specific (orthonormalized) basis. In this case, it is particularly useful to insert the unit operator into the bracket one time or more.

For more information, see Resolution of the identity,

•     1 = ∫ dx |x⟩⟨x| = ∫ dp |p⟩⟨p|, where |p⟩ = ∫ dx e^ixp/ħ|x⟩/√2πħ.

Since ⟨x′|x⟩ = δ(x − x′), plane waves follow, ⟨x|p⟩ = e^ixp/ħ/√2πħ.^[7]

Notation used by mathematicians

The object physicists are considering when using bra–ket notation is a Hilbert space (a complete inner product space).

Let H be a Hilbert space and h ∈ H a vector in H. What physicists would denote by |h⟩ is the vector itself. That is,

${\displaystyle |h\rangle \in {\mathcal {H}}}$.

Let H* be the dual space of H. This is the space of linear functionals on H. The isomorphism Φ : H → H* is defined by Φ(h) = φ_h, where for every g ∈ H we define

${\displaystyle \phi _{h}(g)={\mbox{IP}}(h,g)=(h,g)=\langle h,g\rangle =\langle h|g\rangle }$,

where IP(·,·), (·,·), ⟨·,·⟩ and ⟨·|·⟩ are just different notations for expressing an inner product between two elements in a Hilbert space (or for the first three, in any inner product space). Notational confusion arises when identifying φ_h and g with ⟨h| and |g⟩ respectively. This is because of literal symbolic substitutions. Let φ_h = H = ⟨h| and let g = G = |g⟩. This gives

${\displaystyle \phi _{h}(g)=H(g)=H(G)=\langle h|(G)=\langle h|{\bigl (}|g\rangle {\bigr )}\,.}$

One ignores the parentheses and removes the double bars. Some properties of this notation are convenient since we are dealing with linear operators and composition acts like a ring multiplication.

Moreover, mathematicians usually write the dual entity not at the first place, as the physicists do, but at the second one, and they usually use not an asterisk but an overline (which the physicists reserve for averages and the Dirac spinor adjoint) to denote complex conjugate numbers; i.e., for scalar products mathematicians usually write

${\displaystyle (\phi ,\psi )=\int \phi (x)\cdot {\overline {\psi (x)}}\,\mathrm {d} x\,,}$

whereas physicists would write for the same quantity

${\displaystyle \langle \psi |\phi \rangle =\int \mathrm {d} x\,\psi ^{*}(x)\cdot \phi (x)\,.}$