Bra-ket notation is a standard notation for describing quantum states in the theory of quantum mechanics 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 bracket, , consisting of a left part, , called the bra (pronounced /ˈbrɑː/), and a right part, , called the ket (pronounced /ˈkɛt/). The notation was introduced in 1930 by Paul Dirac, and is also known as Dirac notation.
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 to collapse into the state
- 1 Bras and kets
- 2 Linear operators
- 3 Properties
- 4 Composite bras and kets
- 5 Representations in terms of bras and kets
- 6 The unit operator
- 7 Notation used by mathematicians
- 8 References and notes
- 9 Further reading
- 10 External links
Bras and kets
Usage in quantum mechanics
In quantum mechanics, the state of a physical system is identified with a ray in a complex separable Hilbert space, , or, equivalently, by a point in the projective Hilbert space of the system. Each vector in the ray is called a "ket" and written as , which would be read as "ket psi". (The can be replaced by any symbols, letters, numbers, or even words—whatever serves as a convenient label for the ket.)
The ket can be viewed as a column vector and (given a basis for the Hilbert space) written out in coordinates,
when the considered Hilbert space is finite-dimensional. In infinite-dimensional spaces there are infinitely many coordinates and the ket may be written in complex function notation, by prepending it with a bra (see below). For example,
Every ket has a dual bra, written as . For example, the bra corresponding to the ket above would be the row vector
This is a continuous linear functional from to the complex numbers , defined by:
for all kets , where denotes the inner product defined on the Hilbert space. Here the origin of the bra-ket notation becomes clear: when we drop the parentheses (as is common with linear functionals) and meld the bars together we get , which is common notation for an inner product in a Hilbert space. This combination of a bra with a ket to form a complex number is called a bra-ket or bracket.
The bra is simply the conjugate transpose (also called the Hermitian conjugate) of the ket and vice versa. The notation is justified by the Riesz representation theorem, which states that a Hilbert space and its dual space are isometrically conjugate isomorphic. Thus, each bra corresponds to exactly one ket, and vice versa. More precisely, if
is the Riesz isomorphism between and its dual space, then
Note that this only applies to states that are actually vectors in the Hilbert space. Non-normalizable states, such as those whose wavefunctions are Dirac delta functions or infinite plane waves, do not technically belong to the Hilbert space. So if such a state is written as a ket, it will not have a corresponding bra according to the above definition. This problem can be dealt with in either of two ways. First, since all physical quantum states are normalizable, one can carefully avoid non-normalizable states. Alternatively, the underlying theory can be modified and generalized to accommodate such states, as in the Gelfand–Naimark–Segal construction or rigged Hilbert spaces. In fact, physicists routinely use bra-ket notation for non-normalizable states, taking the second approach either implicitly or explicitly.
In quantum mechanics the expression (mathematically: the coefficient for the projection of onto ) is typically interpreted as the probability amplitude for the state to collapse into the state
The advantage of the bra-ket notation over explicit wave function algebra is the possibility of expressing operations on quantum states independent of a basis. For example the time-independent Schrödinger equation is simply expressed as
The operators can be conveniently expressed in different bases (see next section for the operations used in these formulas : action of a linear operator, outer product of a ket and a bra):
(For a rigorous definition of basis with a continuous set of indices and consequently for a rigorous definition of position and momentum basis see )
(For a rigorous statement of the expansion of an S-diagonalizable operator - observable - in its eigenbasis or in another basis see )
The wave functions in real, momentum or reciprocal space can be retrieved as needed:
and all basis conversions can be performed via the relations such as
More general uses
Bra-ket notation can be used even if the vector space is not a Hilbert space. In any 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.
If A : H → H is a linear operator, we can apply A to the ket to obtain the ket . 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.
Operators can also be viewed as acting on bras from the right hand side. Composing the bra with the operator A results in the bra , defined as a linear functional on H by the rule
This expression is commonly written as (cf. energy inner product)
Note that the second symbol | is completely optional, i.e. , since is in itself a ket and may be written .
If the same state vector appears on both bra and ket side, 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 , written as
A convenient way to define linear operators on H is given by the outer product: if is a bra and is a ket, the outer product
denotes the rank-one operator that maps the ket to the ket (where is a scalar multiplying the vector ). One of the uses of the outer product is to construct projection operators. Given a ket of norm 1, the orthogonal projection onto the subspace spanned by is
Just as kets and bras can be transformed into each other (making into ) the element from the dual space corresponding with is where A† denotes the Hermitian conjugate (or adjoint) of the operator A.
It is usually taken as a postulate or axiom of quantum mechanics, that any operator corresponding to an observable quantity (shortly called observable) is self-adjoint, that is, it satisfies A† = A. Then the identity
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.
- Since bras are linear functionals,
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:
and so forth. The expressions can thus be written, unambiguously, with no parentheses whatsoever. Note that the associative property does not hold for expressions that include non-linear operators, such as the antilinear time reversal operator in physics.
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.,
- 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:
- Inner products:
- Matrix elements:
- Outer products:
Composite bras and kets
Two Hilbert spaces V and W may form a third space 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 is a ket in V and is a ket in W, the direct product of the two kets is a ket in . This is written variously as
- or or or
Representations in terms of bras and kets
In quantum mechanics, it is often convenient to work with the projections of state vectors onto a particular basis, rather than the vectors themselves. The reason is that the former are simply complex numbers, and can be formulated in terms of partial differential equations (see, for example, the derivation of the position-basis Schrödinger equation). This process is very similar to the use of coordinate vectors in linear algebra.
For instance, the Hilbert space of a zero-spin point particle is spanned by a position basis , where the label x extends over the set of position vectors. Starting from any ket in this Hilbert space, we can define a complex scalar function of x, known as a wavefunction:
It is then customary to define linear operators acting on wavefunctions in terms of linear operators acting on kets, by
For instance, the momentum operator p has the following form:
One occasionally encounters an expression like
This is something of an abuse of notation, though a fairly common one. 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:
For further details, see rigged Hilbert space.
The unit operator
with the inner product on the Hilbert space. From the commutativity of kets with (complex) scalars now follows that
must be the unit operator, which sends each vector to itself. This can be inserted in any expression without affecting its value, for example
where in the last identity Einstein summation convention has been used.
In quantum mechanics it often occurs that little or no information about the inner product of two arbitrary (state) kets is present, while it is possible to say something about the expansion coefficients and of those vectors with respect to a chosen (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).
Notation used by mathematicians
Let be a Hilbert space and is a vector in . What physicists would denote as is the vector itself. That is
Let be the dual space of . This is the space of linear functionals on . The isomorphism is defined by Φ(h) = ϕh where for all we have
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 ϕh and g with and respectively. This is because of literal symbolic substitutions. Let and let . This gives
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.
References and notes
- ^ PAM Dirac (1982). The principles of quantum mechanics (Fourth Edition ed.). Oxford UK: Oxford University Press. p. 18 ff. ISBN 0198520115. http://books.google.com/books?id=XehUpGiM6FIC&printsec=frontcover&dq=intitle:quantum+intitle:mechanics+inauthor:dirac#PPA20,M1.
- ^ Carfì, David (2007). "TOPOLOGICAL CHARACTERIZATIONS OF S-LINEARITY". AAPP-PHYSICAL, MATHEMATICAL AND NATURAL SCIENCES 85 (2): 1–16. doi:10.1478/C1A0702005. http://cab.unime.it/journals/index.php/AAPP/article/view/407.
- ^ Carfì, David (2005). "S-DIAGONALIZABLE OPERATORS IN QUANTUM MECHANICS". Glasnik Matematicki 40 (2): 261–301. doi:10.3336/gm.40.2.08. http://web.math.hr/glasnik/vol_40/no2_08.html.
- ^ Carfì, David (April 2003). "Dirac-orthogonality in the space of tempered distributions". Journal of Computational and Applied Mathematics 153 (1-2): 99–107. Bibcode 2003JCoAM.153...99C. doi:10.1016/S0377-0427(02)00634-9. http://portal.acm.org/citation.cfm?id=774918.
- ^ Carfì, David (April 2003). "Some properties of a new product in the space of tempered distributions". Journal of Computational and Applied Mathematics 153 (1-2): 109–118. Bibcode 2003JCoAM.153..109C. doi:10.1016/S0377-0427(02)00635-0. http://portal.acm.org/citation.cfm?id=774919.
- ^ Lecture notes by Robert Littlejohn, eqns 12 and 13
- Feynman, Leighton and Sands (1965). The Feynman Lectures on Physics Vol. III. Addison-Wesley. ISBN 0-201-02115-3.
- Richard Fitzpatrick, "Quantum Mechanics: A graduate level course", The University of Texas at Austin.
- Robert Littlejohn, Lecture notes on "The Mathematical Formalism of Quantum mechanics", including bra-ket notation.
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