Canonical commutation relation

In physics, the canonical commutation relation is the relation between canonical conjugate quantities (quantities which are related by definition such that one is the Fourier transform of another), for example:
between the position x and momentum p_{x} in the x direction of a point particle in one dimension, where [x,p_{x}] = xp_{x} − p_{x}x is the commutator of x and p_{x}, i is the imaginary unit, and ħ is the reduced Planck's constant h /2π . This relation is attributed to Max Born, and it was noted by E. Kennard (1927) to imply the Heisenberg uncertainty principle.
Contents
Relation to classical mechanics
By contrast, in classical physics, all observables commute and the commutator would be zero. However, an analogous relation exists, which is obtained by replacing the commutator with the Poisson bracket multiplied by i ħ:
This observation led Dirac to propose that the quantum counterparts of classical observables f, g satisfy
In 1946, Hip Groenewold demonstrated that a general systematic correspondence between quantum commutators and Poisson brackets could not hold consistently. However, he did appreciate that such a systematic correspondence does, in fact, exist between the quantum commutator and a deformation of the Poisson bracket, the Moyal bracket, and, in general, quantum operators and classical observables and distributions in phase space. He thus finally elucidated the correspondence mechanism, Weyl quantization, that underlies an alternate equivalent mathematical approach to quantization known as deformation quantization.
Representations
According to the standard mathematical formulation of quantum mechanics, quantum observables such as x and p should be represented as selfadjoint operators on some Hilbert space. It is relatively easy to see that two operators satisfying the canonical commutation relations cannot both be bounded. The canonical commutation relations can be made tamer by writing them in terms of the (bounded) unitary operators e ^{− ikx} and e ^{− iap}, which admit finitedimensional representations as well. The resulting braiding relations for these are the socalled Weyl relations. The uniqueness of the canonical commutation relations between position and momentum is guaranteed by the Stonevon Neumann theorem. The group associated with these commutation relations is called the Heisenberg group.
Generalizations
The simple formula
 ,
valid for the quantization of the simplest classical system, can be generalized to the case of an arbitrary Lagrangian .^{[1]} We identify canonical coordinates (such as x in the example above, or a field φ(x) in the case of quantum field theory) and canonical momenta π_{x} (in the example above it is p, or more generally, some functions involving the derivatives of the canonical coordinates with respect to time):
 .
This definition of the canonical momentum ensures that one of the EulerLagrange equations has the form
 .
The canonical commutation relations then amount to
 ,
where δ_{ij} is the Kronecker delta.
Further, it can be easily shown that
 .
Gauge invariance
Canonical quantization is applied, by definition, on canonical coordinates. However, in the presence of an electromagnetic field, the canonical momentum p is not gauge invariant. The correct gaugeinvariant momentum (or "kinetic momentum") is
 (SI units) (cgs units),
where q is the particle's electric charge, A is the vector potential, and c is the speed of light. Although the quantity p_{kin} is the "physical momentum", in that it is the quantity to be identified with momentum in laboratory experiments, it does not satisfy the canonical commutation relations; only the canonical momentum does that. This can be seen as follows.
The nonrelativistic Hamiltonian for a quantized charged particle of mass m in a classical electromagnetic field is (in cgs units)
where A is the threevector potential and ϕ is the scalar potential. This form of the Hamiltonian, as well as the Schroedinger equation , the Maxwell equations and the Lorentz force law are invariant under the gauge transformation
 ,
where
and Λ = Λ(x,t) is the gauge function.
The canonical angular momentum is
and obeys the canonical quantization relations
defining the Lie algebra for so(3), where ε_{ijk} is the LeviCivita symbol. Under gauge transformations, the angular momentum transforms as
The gaugeinvariant angular momentum (or "kinetic angular momentum") is given by
 ,
which has the commutation relations
where
is the magnetic field. The inequivalence of these two formulations shows up in the Zeeman effect and the AharonovBohm effect.
Angular momentum operators
where is the LeviCivita symbol and simply reverses the sign of the answer under pairwise interchange of the indices. An analogous relation holds for the spin operators.
All such nontrivial commutation relations for pairs of operators lead to corresponding uncertainty relations,^{[2]} involving positive semidefinite expectation contributions by their respective commutators and anticommutators. In general, for two Hermitian operators A and B, consider expectation values in a system in the state ψ, the variances around the corresponding expectation values being (ΔA)^{2} ≡ 〈 (A −<A>)^{2} 〉, etc.
Then
where [A,B] ≡ AB−BA is the commutator of A and B, and {A,B} ≡ AB+BA is the anticommutator. This follows through use of the Cauchy–Schwarz inequality, since 〈A^{2}〉 〈B^{2}〉 ≥ 〈AB〉^{2}, and AB = ([A,B] + {A,B}) /2 ; and similarly for the shifted operators A−〈A〉 and B−〈B〉 . Judicious choices for A and B yield Heisenberg's familiar uncertainty relation, for x and p, as usual; or, here, L_{x} and L_{y} , in angular momentum multiplets, ψ = l , m 〉 , useful constraints such as l (l+1) ≥ m (m+1), and hence l ≥ m, among others.
See also
 Canonical quantization
 CCR algebra
 Lie derivative
 Moyal bracket
References
 ^ Townsend, J. S. (2000). A Modern Approach to Quantum Mechanics. Sausalito, CA: University Science Books. ISBN 1891389130.
 ^ Robertson, H. P. (1929). "The Uncertainty Principle". Physical Review 34 (1): 163–164. Bibcode 1929PhRv...34..163R. doi:10.1103/PhysRev.34.163.
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