Chemistry - CCR/CAR algebra

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In quantum field theory, if V is a real vector space equipped with a nonsingular real antisymmetric bilinear form (,) (i.e. a symplectic vector space), the unital *-algebra generated by elements of V subject to the relations

f g - g f = i (f, g)

f*=f

for any f, g in V is called the canonical commutation relations (CCR) algebra. The uniqueness of the representations of this algebra when V is finite dimensional is discussed in the Stone-von Neumann theorem.

There is also a corresponding unital C*-algebra, often referred to as the Weyl form of the algebra, generated by e^if subject to

$e^{ic_1 f}e^{ic_2 f}=e^{i(c_1+c_2) f}$

e i f e i g = e - i (f, g) e i g e i f

(e^if)*=e^-if

for real numbers c₁, c₂.

If V is equipped with a nonsingular real symmetric bilinear form (,) instead, the unital *-algebra generated by the elements of V subject to the relations

f g + g f = (f, g)

f*=f

for any f, g in V is called the canonical anticommutation relations (CAR) algebra.

If V is a real Z₂-graded vector space equipped with a nonsingular antisymmetric bilinear superform (,) (i.e. (g,f)=-(-1)^|f||g|(g,f) ) such that (f,g) is real if either f or g is an even element and imaginary if both of them are odd, the unital *-algebra generated by the elements of V subject to the relations

f g - ( - 1) | f | | g | g f = i (f, g)

f*=f, g*=g

for any two pure elements f, g in V is the obvious super generalization which unifies CCRs with CARs.

CCR/CAR algebras only describe free fields, thanks to Haag's theorem.

CCR/CAR algebra

See also