CCR/CAR algebra - Definition

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

<math>fg-gf=i(f,g)<math>

f*=f

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

There is also a corresponding unital C*-algebra generated by e^if subject to

<math>e^{ic_1 f}e^{ic_2 f}=e^{i(c_1+c_2) f}<math>

<math>e^{if}e^{ig}=e^{-i(f,g)}e^{ig}e^{if}<math>

(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

<math>fg+gf=(f,g)<math>

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

<math>fg-(-1)^{|f||g|}gf=i(f,g)<math>

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.

See also canonical commutation relation, Bose-Einstein statistics, Fermi-Dirac statistics, Bogoliubov transformation