CCR and CAR algebras

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In mathematics and physics CCR algebras (after canonical commutation relations) and CAR algebras (after canonical anticommutation relations) arise from the quantum mechanical study of bosons and fermions respectively. They play a prominent role in quantum statistical mechanics[1] and quantum field theory.

CCR and CAR as *-algebras[edit]

Let be 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 subject to the relations

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

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

for any in is called the canonical anticommutation relations (CAR) algebra.

The C*-algebra of CCR[edit]

There is a distinct, but closely related meaning of CCR algebra, called the CCR C*-algebra. Let be a real symplectic vector space with nonsingular symplectic form . In the theory of operator algebras the CCR algebra over is the unital C*-algebra generated by elements subject to

These are called the Weyl form of the canonical commutation relations and, in particular, they imply that each is unitary and . It is well known that the CCR algebra is a simple non-separable algebra and is unique up to isomorphism.[2]

When is a Hilbert space and is given by the imaginary part of the inner-product, the CCR algebra is faithfully represented on the symmetric Fock space over by setting

for any . The field operators are defined for each as the generator of the one-parameter unitary group on the symmetric Fock space. These are self-adjoint unbounded operators, however they formally satisfy

As the assignment is real-linear, so the operators define a CCR algebra over in the sense of Section 1.

The C*-algebra of CAR[edit]

Let be a Hilbert space. In the theory of operator algebras the CAR algebra is the unique C*-completion of the complex unital *-algebra generated by elements subject to the relations

for any , . When is separable the CAR algebra is an AF algebra and in the special case is infinite dimensional it is often written as .[3]

Let be the antisymmetric Fock space over and let be the orthogonal projection onto antisymmetric vectors:

The CAR algebra is faithfully represented on by setting

for all and . The fact that these form a C*-algebra is due to the fact that creation and annihilation operators on antisymmetric Fock space are bona-fide bounded operators. Moreover, the field operators satisfy

giving the relationship with Section 1.

Superalgebra generalization[edit]

Let be a real -graded vector space equipped with a nonsingular antisymmetric bilinear superform (i.e. ) such that is real if either or is an even element and imaginary if both of them are odd. The unital *-algebra generated by the elements of subject to the relations

for any two pure elements in is the obvious superalgebra generalization which unifies CCRs with CARs: if all pure elements are even, one obtains a CCR, while if all pure elements are odd, one obtains a CAR.

In mathematics, the abstract structure of the CCR and CAR algebras, over any field, not just the complex numbers, is studied by the name of Weyl and Clifford algebras, where many significant results have accrued. One of these is that the graded generalizations of Weyl and Clifford algebras allow the basis-free formulation of the canonical commutation and anticommutation relations in terms of a symplectic and a symmetric non-degenerate bilinear form. In addition the binary elements in this graded Weyl algebra give a basis-free version of the commutation relations of the symplectic and indefinite orthogonal Lie algebras.[4]

See also[edit]


  1. ^ Bratteli, Ola; Robinson, Derek W. (1997). Operator Algebras and Quantum Statistical Mechanics: v.2. Springer, 2nd ed. ISBN 978-3-540-61443-2. 
  2. ^ Petz, Denes (1990). An Invitation to the Algebra of Canonical Commutation Relations. Leuven University Press. ISBN 978-90-6186-360-1. 
  3. ^ Evans, David E.; Kawahigashi, Yasuyuki (1998). Quantum Symmetries in Operator Algebras. Oxford University Press. ISBN 978-0-19-851175-5. .
  4. ^ Roger Howe (1989). "Remarks on Classical Invariant Theory". Transactions of the American Mathematical Society. 313: 539–570. JSTOR 2001418. doi:10.1090/S0002-9947-1989-0986027-X.