In mathematics, the theta representation is a particular representation of the Heisenberg group of quantum mechanics. It gains its name from the fact that the Jacobi theta function is invariant under the action of a discrete subgroup of the Heisenberg group. The representation was popularized by David Mumford.

Construction

The theta representation is a representation of the continuous Heisenberg group H_3(\R) over the field of the real numbers. In this representation, the group elements act on a particular Hilbert space. The construction below proceeds first by defining operators that correspond to the Heisenberg group generators. Next, the Hilbert space on which these act is defined, followed by a demonstration of the isomorphism to the usual representations.

Group generators

Let f(z) be a holomorphic function, let a and b be real numbers, and let \tau be an arbitrary fixed complex number in the upper half-plane; that is, so that the imaginary part of \tau is positive. Define the operators Sa and Tb such that they act on holomorphic functions as and It can be seen that each operator generates a one-parameter subgroup: and However, S and T do not commute: Thus we see that S and T together with a unitary phase form a nilpotent Lie group, the (continuous real) Heisenberg group, parametrizable as where U(1) is the unitary group. A general group element then acts on a holomorphic function f(z) as where U(1) = Z(H) is the center of H, the commutator subgroup [H, H]. The parameter \tau on serves only to remind that every different value of \tau gives rise to a different representation of the action of the group.

Hilbert space

The action of the group elements is unitary and irreducible on a certain Hilbert space of functions. For a fixed value of τ, define a norm on entire functions of the complex plane as Here, \Im \tau is the imaginary part of \tau and the domain of integration is the entire complex plane. Let be the set of entire functions f with finite norm. The subscript \tau is used only to indicate that the space depends on the choice of parameter \tau. This forms a Hilbert space. The action of given above is unitary on, that is, preserves the norm on this space. Finally, the action of on is irreducible. This norm is closely related to that used to define Segal–Bargmann space.

Isomorphism

The above theta representation of the Heisenberg group is isomorphic to the canonical Weyl representation of the Heisenberg group. In particular, this implies that and L^2(\R) are isomorphic as H-modules. Let stand for a general group element of H_3(\R). In the canonical Weyl representation, for every real number h, there is a representation \rho_h acting on L^2(\R) as for x\in\R and Here, h is the Planck constant. Each such representation is unitarily inequivalent. The corresponding theta representation is:

Discrete subgroup

Define the subgroup as The Jacobi theta function is defined as It is an entire function of z that is invariant under This follows from the properties of the theta function: and when a and b are integers. It can be shown that the Jacobi theta is the unique such function.