# Heisenberg group

In mathematics, the **Heisenberg group**, named after Werner Heisenberg, is a group of 3×3 upper triangular matrices of the form

Elements *a*,*b*,*c* can be taken from some (arbitrary) commutative ring.

## Contents

## Examples

(i) If *a*,*b*,*c* are real numbers (in the ring **R**) then we get the **continuous Heisenberg group**. It is a nilpotent Lie group.

(ii) If *a*,*b*,*c* are integers (in the ring **Z**) then we get the **discrete Heisenberg group** H_{3}. It is a non-abelian nilpotent group. It has two generators

and relations

- ,

where

is the generator of the center of H_{3}. By Bass' theorem, it has a polynomial growth rate of order 4.

(iii) If one takes *a*,*b*,*c* in **Z**/*p* **Z**, then we get the **Heisenberg group modulo** * p*.
It is a group of order

*p*

^{3}with two generators,

*x*,

*y*and relations

- .

## General Heisenberg group

There are more general **Heisenberg groups** H_{n}. We begin by discussing the Real Heisenberg group of dimension 2*n*+1, for any integer *n* ≥ 1. As a group of matrices, H_{n} (or H_{n}(**R**) to indicate this is the Heisenberg group over the ring **R**) is defined as the group of square matrices of size *n*+2 with entries in **R**:

where *a* is a row vector of length *n*, *b* is a column vector of length *n* and 1_{n} is
the identity matrix of size *n*. This is indeed a group, as is shown by the multiplication:

and

The Heisenberg group is a connected, simply-connected Lie group whose Lie algebra consists of matrices

where *a* is a row vector of length *n*, *b* is a column vector of length *n* and 0_{n} is
the zero matrix of size *n*.
The exponential map is given by the following expression

By choosing a basis e_{1}, ..., e_{n} of **R**^{n}, and letting

the Lie algebra can also be characterized by the canonical commutation relations

where *p*_{1}, .., *p*_{n}, *q*_{1}, .., *q*_{n}, *z* are generators. In particular, *z* is a *central* element of the Heisenberg Lie algebra. Note that the Lie algebra of the Heisenberg group is nilpotent. The exponential map of a nilpotent Lie algebra is a diffeomorphism between the Lie algebra and the unique associated connected, simply-connected Lie group.

This group occurs not only in quantum mechanics but in the theory of theta functions; it is also used in Fourier analysis. This group is also used in some formulations of the Stone-von Neumann theorem.

The above discussion (aside from statements referring to dimension and Lie group) applies if we replace **R** by any commutative ring **A**. The corresponding group is denoted H_{n}(**A**). Under the additional assumption that the prime 2 is invertible in the ring **A** the exponential map is also defined, since it reduces to a finite sum and has the form above (i.e. **A** could be a ring **Z**/**pZ** with an odd prime p or any field of characteristic 0).

## The connection with the Weyl algebra

The Lie algebra of the Heisenberg group was described above as a Lie algebra of matrices. We now apply the Poincaré-Birkhoff-Witt theorem, to determine the universal enveloping algebra . Among other properties, the universal enveloping algebra is an associative algebra into which injectively imbeds. By Poincaré-Birkhoff-Witt, it is the free vector space generated by the monomials

where the exponents are all non-negative. Thus consists of real polynomials

with the commutation relations

is closely related to the algebra of differential operators on **R**^{n} with polynomial coefficients, since any such operator has a unique representation in the form:

This algebra is called the Weyl algebra. It follows from abstract nonsense that the Weyl algebra *W _{n}* is a quotient of . However, this also easy to see directly from the above representations; viz, by the mapping

## Weyl's view of quantum mechanics

The application that led Hermann Weyl to an explicit introduction of the Heisenberg group was the question of why the Schrödinger picture and Heisenberg picture are physically equivalent. Abstractly there is a good explanation: the group *H _{n}* is a central extension of

**R**

^{2n}by a copy of

**R**, and as such is a semidirect product. Its representation theory is relatively simple (a special case of the later Mackey theory), and in particular there is a uniqueness result for unitary representations with given action of the central element

*z*(in the Lie algebra) or the one-parameter subgroup it creates under the exponential map, which is the central extension. This abstract uniqueness accounts for the equivalence of the two physical pictures.

The same uniqueness result was used by David Mumford for discrete Heisenberg groups, in his theory of abelian varieties. This is a large generalization of the approach used in Jacobi's elliptic functions, which is the case of the modulo 2 Heisenberg group, of order 8.

## As a sub-Riemannian manifold

The three-dimensional Heisenberg group *H*_{3}(**R**) on the reals can also be understood to be a smooth manifold, and specifically, a simple example of a sub-Riemannian manifold. Given a point *p*=(*x*,*y*,*z*) in **R**^{3}, define a differential 1-form Θ at this point as

- .

This one-form belongs to the cotangent bundle of **R**^{3}; that is,

is a map on the tangent bundle. Let

It can be seen that *H* is a subbundle of the tangent bundle T**R**^{3}. A cometric on *H* is given by projecting vectors to the two-dimensional space spanned by vectors in the *x* and *y* direction. That is, given vectors and in T**R**^{3}, the inner product is given by

The resulting structure turns *H* into the manifold of the Heisenberg group. An orthonormal frame on the manifold is given by the Lie vector fields

which obey the relations [*X*,*Y*]=*Z* and [*X*,*Z*]=[*Y*,*Z*]=0. Being Lie vector fields, these form a left-invariant basis for the group action. The geodesics on the manifold are spirals, projecting down to circles in two dimensions. That is, if

is a geodesic curve, then the curve is an arc of a circle, and

with the integral limited to the two-dimensional plane. That is, the height of the curve is proportional to the area of the circle subtended by the circular arc, which follows by Stokes theorem.

## References

- Richard Montgomery,
*A Tour of Subriemannian Geometries, Their Geodesics and Applications (Mathematical Surveys and Monographs, Volume 91)*, (2002) American Mathematical Society, ISBN 0-8218-1391-9.