# Heisenberg group

(Redirected from Discrete Heisenberg group)

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

[itex]\begin{pmatrix}
1 & a & c\\
0 & 1 & b\\
0 & 0 & 1\\


\end{pmatrix}. [itex]

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 H3. It is a non-abelian nilpotent group. It has two generators

[itex]x=\begin{pmatrix}
1 & 1 & 0\\
0 & 1 & 0\\
0 & 0 & 1\\


\end{pmatrix},\ \ y=\begin{pmatrix}

1 & 0 & 0\\
0 & 1 & 1\\
0 & 0 & 1\\


\end{pmatrix}[itex] and relations

[itex] z^{}_{}=xyx^{-1}y^{-1},\ xz=zx,\ yz=zy [itex],

where

[itex]z=\begin{pmatrix}
1 & 0 & 1\\
0 & 1 & 0\\
0 & 0 & 1\\


\end{pmatrix}[itex] is the generator of the center of H3. 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 p3 with two generators, x, y and relations

[itex] z^{}_{}=xyx^{-1}y^{-1},\ x^p=y^p=z^p=1,\ xz=zx,\ yz=zy [itex].

## General Heisenberg group

There are more general Heisenberg groups Hn. We begin by discussing the Real Heisenberg group of dimension 2n+1, for any integer n ≥ 1. As a group of matrices, Hn (or Hn(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:

[itex] \begin{bmatrix} 1 & a & c \\ 0 & 1_n & b \\ 0 & 0 & 1 \end{bmatrix} [itex]

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

[itex] \begin{bmatrix} 1 & a & c \\ 0 & 1_n & b \\ 0 & 0 & 1 \end{bmatrix} \times \begin{bmatrix}1 & a' & c' \\ 0 & 1_n & b' \\ 0 & 0 & 1 \end{bmatrix} = \begin{bmatrix} 1 & a+ a' & c+c' +a b' \\ 0 & 1_n & b+b' \\ 0 & 0 & 1 \end{bmatrix} [itex]

and

[itex] \begin{bmatrix} 1 & a & c \\ 0 & 1_n & b \\ 0 & 0 & 1 \end{bmatrix} \times \begin{bmatrix}1 & -a & -c +a b\\ 0 & 1_n & -b \\ 0 & 0 & 1 \end{bmatrix} = \begin{bmatrix} 1 & 0 & 0 \\ 0 & 1_n & 0 \\ 0 & 0 & 1 \end{bmatrix}. [itex]

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

[itex] \begin{bmatrix} 0 & a & c \\ 0 & 0_n & b \\ 0 & 0 & 0 \end{bmatrix}, [itex]

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

[itex] \exp \begin{bmatrix} 0 & a & c \\ 0 & 0_n & b \\ 0 & 0 & 0 \end{bmatrix} = \sum_{k=0}^\infty \frac{1}{k!}\begin{bmatrix} 0 & a & c \\ 0 & 0_n & b \\ 0 & 0 & 0 \end{bmatrix}^k = \begin{bmatrix} 1 & a & c + {1\over 2}a b\\ 0 & 1_n & b \\ 0 & 0 & 1 \end{bmatrix}. [itex]

By choosing a basis e1, ..., en of Rn, and letting

[itex] p_i = \begin{bmatrix} 0 & \operatorname{e}_i & 0 \\ 0 & 0_n & 0 \\ 0 & 0 & 0 \end{bmatrix} [itex]
[itex] q_j = \begin{bmatrix} 0 & 0 & 0 \\ 0 & 0_n & \operatorname{e}_j^{\mathrm{T}} \\ 0 & 0 & 0 \end{bmatrix} [itex]
[itex] z = \begin{bmatrix} 0 & 0 & 1\\ 0 & 0_n & 0 \\ 0 & 0 & 0 \end{bmatrix} [itex]

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

[itex] [p_i, q_j] = \delta_{ij}z \quad [itex]
[itex] [p_i, z] = 0 \quad [itex]
[itex] [q_j, z] = 0 \quad [itex]

where p1, .., pn, q1, .., qn, 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 Hn(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 [itex] \mathfrak{h}_n[itex] 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 [itex] \mathfrak{U}(\mathfrak{h}_n)[itex]. Among other properties, the universal enveloping algebra is an associative algebra into which [itex] \mathfrak{h}_n[itex] injectively imbeds. By Poincaré-Birkhoff-Witt, it is the free vector space generated by the monomials

[itex] z^j p_1^{k_1} p_2^{k_2} \cdots p_n^{k_n} q_1^{\ell_1} q_2^{\ell_2} \cdots q_n^{\ell_n}[itex]

where the exponents are all non-negative. Thus [itex]\mathfrak{U}(\mathfrak{h}_n)[itex] consists of real polynomials

[itex] \sum_{\vec{k} \vec{\ell}} c_{j \ \vec{k} \ \vec{\ell}}\quad z^j p_1^{k_1} p_2^{k_2} \cdots p_n^{k_n} q_1^{\ell_1} q_2^{\ell_2} \cdots q_n^{\ell_n}[itex]

with the commutation relations

[itex] p_k p_\ell = p_\ell p_k, \quad q_k q_\ell = q_\ell q_k, \quad p_k q_\ell - q_\ell p_k = \delta_{k \ell} z, \quad z p_k - p_k z =0, \quad z q_k - q_k z =0 [itex]

[itex]\mathfrak{U}(\mathfrak{h}_n)[itex] is closely related to the algebra of differential operators on Rn with polynomial coefficients, since any such operator has a unique representation in the form:

[itex] P = \sum_{\vec{k} \vec{\ell}} c_{\vec{k} \vec{\ell}}\quad \partial_{x_1}^{k_1} \partial_{x_2}^{k_2} \cdots \partial_{x_n}^{k_n} x_1^{\ell_1} x_2^{\ell_2} \cdots x_n^{\ell_n}[itex]

This algebra is called the Weyl algebra. It follows from abstract nonsense that the Weyl algebra Wn is a quotient of [itex]\mathfrak{U}(\mathfrak{h}_n)[itex]. However, this also easy to see directly from the above representations; viz, by the mapping

[itex] z^j p_1^{k_1} p_2^{k_2} \cdots p_n^{k_n} q_1^{\ell_1} q_2^{\ell_2} \cdots q_n^{\ell_n} \rightarrow \partial_{x_1}^{k_1} \partial_{x_2}^{k_2} \cdots \partial_{x_n}^{k_n} x_1^{\ell_1} x_2^{\ell_2} \cdots x_n^{\ell_n}.[itex]

## 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 Hn is a central extension of R2n 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 H3(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 R3, define a differential 1-form Θ at this point as

[itex]\Theta_p=dz -\frac{1}{2}\left(xdy - ydx\right)[itex].

This one-form belongs to the cotangent bundle of R3; that is,

[itex]\Theta_p:T_p\mathbb{R}^3\to\mathbb{R}[itex]

is a map on the tangent bundle. Let

[itex]H_p = \{ v\in T_p\mathbb{R}^3 \; s.t.\;\; \Theta_p(v) = 0 \}[itex]

It can be seen that H is a subbundle of the tangent bundle TR3. 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 [itex]v=(v_1,v_2,v_3)[itex] and [itex]w=(w_1,w_2,w_3)[itex] in TR3, the inner product is given by

[itex]\langle v,w\rangle = v_1w_1+v_2w_2[itex]

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

[itex]X=\frac{\partial}{\partial x} - \frac{1}{2} y\frac{\partial}{\partial z}[itex]
[itex]Y=\frac{\partial}{\partial y} + \frac{1}{2} x\frac{\partial}{\partial z}[itex]
[itex]Z=\frac{\partial}{\partial z}[itex]

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

[itex]\gamma(t)=(x(t),y(t),z(t))[itex]

is a geodesic curve, then the curve [itex]c(t)=(x(t),y(t))[itex] is an arc of a circle, and

[itex]z(t)=\frac{1}{2}\int_c xdy-ydx[itex]

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.

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