# nLab Heisenberg group

group theory

### Cohomology and Extensions

#### Symplectic geometry

symplectic geometry

higher symplectic geometry

# Contents

## Idea

A Heisenberg group is a Lie group integrating a Heisenberg Lie algebra.

There are several such, and so the conventions in the literature vary slightly as to which one to pick by default.

The Heisenberg group historically originates in and still has its strongest ties to quantum physics: there it is a group of unitary operators acting on the space of states induced from those observables on a linear phase space – a symplectic vector space – which are given by linear or by constant functions. So any Heisenberg group is a subgroup of a group of observables in certain simple examples of quantum mechanical systems.

## Definition

A Heisenberg group is a Lie group whose Lie algebra is a Heisenberg Lie algebra.

We spell out some special cases in detail.

### $H_3$ in components

The simplest non-trivial example of a Heisenberg group is the unique simply connected Lie integration of the Heisenberg Lie algebra $Heis(\mathbb{R}^2, \omega = d p \wedge dq)$ on the the canonical 2-dimensional symplectic vector space $\mathbb{R}^2$ with canonical coordinates $(p,q)$.

This Heisenberg Lie algebra is generated from 3 elements, here to be denoted $\mathbf{p}, \mathbf{q},\mathbf{e}$, subject to the single non-trivial Lie bracket

$[\mathbf{q}, \mathbf{p}] = \mathbf{e} \,.$

The corresponding Heisenberg group is usually denoted $H_3$:

$H_3 =: \exp Heis(\mathbb{R}^2 , d p \wedge d q) \,.$

The underlying smooth manifold of this Lie group is the Cartesian space $\mathbb{R}^3$. A general element may be written as

$g_{a,b,c} = \exp(a \mathbf{q} + b \mathbf{p} )\exp(c \mathbf{e})$

with $a, b , c\in \mathbb{R}$. In terms of this notation the product in the group is given (by the Baker-Campbell-Hausdorff formula) by

$\exp(a_1 \mathbf{q} + b_1 \mathbf{p}) \exp(c_1 \mathbf{e}) \cdot \exp(a_2 \mathbf{q} + b_2 \mathbf{p}) \exp(c_2 \mathbf{e}) = \exp((a_1 + b_1) \mathbf{q} + (b_1 + c_1) \mathbf{p}) \exp(c_1 + c_2 - \frac{1}{2}(a_1 b_2 - a_2 b_1) \mathbf{e}) \,.$

A discrete quotient group of this, which still has the same Lie algebra, has as underlying manifold $\mathbb{R}^2 \times U(1)$ (the second factor being the circle group), with the projection

$\mathbb{R}^2 \times \mathbb{R} \to \mathbb{R}^2 \times U(1)$

being quotienting by $\mathbb{Z}$: $U(1) \simeq \mathbb{R} / \mathbb{Z}$.

If the circle group is instead thought of as the unit circle in the complex plane, then this quotient is thought of as the exponential map $\exp(2 \pi i (-)) : \mathbb{R} \to U(1)$. In terms of this the group elements in the quotient read

$g_{a, b, c} = \exp(a \mathbf{q} + b \mathbf{p} )\exp(2 \pi i c )$

and their product is

$\exp(a_1 \mathbf{q} + b_1 \mathbf{p}) \exp(2 \pi i c_1 ) \cdot \exp(a_2 \mathbf{q} + b_2 \mathbf{p}) \exp(2 \pi i c_2 ) = \exp((a_1 + b_1) \mathbf{q} + (b_1 + c_1) \mathbf{p}) \exp(2 \pi i(c_1 + c_2 - \frac{1}{2}(a_1 b_2 - a_2 b_1))) \,.$

While the Lie algebra is still the same real Heisenberg Lie algebra as before, it is now suggestive to write the Lie bracket as

$[q,p] = i \,.$

This is the way the relation appears in texts on quantum physics.

### For a symplectic vector space

Generally, there is a Heisenberg group $H(V, \omega)$ associated to any symplectic vector space $(V, \omega)$.

Regard $V$ with its abelian group structure underlying its vector space structure.

The Heisenberg group $H(V,\omega)$ is the space $V \times U(1)$ (for $U(1)$ the circle group) equipped with the group product

$(v_1, c_1) \cdot (v_2, c_2) = (v_1 + v_2, c_1 c_2 \exp(2 \pi i \omega(v,w))) \,.$

## Properties

### Relation to Poisson algebra

A symplectic vector space $(V, \omega)$ is in particular a symplectic manifold. Accordingly its algebra of smooth functions $C^\infty(V)$ is a Poisson algebra. The Lie algebra underlying the Poisson algebra contains the Heisenberg Lie algebra as the subspace with is the direct sum of the linear functions $V^* \hookrightarrow C^\infty(V)$ and the constant functions $\marthbb{R} \hookrightarrow C^\infty(V)$.

For more details in this at Heisenberg Lie algebra the section Relation to Poisson algebra.

### Relation to symplectomorphisms

By the above, the Heisenberg group is a subgroup of the group that integrates the Poisson bracket. The latter is a central extension of the group of Hamiltonian symplectomorphisms.

(Of course, on a contractible symplectic manifold such as a symplectic vector space, every symplectomorphism is automatically a Hamiltonian symplectomorphism.)

### Cocycle and extension

The additive group on the Cartesian space $\mathbb{R}^2$ with group operation

$(a,b) + (a',b') = (a + a' , b + b')$

carries a degree-2 group cocycle $\omega$ with values in $\mathbb{R}$ given by

$\omega : ((a_1,b_1), (a_2,b_2)) \mapsto a_1 \cdot b_2 \,.$

The cocycle condition for this is the identity

$a_1 \cdot (b_2 + b_3) + a_2 \cdot b_3 = a_1 \cdot b_2 + (a_1 + a_2) \cdot b_3$

The Heisenberg group $H_3$ is the group extension of $\mathbb{R}^2$ by this cocycle.

### Automorphism group

The automorphism group of the Heisenberg group is the symplectic group.

## References

An original account is in

• Bertram Kostant, Quantization and unitary representations, in Lectures in modern analysis and applications III. Lecture Notes in Math. 170 (1970), Springer Verlag, 87—208

A textbook account is in section II.3 of

• Jean-Luc Brylinski, Loop spaces, characteristic classes and geometric quantization, Birkhäuser (1993)

Discussion in the context of geometric quantization is in

• Geometric quantization II, Prequantization and the Heisenberg group (pdf)

Discussion of the representation theory of the infinite-dimensional Heisenberg group is in

• Günther Hörmann, Representations of the infinite dimensional Heisenberg group, PhD thesis, Vienna 1993 (pdf)

Revised on June 20, 2013 20:47:07 by Urs Schreiber (131.174.42.16)