The Symmetry Group of Gaussian States in $L^2 (\mathbb{R}^n)$

Abstract

This is a continuation of the expository article \cite{krp} with some new remarks. Let $S_n$ denote the set of all Gaussian states in the complex Hilbert space $L^2 (\mathbb{R}^n),$ $K_n$ the convex set of all momentum and position covariance matrices of order $2n$ in Gaussian states and let $\mathcal{G}_n$ be the group of all unitary operators in $L^2 (\mathbb{R}^n)$ conjugations by which leave $S_n$ invariant. Here we prove the following results. $K_n$ is a closed convex set for which a matrix $S$ is an extreme point if and only if $S=\frac{1}{2} L^{T} L$ for some $L$ in the symplectic group $Sp (2n, \mathbb{R}).$ Every element in $K_n$ is of the form $\frac{1}{2} (L^{T} L + M^{T} M)$ for some $L,M$ in $Sp (2n, \mathbb{R}).$ Every Gaussian state in $L^2 (\mathbb{R}^n)$ can be purified to a Gaussian state in $L^2 (\mathbb{R}^{2n}).$ Any element $U$ in the group $\mathcal{G}_n$ is of the form $U = \lambda W ({\bm {\alpha}}) \Gamma (L)$ where $\lambda$ is a complex scalar of modulus unity, ${\bm {\alpha}} \in \mathbb{C}^n,$ $L \in Sp (2n, \mathbb{R}),$ $W({\bm {\alpha}})$ is the Weyl operator corresponding to ${\bm {\alpha}} $ and $\Gamma (L)$ is a unitary operator which implements the Bogolioubov automorphism of the Lie algebra generated by the canonical momentum and position observables induced by the symplectic linear transformation $L.$