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$SU(p,q)$ coherent states and a Gaussian de Finetti theorem

Abstract : We prove a generalization of the quantum de Finetti theorem when the local space is an infinite-dimensional Fock space. In particular, instead of considering the action of the permutation group on $n$ copies of that space, we consider the action of the unitary group $U(n)$ on the creation operators of the $n$ modes and define a natural generalization of the symmetric subspace as the space of states invariant under unitaries in $U(n)$. Our first result is a complete characterization of this subspace, which turns out to be spanned by a family of generalized coherent states related to the special unitary group $SU(p,q)$ of signature $(p,q)$. More precisely, this construction yields a unitary representation of the noncompact simple real Lie group $SU(p,q)$. We therefore find a dual unitary representation of the pair of groups $U(n)$ and $SU(p,q)$ on an $n(p+q)$-mode Fock space. The (Gaussian) $SU(p,q)$ coherent states resolve the identity on the symmetric subspace, which implies a Gaussian de Finetti theorem stating that tracing over a few modes of a unitary-invariant state yields a state close to a mixture of Gaussian states. As an application of this de Finetti theorem, we show that the $n\times n$ upper-left submatrix of an $n\times n$ Haar-invariant unitary matrix is close in total variation distance to a matrix of independent normal variables if $n^3 =O(m)$.
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Contributor : Anthony Leverrier <>
Submitted on : Wednesday, November 29, 2017 - 9:33:02 PM
Last modification on : Tuesday, December 24, 2019 - 3:26:01 PM

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Anthony Leverrier. $SU(p,q)$ coherent states and a Gaussian de Finetti theorem. Journal of Mathematical Physics, American Institute of Physics (AIP), 2018, 59, pp.042202. ⟨10.1063/1.5007334⟩. ⟨hal-01652084⟩



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