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I am reading a paper about Gaussian quantum states and the mathematical formalism used to describe them. At one point the authors say "An important example of a Gaussian state is the maximally entangled state $\Phi$. In their endnotes, they then note that

"Although this maximally entangled state does not belong to the Hilbert space, it can always be considered as a limit of a proper pure state."

Why does the maximally entangled state not belong to the Hilbert space? Is it in some sense ill-defined as a pure state?

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The maximally entangled state in the Gaussian framework is the state that one gets when taking the limit of infinite energy/photon number of a particular class of state (see equation (4) of the paper you linked). From equation equation (2) it is clear that such a state is not bounded, and thus is not in $\mathcal{H}=L^2(\mathbb{R}^n)$.

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  • $\begingroup$ So...in what sense is this a "state"? $\endgroup$
    – ACuriousMind
    Commented Jan 31, 2017 at 13:54

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