compact positive operator

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let‎ ‎$ T \in K ( H ) $ ‎be‎ ‎$ T ‎\geqslant ‎0‎ $‎.‎ ‎

What is the best way to show the following statement?

There is a compact, positive and unique operator ‎$ A$‎ so that ‎$ A‎^{2} =‎‎ ‎T‎ $.‎

( K ( H ) defined compact operator)