On the best constants in noncommutative Khintchine-type inequalities

We obtain new proofs with improved constants of the Khintchine-type inequality with matrix coefficients in two cases. The first case is the Pisier and Lust-Piquard noncommutative Khintchine inequality for p=1, where we obtain the sharp lower bound of in the complex Gaussian case and for the sequence of functions . The second case is Junge's recent Khintchine-type inequality for subspaces of the operator space RC, which he used to construct a cb-embedding of the operator Hilbert space OH into the predual of a hyperfinite factor. Also in this case, we obtain a sharp lower bound of . As a consequence, it follows that any subspace of a quotient of (RC)^* is cb-isomorphic to a subspace of the predual of the hyperfinite factor of type III₁, with cb-isomorphism constant. In particular, the operator Hilbert space OH has this property.