Entangled states close to the maximally mixed state

We give improved upper bounds on the radius of the largest ball of separable states of an m-qubit system around the maximally mixed state. The ratio between the upper bound and the best known lower bound (Hildebrand, quant.ph/0601201) thus shrinks to a constant c = p 34/27 ≈ 1.122, as opposed to a term of order √ m logm for the best upper bound known previously (Aubrun and Szarek, quant.ph/0503221). We give concrete examples of separable states on the boundary to entanglement which realize these upper bounds. As a by-product, we compute the radii of the largest balls that fit into the projective tensor product of four unit balls in R and in the projective tensor product of an arbitrary number of unit balls in Rn for n = 2, 4, 8.