  We investigate the power of interaction in two player quantum communication
protocols. Our main result is a rounds-communication hierarchy for the pointer
jumping function $f_k$. We show that $f_k$ needs quantum communication
$\Omega(n)$ if Bob starts the communication and the number of rounds is limited
to $k$ (for any constant $k$). Trivially, if Alice starts, $O(k\log n)$
communication in $k$ rounds suffices. The lower bound employs a result relating
the relative von Neumann entropy between density matrices to their trace
distance and uses a new measure of information. We also describe a classical
probabilistic $k$ round protocol with communication
$O(n/k\cdot(\log^{(k/2)}n+\log k)+k\log n)$ in which Bob starts the
communication. Furthermore as a consequence of the lower bound for pointer
jumping we show that any $k$ round quantum protocol for the disjointness
problem needs communication $\Omega(n^{1/k})$ for $k=O(1)$.
