  The stochastic block model (SBM) is an important generative model for random
graphs in network science and machine learning, useful for benchmarking
community detection (or clustering) algorithms. The symmetric SBM generates a
graph with $2n$ nodes which cluster into two equally sized communities. Nodes
connect with probability $p$ within a community and $q$ across different
communities. We consider the case of $p=a\ln (n)/n$ and $q=b\ln (n)/n$. In this
case, it was recently shown that recovering the community membership (or label)
of every node with high probability (w.h.p.) using only the graph is possible
if and only if the Chernoff-Hellinger (CH) divergence
$D(a,b)=(\sqrt{a}-\sqrt{b})^2 \geq 1$. In this work, we study if, and by how
much, community detection below the clustering threshold (i.e. $D(a,b)<1$) is
possible by querying the labels of a limited number of chosen nodes (i.e.,
active learning). Our main result is to show that, under certain conditions,
sampling the labels of a vanishingly small fraction of nodes (a number
sub-linear in $n$) is sufficient for exact community detection even when
$D(a,b)<1$. Furthermore, we provide an efficient learning algorithm which
recovers the community memberships of all nodes w.h.p. as long as the number of
sampled points meets the sufficient condition. We also show that recovery is
not possible if the number of observed labels is less than $n^{1-D(a,b)}$. The
validity of our results is demonstrated through numerical experiments.
