  We show that Coulomb drag in ultra-clean graphene double layers can be used
for controlling the on/off ratio for current flow by tunning the external gate
voltage. Hence, although graphene remains semi-metallic, the double layer
graphene system can be tuned from conductive to a highly resistive state. We
show that our results explain previous data of Coulomb drag in double layer
graphene samples in disordered SiO2 substrates.
