  Controlling and manipulating individual quantum systems in solids underpins
the growing interest in development of scalable quantum technologies. Recently,
hexagonal boron nitride has garnered significant attention in quantum photonic
applications due to its ability to host optically stable quantum emitters.
However, the large band gap of hBN and the lack of efficient doping inhibits
electrical triggering and limits opportunities to study electrical control of
emitters. Here, we show an approach to electrically modulate quantum emitters
in n hBN graphene van der Waals heterostructure. We show that quantum emitters
in hBN can be reversibly activated and modulated by applying a bias across the
device. Notably, a significant number of quantum emitters are intrinsically
dark, and become optically active at non-zero voltages. To explain the results,
we provide a heuristic electrostatic model of this unique behaviour. Finally,
employing these devices we demonstrate a nearly coherent source with linewidths
of 160 MHz. Our results enhance the potential of hBN for tuneable solid state
quantum emitters for the growing field of quantum information science.
