  Electronic spectroscopy of zero-dimensional (0D) quantum systems, including
point defects in solids, atomic states, and small molecules, is a critical tool
for developing a fundamental understanding of these systems, with applications
ranging from solid-state and molecular materials development to emerging
technologies rooted in quantum information science. Toward this end, scanning
tunneling spectroscopy (STS) has demonstrated atomic-scale sensitivity, but is
not easily scalable for applications, whereas device-based approaches rely on
embedding these systems within a solid-state tunnel junction (TJ) and are not
generally applicable. Here we demonstrate an all-electrical readout mechanism
for these quasi-0D states that is modular and general, dramatically expanding
the phase space of accessible quantum systems and providing an approach that is
amenable to scaling and integration with other solid-state quantum
technologies. Our approach relies on the creation of high-quality tunnel
junctions via the mechanical exfoliation and stacking of multi-layer graphene
(MLG) and hexagonal boron nitride (hBN) to encapsulate the target quantum
system (QS) in an MLG/hBN/QS/hBN/MLG heterostructure. This structure allows for
electronic spectroscopy and readout of candidate quantum systems through a
combination of Coulomb and spin-blockade, providing access to entire classes of
quantum systems that have previously only been accessible via optical
spectroscopy or magnetic resonance measurements of large ensembles, if at all.
