  Gate-tunable quantum-mechanical tunnelling of particles between a quantum
confined state and a nearby Fermi reservoir of delocalized states has
underpinned many advances in spintronics and solid-state quantum optics. The
prototypical example is a semiconductor quantum dot separated from a gated
contact by a tunnel barrier. This enables Coulomb blockade, the phenomenon
whereby electrons or holes can be loaded one-by-one into a quantum dot.
Depending on the tunnel-coupling strength, this capability facilitates single
spin quantum bits or coherent many-body interactions between the confined spin
and the Fermi reservoir. Van der Waals (vdW) heterostructures, in which a wide
range of unique atomic layers can easily be combined, offer novel prospects to
engineer coherent quantum confined spins, tunnel barriers down to the atomic
limit or a Fermi reservoir beyond the conventional flat density of states.
However, gate-control of vdW nanostructures at the single particle level is
needed to unlock their potential. Here we report Coulomb blockade in a vdW
heterostructure consisting of a transition metal dichalcogenide quantum dot
coupled to a graphene contact through an atomically thin hexagonal boron
nitride (hBN) tunnel barrier. Thanks to a tunable Fermi reservoir, we can
deterministically load either a single electron or a single hole into the
quantum dot. We observe hybrid excitons, composed of localized quantum dot
states and delocalized continuum states, arising from ultra-strong
spin-conserving tunnel coupling through the atomically thin tunnel barrier.
Probing the charged excitons in applied magnetic fields, we observe large
gyromagnetic ratios (~8). Our results establish a foundation for engineering
next-generation devices to investigate either novel regimes of Kondo physics or
isolated quantum bits in a vdW heterostructure platform.
