  Quantum dots are semiconductor nano-structures where particle motion is
confined in all three spatial dimensions. Since their first experimental
realization, nanocrystals confining the quanta of polarization waves, termed
excitons, have found numerous applications in fields ranging from single photon
sources for quantum information processing to commercial displays. A major
limitation to further extending the range of potential applications has been
the large inhomogeneity in, and lack-of tunability of, exciton energy that is
generic to quantum dot materials. Here, we address this challenge by
demonstrating electrically-defined quantum dots for excitons in monolayer
semiconductors where the discrete exciton energies can be tuned using applied
gate voltages. Resonance fluorescence measurements show strong spectral jumps
and blinking of these resonances, verifying their zero-dimensional nature. Our
work paves the way for realizing quantum confined bosonic modes where nonlinear
response would arise exclusively from exciton--exciton interactions.
