  Strongly bound excitons and many-body interactions between them determine
light-matter interactions in van der Waals (vdW) heterostructures of 2D
semiconductors. Unlike fundamental particles, quasiparticles in condensed
matter, such as excitons, can be tailored to alter their interactions and
realize emergent quantum phases. Here, using a WS$_2$/WSe$_2$/WS$_2$
heterotrilayer, we create a quantum superposition of oppositely oriented
dipolar excitons - a quadrupolar exciton - wherein an electron is
layer-hybridized in WS$_2$ layers while the hole localizes in WSe$_2$. In
contrast to dipolar excitons, symmetric quadrupolar excitons only redshift in
an out-of-plane electric field, consistent with ab initio calculations,
regaining dipolar characteristics at higher fields. Electric field tunes the
hybridization and allows for lifetime control through modification of the
excitonic wavefunction. Lack of density-dependent blue shift of heterotrilayer
excitons compared to dipolar excitons is consistent with quadrupolar
interactions. Our results present vdW heterotrilayers as a field-tunable
platform to engineer light-matter interactions and explore quantum phase
transitions between spontaneously ordered many-exciton phases.
