  A hallmark of wave-matter duality is the emergence of quantum-interference
phenomena when an electronic transition follows different trajectories. Such
interference results in asymmetric absorption lines such as Fano resonances,
and gives rise to secondary effects like electromagnetically induced
transparency (EIT) when multiple optical transitions are pumped. Few
solid-state systems show quantum interference and EIT, with quantum-well
intersubband transitions in the IR offering the most promising avenue to date
to devices exploiting optical gain without inversion. Quantum interference is
usually hampered by inhomogeneous broadening of electronic transitions, making
it challenging to achieve in solids at visible wavelengths and elevated
temperatures. However, disorder effects can be mitigated by raising the
oscillator strength of atom-like electronic transitions - excitons - which
arise in monolayers of transition-metal dichalcogenides (TMDCs). Quantum
interference, probed by second-harmonic generation (SHG), emerges in monolayer
WSe2, without a cavity, splitting the SHG spectrum. The splitting exhibits
spectral anticrossing behaviour, and is related to the number of Rabi flops the
strongly driven system undergoes. The SHG power-law exponent deviates strongly
from the canonical value of 2, showing a Fano-like wavelength dependence which
is retained at room temperature. The work opens opportunities in solid-state
quantum-nonlinear optics for optical mixing, gain without inversion and
quantum-information processing.
