  A fundamental prediction of quantum mechanics is that there are random
fluctuations everywhere in a vacuum because of the zero-point energy.
Remarkably, quantum electromagnetic fluctuations can induce a measurable force
between neutral objects, known as the Casimir effect, which has attracted broad
interests. The Casimir effect can dominate the interaction between
microstructures at small separations and has been utilized to realize nonlinear
oscillation, quantum trapping, phonon transfer, and dissipation dilution.
However, a non-reciprocal device based on quantum vacuum fluctuations remains
an unexplored frontier. Here we report quantum vacuum mediated non-reciprocal
energy transfer between two micromechanical oscillators. We modulate the
Casimir interaction parametrically to realize strong coupling between two
oscillators with different resonant frequencies. We engineer the system's
spectrum to have an exceptional point in the parameter space and observe the
asymmetric topological structure near it. By dynamically changing the
parameters near the exceptional point and utilizing the non-adiabaticity of the
process, we achieve non-reciprocal energy transfer with high contrast. Our work
represents an important development in utilizing quantum vacuum fluctuations to
regulate energy transfer at the nanoscale and build functional Casimir devices.
