  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 many
demonstrations. 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 a Casimir diode that achieves quantum
vacuum mediated non-reciprocal topological 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 include an exceptional point
in the parameter space and observe the topological structure near it. By
dynamically changing the parameters and encircling the exceptional point, we
achieve non-reciprocal topological 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.
