  The dynamics of three interacting objects has been investigated extensively
in Newtonian gravitational physics (often termed the three-body problem), and
is important for many quantum systems, including nuclei, Efimov states, and
frustrated spin systems. However, the dynamics of three macroscopic objects
interacting through quantum vacuum fluctuations (virtual photons) is still an
unexplored frontier. Here, we report the first observation of Casimir
interactions between three isolated macroscopic objects. We propose and
demonstrate a three terminal switchable architecture exploiting opto-mechanical
Casimir interactions that can lay the foundations of a Casimir transistor.
Beyond the paradigm of Casimir forces between two objects in different
geometries, our Casimir transistor represents an important development for
control of three-body virtual photon interactions and will have potential
applications in sensing and information processing with the Casimir effect.
