  Nonlinearity in macroscopic mechanical system plays a crucial role in a wide
variety of applications, including signal transduction and processing,
synchronization, and building logical devices. However, it is difficult to
generate nonlinearity due to the fact that macroscopic mechanical systems
follow the Hooke's law and response linearly to external force, unless strong
drive is used. Here we propose and experimentally realize a record-high
nonlinear response in macroscopic mechanical system by exploring the
anharmonicity in deforming a single chemical bond. We then demonstrate the
tunability of nonlinear response by precisely controlling the chemical bonding
interaction, and realize a cubic elastic constant of \mathversion{bold}$2
\times 10^{18}~{\rm N}/{\rm m^3}$, many orders of magnitude larger in strength
than reported previously. This enables us to observe vibrational bistate
transitions of the resonator driven by the weak Brownian thermal noise at 6~K.
This method can be flexibly applied to a variety of mechanical systems to
improve nonlinear responses, and can be used, with further improvements, to
explore macroscopic quantum mechanics.
