  Broadband homodyne detection of the light transmitted by a Fabry-Perot cavity
containing a strongly-coupled $^{133}$Cs atom is used to probe the dynamic
optical response in a regime where semiclassical theory predicts bistability
but strong quantum corrections should apply. While quantum fluctuations
destabilize true equilibrium bistability, our observations confirm the
existence of metastable states with finite lifetimes and a hysteretic response
is apparent when the optical drive is modulated on comparable timescales. Our
experiment elucidates remnant semiclassical behavior in the attojoule ($\sim10$
photon) regime of single-atom cavity QED, of potential significance for
ultra-low power photonic signal processing.
