  We observe and characterize intrinsic optical bistability in a dilute Rydberg
vapor. The bistability is characterized by sharp jumps between states of low
and high Rydberg occupancy with jump up and down positions displaying
hysteresis depending on the direction in which the control parameter is
changed. We find that the shift in frequency of the jump point scales with the
fourth power of the principal quantum number. Also, the width of the hysteresis
window increases with increasing principal quantum number, before reaching a
peak and then closing again. The experimental results are consistent with
predictions from a simple theoretical model based on semiclassical
Maxwell-Bloch equations including the effect of broadening and frequency
shifts. These results provide insight to the dynamics of driven dissipative
systems.
