  We experimentally demonstrate an optical bistability between two hyperfine
atomic ground states, using a single mode of an optical resonator in the
collective strong coupling regime. Whereas in the familiar case, the bistable
region is created through atomic saturation, we report an effect between states
of high quantum purity, which is essential for future information storage. The
nonlinearity of the transitions arise from cavity-assisted pumping between
ground states of cold, trapped atoms and the stability depends on the intensity
of two driving lasers. We interpret the phenomenon in terms of the recent
paradigm of first-order, driven-dissipative phase transitions, where the
transmitted and driving fields are understood as the order and control
parameters, respectively. The saturation-induced bistability is recovered for
infinite drive in one of the controls. The order of the transition is confirmed
experimentally by hysteresis in the order parameter when either of the two
control parameters is swept repeatedly across the bistability region and the
underlying phase diagram is predicted in line with semiclassical mean-field
theory.
