  We experimentally demonstrate collective strong coupling, optical
bi-stability (OB) and all-optical switching in a system consisting of ultracold
85Rb atoms, trapped in a dark magneto-optical trap (DMOT), coupled to an
optical Fabry-Perot cavity. The strong coupling is established by measuring the
vacuum Rabi splitting (VRS) of a weak on-axis probe beam. The dependence of VRS
on the probe beam power is measured and bi-stability in the cavity transmission
is observed. We demonstrate control over the transmission of the probe beam
through the atom-cavity system using a free-space off-axis control beam and
show that the cavity transmission can be switched on and off in micro-second
timescales using micro-Watt control powers. The utility of the system as a tool
for sensitive, in-situ and rapid measurements is envisaged.
