Ensemble studies of protein unfolding are typically modeled as two state reactions with a well-defined rate constant. Here we examine whether
this view also holds at the single molecule level. We have developed force-clamp spectroscopy to follow the end-to-end length of single small
proteins during their folding reaction. We first measure the kinetics of unfolding of the protein ubiquitin under a constant force and discover a
surprisingly broad distribution of unfolding rates that follows a power law with no characteristic mean. The structural fluctuations that give
rise to this distribution reveal the architecture of the protein's energy landscape. Following models of glassy dynamics, this complex kinetics
implies large fluctuations in the energies of the folded protein, characterized by an exponential distribution with a width of 5-10 kbT. Our
results predict the existence of a ``glass transition'' force below which the folded conformations interconvert between local minima on multiple
time-scales. Both the unfolding as well as the folding pathways captured by force-clamp spectroscopy are much more complex than the two state model
that is commonly used to interpret such data in classical protein biochemistry. Our results point to the necessity of using statistical
mechanics to fully describe the folding of proteins under a stretching force (Brujic et al, Nature Physics, 2, 282-286, 2006).