Minimally entangled typical quantum states at finite temperature

We introduce a class of states, called minimally entangled typical thermal states (METTS), designed to resemble a typical state of a quantum system at finite temperature with a bias towards classical (minimally entangled) properties. These states reveal in an intuitive way properties such as short-range order which may be hidden in correlation functions. An algorithm is presented which, when used with the density matrix renormalization group (DMRG), is faster by a factor of $10^3 - 10^{10}$ than previous heat-bath approaches for thermally averaged quantities.