Quantum Simulators at Negative Absolute Temperatures
  We propose that negative absolute temperatures in ultracold atomic clouds in optical lattices can be used to simulate quantum systems in new regions of phase diagrams. First we discuss how the attractive SU(3) Hubbard model in three dimensions can be realized using repulsively interacting 173-Yb atoms, then we consider how an antiferromagnetic S=1 spin chain could be simulated using spinor 87-Rb or 23-Na atoms. The general idea to achieve negative absolute temperatures is to reverse the sign of the external harmonic potential. Energy conservation in a deep optical lattice imposes a constraint on the dynamics of the cloud, which can relax toward a T<0 state. As the process is strongly non-adiabatic, we estimate the change of the entropy. 
Akos Rapp
cond-mat.quant-gas