Valery G. Rousseau

The universality of time dilation is sometimes misunderstood, with claims that it applies exclusively to light clocks and not to mechanical clocks or other material objects, including biological entities. This leads to incorrect interpretations of experiments like the Hafele–Keating experiment, which actually confirm relativistic effects through the use of atomic clocks. To address this issue, we propose a thought experiment, inspired by Schrödinger’s cat, involving a ‘Sync-or-Die clock’, a device that combines a light clock and a mechanical stopwatch, with the life of Einstein’s cat, Tiger, depending on their synchronization. A straightforward analysis shows that both clocks experience time dilation equally, reinforcing the fact that time dilation is not limited to clocks based on light propagation but is a universal feature of Special Relativity.

Cold atomic gases have proven capable of emulating a number of fundamental condensed matter phenomena including Bose-Einstein condensation, the Mott transition, Fulde-Ferrell-Larkin-Ovchinnikov pairing, and the quantum Hall effect. Cooling to a low enough temperature to explore magnetism and exotic superconductivity in lattices of fermionic atoms remains a challenge. We propose a method to produce a low temperature gas by preparing it in a disordered potential and following a constant entropy trajectory to deliver the gas into a nondisordered state which exhibits these incompletely understood phases. We show, using quantum Monte Carlo simulations, that we can approach the Néel temperature of the three-dimensional Hubbard model for experimentally achievable parameters. Recent experimental estimates suggest the randomness required lies in a regime where atom transport and equilibration are still robust.

Diverse solid and supersolid phases of bosons in a triangular lattice

We study, using quantum Monte Carlo simulations, the bosonic Kondo-Hubbard model in a two-dimensional square lattice. We explore the phase diagram and analyze the mobility of particles and magnetic properties. At unit filling, the transition from a paramagnetic Mott insulator to a ferromagnetic superfluid appears continuous, contrary to what was predicted with mean field. For double occupation per site, both the Mott insulating and superfluid phases are ferromagnetic and the transition is still continuous. Multiband tight-binding Hamiltonians can be realized in optical lattice experiments, which offer not only the possibility of tuning the different energy scales over wide ranges, but also the option of loading the system with either fermionic or bosonic atoms.

We recently studied a doped two-dimensional bosonic Hubbard model with two hard-core species, with different masses, using quantum Monte Carlo simulations [Hettiarachchilage, Phys. Rev. B 88, 161101(R) (2013)]. Upon doping away from half filling, we find several distinct phases, including a phase-separated ferromagnet with Mott behavior for the heavy species and both Mott-insulating and superfluid behaviors for the light species. Introducing polarization, i.e., an imbalance in the population between species, we find a fully phase-separated ferromagnet. This phase exists for a broad range of temperatures and polarizations. By using finite-size scaling of the susceptibility, we find a critical exponent which is consistent with the two-dimensional Ising universality class. Significantly, since the global entropy of this phase is higher than that of the ferromagnetic phase with single species, its experimental observation in cold atoms may be feasible.