Building a crystal out of quantum bricks

Preparation of strongly interacting many-body phases starting from a weakly interacting quantum gas is known to be very challenging. It involves crossing a phase transition, where the energy gap protecting the ground state becomes vanishingly small for large systems. In a recent publication in Physical Review Letters, we report on a recipe to overcome this problem. In our work, we study the formation of dipolar spin crystals of ultracold polar molecules trapped in an optical lattice. We demonstrate that one can achieve a substantially better growth of the emergent spin crystal using non-adiabatic driving of rotational transitions.

Our proposed technique builds on the dipole blockade of microwave excitations. In the context of an effective spin-1/2 model, the dipole blockade allows to flip particular spins in an optical lattice without a need for single-site addressing. The growth of the spin crystal is started by a short sequence of microwave pulses that create a few flipped spins, serving as a nucleation center of the crystalline phase. After that, continuous microwave driving propagates the boundary of the crystal, adding one additional flipped spin after the other.

We investigate the dynamics under the continuous driving and demonstrate that the growth of the ordered domains is efficient. The analysis of the imperfections and the required experimental parameters shows that structures consisting of more than 1,000 spins can be grown. The average size of the resulting domains scales substantially more favorable compared to the conventional adiabatic preparation.





Reference: Mikhail Lemeshko, Roman V. Krems, and Hendrik Weimer,
"Nonadiabatic Preparation of Spin Crystals with Ultracold Polar Molecules",
Phys. Rev. Lett. 109, 035301 (2012).


About the authors: Misha Lemeshko and Hendrik Weimer are the ITAMP Postdoctoral Fellows at the Harvard-Smithsonian Center for Astrophysics and Harvard Physics Department; Roman Krems is an Associate Professor of Chemistry at the University of British Columbia in Vancouver.