13 January 2014 ICFO in Science

Sketch of fermionic many-body spin-changing dynamics.

Giant spin oscillations in an ultracold Fermi sea Understanding collective behavior of ultra-cold quantum gases is of great interest since it is intimately related to many encountered systems in nature such as human behavior, swarms of birds, traffic jam, sand dunes, neutron stars, fundamental magnetic properties of solids, or even super-fluidity or super-conductivity. In all of these everyday life examples, collective behavior plays a crucial role since all participating objects move, voluntarily or not, synchronously.

The Quantum Optics Theory group at ICFO led by ICREA Prof. Maciej Lewenstein, participated in a study with the Institute of Laser Physics, University of Hamburg - Germany, in which they have been able to observe, for the first time, the collective spin dynamics of ultra-cold fermions by analyzing the microscopic properties of the particles through their local collisions. The researchers were able to observe that at very low temperatures, close to absolute zero, the individual properties of each particle team up and behave coherently as a single identity in spin space. When particles with very high spins collide between each other in local interactions, the resulting individual spins change although, in total, they induce a collective behaviour and thus stabilize the gas through long-lived, large-amplitude spin oscillations. The results obtained from this study have recently been published in Science.

To obtain ultra-cold fermions, the researchers trapped, via the use of an optical dipole trap, a quantum degenerate gas containing potassium atoms, cooled it down to very low temperatures and, with the use of radio-frequency pulses and sweeps, prepared different spin mixtures at very low magnetic fields to induce spin-changing dynamics.

Maciej Lewenstein states that “Fermions, due to Pauli’s principle, are ``individualists´´ - they do not like to behave in a same way. Nevertheless, here they team up to exhibit amazingly robust collective behaviour. ”