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CQOM Colloquium - Yongchang Zhang, IFA and Thorsten Ackemann, University of Strathclyde

Oplysninger om arrangementet


Onsdag 9. oktober 2019,  kl. 15:00 - 17:00


Fys. Aud.


Yongchang Zhang, IFA

Title: Long-range interactions and critical behavior in Bose-Einstein condensates

Abstract: This talk will present recent results on pattern-formation in long-range interacting ultracold atomic systems. The first part will focus on dipolar Bose-Einstein condensates, for which the possibility of obtaining quantum states with self-organized long-ranged ordering can be facilitated by quantum fluctuations which suppress collapse and pave the way for supersolids. We find that quantum fluctuations can alter the order of the phase transition from first- to second order. Furthermore, apart from the usual triangular lattice of density droplets, quantum fluctuations can give rise to a novel quantum state whose density distribution displays a honeycomb structure. The second part will present photon-mediated effective atom-atom interactions via retro-reflecting mirrors. These lead to a rich variety of patterns of self-bound droplet clusters of ultracold matter waves.

15:15-15:30 Questions and coffee/tea and cake

Invited talk

Thorsten Ackemann, University of Strathclyde

Title: Spontaneous magnetic ordering and crystallization in cold atoms via light mediated interactions

Abstract: Recent years have witnessed considerable interest in simulating the dynamics of complex many-body systems by well-controlled cold atom model systems. Among others, magnetic ordering phenomena and supersolids draw a lot of attention. I will discuss an unconventional approach to light mediated atomic interactions using diffraction of light. Diffractive propagation of periodically modulated light fields leads to an exchange between phase and amplitude modulated planes (Talbot effect) which can be used to couple atomic degrees of freedom. In a feedback scheme this can lead to the spontaneous formation of coupled lattices in the light field and the atomic degrees of freedom. In the experiment a cold cloud of Rb atoms placed near a retro-reflecting mirror is driven by a detuned pump laser. In principle, any degree of freedom on the Poincare sphere of light can be used to tailor coupling to specific atomic degrees of freedom. Via optomechanical nonlinearities, the spontaneous emergence atomic crystals (density patterns) is observed experimentally. The formation of a supersolid is predicted when the thermal atoms are replaced by a quantum degenerate gas.  Via optical pumping nonlinearities magnetic ordering of anti-ferromagnetic and ferromagnetic structures is observed and a connection to the transverse (quantum) Ising model exploited. Finally, novel spontaneous structures in quantum coherences linked to the polarization direction are demonstrated, which correspond to spatial undulations of the direction of the principal axes of magnetic quadrupoles.