Tuning short- and long-range interactions in an ultracold chromium gas

有序偶极系统是众所周知的，例如在固体中作为铁磁体，在液体中作为磁流体。偶极量子气体有望呈现出与凝聚态物理和量子光学相关的丰富的新现象。拟议项目的目标是利用铬原子的稀薄气体来创造和表征简并的玻色子和费米子量子气体。由于铬的大磁矩，偶极-偶极相互作用在这个体系中起着重要的作用。通过将铬冷却到量子简并区，我们将能够观察到偶极-偶极相互作用的影响，而到目前为止，只有S波接触相互作用的影响可以被研究，例如在Rb或钠玻色-爱因斯坦凝聚体中。我们的系统提供了通过使用时变磁场来改变偶极-偶极相互作用的可能性，也提供了通过费什巴赫共振来改变接触相互作用的可能性。这使我们能够独立地控制这两种相互作用，并研究它们对量子气体性质的相互作用。我们将研究玻色子和费米子量子简并气体基态中的偶极序。虽然接触相互作用等价于经典光学中的克尔非线性，从而导致观察到物质-波孤子和涡旋，但在经典光学中没有与长程相互作用等效的相互作用。因此，预计偶极物质波将出现新的现象，这在光学上是没有相似之处的。一个例子可能是远距离量子关联的产生。

Ordered dipolar systems are well known, e.g. in the solid as ferromagnets and in the liquid phase as ferrofluids. Dipolar quantum gases are expected to exhibit a rich variety of new phenomena related to condensed matter physics and quantum optics. The goal of the proposed project is the creation and characterisation of degenerate bosonic and fermionic quantum gases using a dilute gas of chromium atoms. Due to the large magnetic moment of chromium, the dipole-dipole interaction plays an important role in this system. By cooling chromium down into the quantum degenerate regime, we will be able to observe the influence of the dipole-dipole interaction while so far only the effects of the s-wave contact interaction could be studied in e.g. rubidium or sodium Bose-Einstein condensates. Our system offers the possibility to change the dipole-dipole interaction by using time-varying magnetic fields and also the contact interaction by using Feshbach resonances. This allows us to independently control both interactions and to investigate their interplay on the properties of a quantum gas. We will investigate the dipolar order in the groundstate of bosonic and fermionic quantum degenerate gases. While the contact interaction is eqivalent to a Kerr nonlinearity in classical optics, which has lead to the observation of matter-wave solitons and vortices, there is no equivalent to the long-range interaction in classical optics. Thus, new phenomena are expected to emerge for a dipolar matter wave, which have no parallels in light optics. One example could be the generation of quantum correlations over a distance.