Collective optical effects of ultra-cold Rydberg atoms in optical cavities

该项目旨在研究里德伯原子的强中程偶极-偶极相互作用对光学腔内超冷原子的集体内外动力学的影响。众所周知，当激光从腔体侧面照射腔体中的原子时，它们会自组织成周期性结构。自组织过程是由光触发的，这些光被集体散射到腔中，被腔镜回收，并机械地作用于原子。这种由空腔介导的原子间相互作用是无限范围的。里德伯原子的额外相互作用使新物理学开始发挥作用。在中间距离，里德伯原子之间的偶极-偶极相互作用抑制了多个里德伯原子的激发，即里德伯封锁。因此，在里德伯泡内，光散射是一个高度非线性的过程，光散射到腔内是单个里德伯泡而不是单个原子的集体现象。我们将研究Rydberg封锁对新出现的自组织结构以及散射到腔内的光的强度和相位的影响。此外，我们将研究额外的相互作用是否会导致原子云的新稳定相，这取决于封锁半径和原子云作为一个整体的大小。内部动力学是由这样一个事实决定的，即腔模式与原子基态到中间能级的跃迁是近共振的，具有集体增强的耦合。中间态又通过强外部耦合激光场耦合到里德伯态。对于中间能级的大失谐，动力学是一个有效的两能级系统，其中原子和腔之间的耦合是由双光子跃迁介导的。因此，耦合强度可以通过耦合激光的强度来调节，而腔只与较低跃迁的光子共振。这一事实是值得注意的，并且与通常情况下腔场与单光子跃迁共振的情况有所不同。利用双光子跃迁，耦合强度可以在时间和空间上进行外部控制，这对于量子技术的应用来说是一个非常有趣的特性，例如量子存储器。我们将通过在基态和里德伯态之间展示腔光和原子双光子跃迁的强集体耦合，为这种未来的应用奠定基础。这将通过使用脉冲激励方案观察真空拉比振荡来实现。

The project aims at studying the influence of strong intermediate-range dipole-dipole interactions of Rydberg atoms on the collective external and internal dynamics of ultracold atoms inside an optical cavity. Atoms in cavities are known to self-organize in periodic structures when they are illuminated from the side of the cavity by laser light. The self-organization process is triggered by the light which is collectively scattered into the cavity, recycled by the cavity mirrors, and mechanically back acts on the atoms. This cavity-mediated interaction between the atoms is infinite-range. New physics comes into play by the additional interactions of Rydberg atoms. At intermediate distance dipole-dipole interactions between Rydberg atoms suppress the excitation of more than one Rydberg atom, i.e. Rydberg blockade. Thus, within a Rydberg bubble light scattering is a highly nonlinear process, and scattering of light into the cavity is a collective phenomenon of the individual Rydberg bubbles rather than of the individual atoms. We will study the effect of Rydberg blockade on the emerging self-organized structures and on the intensity and phase of the light scattered into the cavity. Furthermore, we will study if the additional interaction can lead to new stable phases of the atom cloud, depending on the size of the blockade radius and of the atom cloud as a whole.The internal dynamics is governed by the fact that the cavity mode is near-resonant to the transition from the atomic ground state to an intermediate level, with collectively enhanced coupling. The intermediate state in turn is coupled to a Rydberg state by a strong external coupling laser field. For large detuning from the intermediate level, the dynamics is that of an effective two-level system, where the coupling between the atoms and the cavity is mediated by a two-photon transition. The coupling strength can thus be tuned by the intensity of the coupling laser, whereas the cavity is only resonant to the photons of the lower transition. This fact is remarkable and makes a difference to the usual case where the cavity field is resonant to a single photon transition. Using two-photon transitions the coupling strength can be externally controlled both in time and in space, which is a very interesting feature for applications in quantum technology, as for instance quantum memories. We will lay the fundament for such future applications by demonstrating strong collective coupling of cavity light and atomic two-photon transitions between the ground state and Rydberg states. This will be achieved by observing vacuum Rabi oscillations using pulsed excitation schemes.