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

该项目旨在研究里德堡原子中强的中程偶极-偶极相互作用对光学腔内超冷原子集体外部和内部动力学的影响。当激光从腔的侧面照射时，腔中的原子会以周期性结构自组织。自组织过程是由集体散射到腔内的光触发的，由腔镜循环，并机械地反向作用于原子。这种由腔介导的原子间相互作用是无限范围的。通过里德堡原子的额外相互作用，新物理学开始发挥作用。在中等距离，里德堡原子之间的偶极-偶极相互作用抑制了多个里德堡原子的激发，即里德堡阻塞。因此，在里德堡气泡中，光散射是一个高度非线性的过程，而进入腔体的光散射是单个里德堡气泡而不是单个原子的集体现象。我们将研究里德堡阻塞对新出现的自组织结构的影响，以及对散射入腔的光的强度和相位的影响。此外，我们将研究附加的相互作用是否会导致原子云的新的稳定相，这取决于封锁半径的大小和整个原子云的大小。内部动力学取决于这样一个事实，即腔模与从原子基态到中间能级的跃迁是近共振的，并且集体增强了耦合。中间态又通过强的外部耦合激光场耦合到里德堡态。对于中能级的大失谐，动力学是一个有效的二能级系统，其中原子与腔之间的耦合是通过双光子跃迁来调节的。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，并使不同于通常的情况，即腔场与单光子跃迁共振。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征，例如量子存储器。我们将通过展示腔光和原子双光子跃迁在基态和里德堡态之间的强集体耦合，为未来的应用奠定基础。这将通过使用脉冲激发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.