## Rydberg atoms and quantum chaos

**A Rydberg atom is an atom in which one or more electrons are excited to states with high principal quantum number n.. For example, Rydberg atoms with n ~300 have the Bohr radius of ~5µm, and according to Kepler's third law, an orbital period of ~4 nanoseconds. In such a "slow" time scale, it is possible to tailor arbitrary electric field pulses that allow to control and manipulate the electron wave function as well as to probe the product states. Even though atoms of such mesoscopic scale manifest many features of classical particles, their quantum (wave-like) nature is equally present. Therefore, Rydberg atoms can be used to study quantum chaos. By extending to many-body dynamics, the effects of interactions with the environment on the coherence of quantum state engineering can be studied.**

**Bohr wavepacket **

With recent development in the wavepacket engineering, it has become possible to "design" various wavefunctions using Rydberg atoms. One example is the so-called Bohr wavepacket. A localized electron wavefunction travels along a circular orbit realizing the Bohr model of hydrogen suggested over 100 years ago. The created wavepacket can be further manipulated using external electric fields. The figure shows that the circular orbit can be enlarged by a chirped electric field. The dynamics governing this control scheme resembles that of Trojan asteroids. This work is done in collaboration with the group of Prof. Dunning from Rice University.

Selected publication:

Creating and transporting Trojan wavepackets

B. Wyker et al. Phys. Rev. Lett. 108, 043001 (2012)

**Coherent control of Rydberg atoms**

While the manipulation of wavefunctions in a single Rydberg atom has been studied intensively, the study can be extended to many-body systems. Coherent control of multiple Rydberg atom excitation and creation of dressed states have many potential applications, such as quantum information processsing and quantum simulator. This research is done in collaboration with Prof. Dunning from Rice University.

Selected publications:

Efficient three-photon excitation of quasi-one-dimensional strontium Rydberg atoms with n ~ 300

S. Ye et al.Phys. Rev. A 90 013401 (2014).

Characterizing high-n quasi-one-dimensional strontium Rydberg atoms

M. Hiller et al. Phys. Rev. A 89 0023426 (2014).

Production of very-high-n strontium Rydberg atoms

S. Ye et al. Phys. Rev. A 88 043430 (2013).

**Particle-like states in quantum billiards**

The wave-particle duality of quantum mechanics adresses the inability of classical concepts like "wave" or "particle" to explain the behavior of quantum mechanical systems. We investigate this crossover between the wave nature of quantum mechanics and the particle picture of classical mechanics by shaping waves to behave like particles: the image shows a wave scattering through a rectangular cavity with disorder (see lower panel for the disorder landscape). By suitably shaping the incoming wavefront, the wave can be made to follow the zig-zag orbit of a particle. Further research is directed towards understanding the propagation of waves through disordered media, or at metal surfaces. This research is done in collaboration with the group of Prof. Rotter.

Selected publications:

Generating Particlelike Scattering States in Wave Transport

S. Rotter, P. Ambichl, and F. Libisch Physical Review Letters 106, 120602 (2011)