Scientific adventure

I have solid research background in ultracold atom experiments. I have worked for two years as research assistant at Centre of Quantum Technologies in Singapore followed by three years as PhD researcher at Laboratory Charles Fabry in Palaiseau, France.

(Ultra)Cold atoms

Cold atoms refer to temperature below microKelvin. They are in the gaseous phase and are obtained by employing laser cooling and trapping techniques. At typical temperature of few nanoKelvins, the gas can behave as quantum gas, for example the phase of Bose-Einstein Condensation (BEC). The ultracold gas offers a convenient tool to study quantum phenomena.

My PhD project

My PhD project aimed to study experimentally the phenomenon of Anderson localization with ultracold atoms. The phenomenon is associated with propagation of wave in disordered systems. In our experiment, we realized matter-wave with atomic cloud with temperature of few nanoKelvin, even few hundreds of pikoKelvin. The atoms are in the state of Bose-Enstein condensation. We also realized artificial disorder for the matter wave using laser speckle. In the project, we have developed techniques allowing fine tuning of the matter-wave energy in the disorder.

Anderson localization

It has been studied for more than 60 years! It is named after physicist Philip W. Anderson who firstly accounted for the phenomenon; he also received the Nobel prize in physics in 1977 for his work in this field. Furthermore, it has relevance in many scientific fields. It can apply to electromagnetic wave, acoustic wave, seismic wave, and electronic wave.

In order to understand the phenomenon, we need to consider the expansion of the wave packet in the presence of disorder; the disorder slows down its expansion. Anderson localization manifests as vanishing expansion, i.e. wave localization. Such description provides a new quantum mechanical view of metal-insulator transitions.

In short, the phenomenon presents tremendous theoretical aspects and challenges for experimentalists to realize the appropriate conditions that allow the observation of the localization signature.

Anderson (phase) transition

My PhD work deals with the aspect of phase transition associated with the Anderson localization phenomenon in three-dimensional systems. In certain semiconductor systems, such transition is associated with transition between metallic behavior and insulating behavior.

The phase transition can naively be analogous to transition between water and ice. The metallic behavior is analogous to the liquid phase (water), wave packet "fills up" the space. The insulating behavior is analogous to the solid phase (ice), wave packet is "frozen". We remark certain boundary between the phases.

For our experiment, we are interested in the description of the boundary in terms of the energy of the wave packet. The boundary is called the "mobility edge". Precise and direct measurement of this mobility edge requires high degree of control of the matter wave energy.

Anderson localization of ultracold atoms

In ultracold atom experiments, the localization signature can be directly observed from the imaging of the atomic cloud expansion. For this, we need to deduce the expansion rate from the dependence of the cloud size on the duration of introduction of the laser speckle disorder. The onset of localization can be inferred from the absence of expansion after long duration, typically several seconds. Experiments with ultracold atoms offer the possibility of direct observation with controllable disorder.

Despite several experimental advances in this field, there is a remaining challenge of direct measurement of the mobility edge. In the precedent experiments, there was impeding limitation on the energy resolution due to the broadening effect of the energy distribution resulting from the introduction of the disorder. As consequences, the mobility edge could not be measured directly and the precision was rather poor. It motivates me and my colleagues to develop experimental techniques allowing both resolution and fine-tuning of the energy distribution. It constitutes a spectroscopical approach to study the Anderson transition.

The second feature is the creation of artificial disorder that is selective in spin state. The blue mountainous landscape on the upper level illustrates the disorder that is only felt by the atoms when they are on the upper spin states. This is our trick to avoid the previously mentioned issue on disorder-induced energy broadening.

Lastly, the green arrow represents the application of known radio-frequency transfer technique that allow us to transfer the atoms from the lower level to the upper level at desired energy. The energy resolution can now be made very narrow.