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Experimental Studies of Strongly Interacting Quantum Gases in an Optical Lattice

Periodic Reporting for period 1 - RyM (Experimental Studies of Strongly Interacting Quantum Gases in an Optical Lattice)

Reporting period: 2015-05-01 to 2017-04-30

During the action the research fellow has investigated various aspect of Rydberg dressing scheme in many-body systems and non-equilibrium quantum phenomena in a disordered system. The Rydberg dressing technique is essential in realizing a supersolid phase, and the fellow successfully demonstrates this experimental scheme in many-body systems. Furthermore, coherence in the dressed system is recently observed by collapse and revival dynamics. In the experiment, however, the researcher find out the lifetime of the system is significantly shorter than theoretical predictions, which could be attributed to a complex molecular spectrum in dressed system. After finding suitable parameter window that shows enough lifetime, experimental efforts towards the supersolid phase will be continued. Still, the experimental works provide a stepping stone to realize long-range interacting many-body system with Rydberg atoms.
Furthermore, the fellow has observed a many-body localized phase in two-dimensional Bose gas. The many-body localization (MBL) is a new quantum phase in non-equilibrium state, where initial state memory persists for infinitely long time. The major findings in the experiments are as follows. followings are the main questions that we addressed in the experiments. 1) The observation MBL phase transition in two-dimensions 2) Evidence of diverging length scale near the critical disorder. Our findings show the many-body localization is not a theoretical artifact from numerical errors or finite size effect, but it exists in real system. Especially, we have studied the localized phase in two dimensions, where it is impossible to study with the current state-of-art numerical techniques. Thus, our results provide a guide to the development of MBL theory or new tools for simulating complex quantum dynamics.
1. Project - Many-body localization in 2D
January 2015 - April 2015: Preparing experimental setup to demonstrate many-body localized phase.
November 2015 - December 2015: Take date for 2D many-body localization.
January 2016 - April 2016: Writing a paper.
April 2016 - June 2016: Publication the article into Science.
Summary: We successfully show the possible existence of many-body localization and its phase transition in two dimensions.

2. Project - Rydberg dressing in many-body system
May 2015: Characterization of Rabi frequency of UV transition.
June 2015: Search for Rydberg 33P state and investigate atom loss.
July 2015: Search for Rydberg 31P state and investigate atom loss.
August- September 2015: Ramsey spectroscopy to image dressed potential
October 2015 - February 2016: Preparing a manuscript
March 2016 - July 2016: Publication the article into Nature Physics.
Summary: We directly image the long-range interacting potential and show its control-ability.

3. Project - Rotating BEC towards fractional quantum Hall state
January 2016 - April 2016: Design a new experimental setup for rotation
May 2016 - August 2016: Try to nucleate vortex in BEC.
Summary: We conclude the experiment requires a significant improvement to really explore the vortex nucleation, and therefore shift this project to many-body localization.

4. Project - Coherent dynamics in the dressed system
September 2016: Change the beam size of dressing laser and search for Rydberg 31P state transition line.
October 2016: characterize Rabi frequency.
November - December 2016: Measure collapse and revival dynamics.
January 2017 - April 2017: Preparing a manuscript
Summary: We observe a coherent collapse and revival dynamics in the long-range interacting system.

5. Project - Correlation development in Many-body localized phase
November 2016 - December 2016: We measure the density density correlation at various disorder strength.
Summary: We conclude the correlation is highly affected by the system size, and we plan for increasing system size at least factor of three for the clear interpretation.

6. Project - Many-body localization with finite Bath
January 2017: Make disorder potential under control
February 2017: First signature of delocalization of MBL phase by thermal bath.
March 2017: Investigation of long-time relaxation dynamics of non-equilibrium initial state under disorder.
April 2017: We take take for showing the transition behavior as increasing thermal bath fraction.
Summary: We observe a delocalization of localized phase by including small heat bath and will submit the result into research article.

Overview of the results
During the research periods, I have mainly participated four challenging projects of Rydberg system and explore new field of modern statistical physics.
The main results is published in Nature physics and Science journal, which is highlighted as a cover page. The research fellow presents this result in public seminar and many conferences and workshops.

List of seminar, workshops, and conference.

May 2015: Public seminar at Max-planck institute of quantum optics (Long night of Science, Munich)
September 2015: Finite-Temperature Non-Equilibrium Superfluid Systems
September 2015: 2nd Ultracold Quantum Matter workshop
October 2016: Seminar at Seoul National University
December 2015: Seminar at Pohang university of science and technology
October 2015: Korean Physical society Fall meeting
April 2016: German Physical society Spring meeting
July 2016: International Conference on Atomic Physics
February 2017: Royal society meeting: Breakdown of ergodicity in quantum systems
March 2017: German Physical society Spring meeting
April 2017: Korean Physical society Spring meeting
Project: Rydberg atoms
The long-range interacting many-body system is one of the frontiers in atomic quantum matter systems. The experiments have demonstrated the long-range interactions in a many-body system and its directional controllability. This will play an essential building block for studying exotic phases of matter such as supersolid and spin liquids. Furhtermore, the demonstration of coherent dynamics in the dressed system will be further extended to study non-equilibrium phase like floquet time crystal.

Project: Many-body localization
The major technical progress in this experiment is the well-controlled disorder potential with short-range correlation. In our experiment, we develop a new tool to measure the actual on-site disorder potential. From this measurement, we map out the 2D disorder distribution with its correlation is less than one-lattice site. Since the experimental results are far beyond the numerical calculation with current technique, it provides a wonderful bench mark for inventing more efficient algorithm, new theoretical tools and techniques. Furthermore, systematic studies of the effect of incoherent thermal state in the MBL phase might give an opportunity to build a stable quantum storage device that can work in decoherent environments.
Initially prepared atomic gases keep its shape even after long waiting time