# LL Lab

^{87}Rb Bose-Einstein Condensation

In our lab, we focus on studying many-body entanglement realized by ultracold atoms, which is a powerful tool in both fundamental researches and physical applications.

### In our lab, we focus on studying many-body entanglement realized by ultracold atoms, which is a powerful tool in both fundamental researches and physical applications.

Entanglement can be used to characterize topological orders, which are new phases that cannot be described by the symmetry breaking. There are many interesting, important, and counter-intuitive phenomena that were discovered in the past few years, such as quantum Hall effects, topological insulators, and new types of superconductors. And we want to engineer an artificial, universal, and versatile platform in order to synthesize novel quantum matters that have never been found before.

On the other hand, entanglement is also a useful resource in the applications of quantum metrology and quantum computation. By integrating the quantum entanglement with precision measurement, we can design and build sensors that beyond the standard quantum limit, a fundamental limit of current precision measurement. We have created the world record in year 2015 (Nature 519, 439), that 3000 atoms are all entangled together with a negative probability distribution function measured. In the future, we will work on building larger systems with more particles entangled together and use them in precision measurement. Furthermore, we will work on apparatus as nodes of quantum networks and quantum computers, following the first single-photon transistor (Science 341, 768) we have invented a few years ago.

#### 1. Manipulation of single atom and synthesis of artificial materials

This platform is to realize the ability of manipulating each individual atom at low entropy, which can be achieved by optical tweezers and a new method of laser cooling (Science 358, 1078). By cooling one atom directly in each optical tweezers, we can re-assemble the atomic ensemble back to a condensate that has the absolute zero temperature. The symmetry of atoms array is no longer limited by the standing wave of optical lattices and it could be any type such as hexagonal lattice, five-folds quasi-crystal, and lattices with defects. Combining the techniques of quantum gas microscopes and Feshbach resonance, we can even create any type of materials according to our design with adjustable mutual interactions and correlations.

By integrating the ideas of machine learning (arXiv 1803.01786, in review), we can further recognize novel quantum phases with the assistance of artificial intelligence.

#### 2. Quantum metrology and sensing with entanglement

This experiment is to apply the quantum entanglement in metrology and sensing. We will design a new generation of optical resonators (Febry-Pérot cavities) to realize strong interaction between single photon and single atom. By using the cavity as a medium, each individual atom can interact with the other atoms with an infinite interaction-range, while the photons will also interact with each other strongly (Phys. Rev. Lett. 113, 113603). With the assistance of mutual strong interaction, the system will evolve into a deep-entangled state. The uncertainty during the measurement goes beyond the standard quantum limit that is defined by the central limit theorem in math. The entanglement will help us to beat the classical statistics and push the metrology into a new regime – the quantum era.

Meanwhile, by combining the up-to-date techniques such as digit mirror devices, quantum gas microscopes, and quantum devices, we can probe each atom individually or collectively. This will help us to figure out open questions in quantum physics, e.g., how entanglement (or information) is created, spread or annihilated.