Pan Jianwei of the University of Science and Technology of China and his colleagues Chen Yuxi, Yao Xingcan and Tsinghua University, and the research team of Renmin University Qiqi and Zhang Wei have made important progress in the research of ultra-cold atomic quantum simulation: they are in Bose-Ein for the first time. An extremely wide d-wave shape-scattering resonance was observed in the Stein condensate, and the existence of the super-flow of the d-wave molecule was indirectly proved.
This experiment has brought new opportunities and challenges to the ultra-cold atom quantum simulation research, and paved the way for the study of low-order and multi-body quantum physics dominated by high-order partial wave interactions. Recently, the results were published in the long-formed international journal "Nature Physics 15, 570– 576 (2019)].
Collision scattering between particles is a fundamental and important interaction. Whether it is the generation of elements at the beginning of the universe or the chemical reactions in daily life, in principle, it can be described by the quantum theory of scattering.
According to the symmetry of the scattering wave function, we can divide the scattering process into isotropic s-waves and anisotropic p-waves, d-waves and other high-order partial waves. Compared with s-wave scattering, quantum multibody systems dominated by higher-order partial waves exhibit more interesting phenomena, such as p-wave superfluids in 氦3, and d-wave Cooper pairs in copper oxide high-temperature superconductors. And a wide variety of biological, chemical and other dynamic processes. Unfortunately, due to the complexity of the scattering process of high-order partial waves, the resources required for theoretical calculations greatly exceed the capabilities of classical calculations, which seriously hinders our understanding of related physical phenomena.
Thanks to the purity of the system and the rich handling and detection technology, ultra-cold atomic quantum simulation provides a new tool for solving these problems. For example, using the s-wave scattering resonance between atoms, we can precisely adjust the intensity and form of interaction between ultra-cold atoms, thus achieving novel quantum phenomena such as Bose supernova, quantum fireworks, and Fermi superfluid and supercooled. Important quantum states such as molecules.
However, due to the difficulties in theory and experiment, more important high-order partial-wave resonance studies are still rarely carried out. There are mainly the following problems: 1) Ultra-cold atoms under quantum degeneracy often do not have enough The kinetic energy crosses the centrifugal barrier of the high-order partial wave, so that strong interaction cannot be achieved. 2) The high-order partial-wave resonance is usually extremely narrow, and the existing experimental methods cannot use it to accurately control the interaction between atoms; 3) the atomic group The life near the resonance is very short and no effective research can be carried out.
In this work, the research team first observed an extremely wide d-wave shape resonance in the Bose-Einstein condensate. They found that near the scattering resonance (strong interaction zone), the lifetime of Bose Einstein condensates is still hundreds of milliseconds, much larger than the equilibrium time of multibody systems. Therefore, the d-wave resonance has three elements of super-cold, wide resonance bandwidth and long life, which provides an excellent platform for quantum simulation research based on d-wave interaction.
In further research, it was found that when the scanning magnetic field passes through the resonance point at a certain rate, the system realizes the coherent transformation between the atom and the d-wave molecule, spontaneously exhibiting a long-life (second order) collective excitation oscillation. By carefully measuring the relationship between the frequency, amplitude, atomic number of the collective oscillation and the scanning magnetic field rate, the research team proved that there are already a large number of ultra-cold d-wave molecules inside the system.
Although there is currently no technology for direct detection of this d-wave molecule, researchers can only obtain information on the number of d-wave molecules by fitting the collective excitation. Their preliminary results clearly indicate that these d-wave molecules have formed a A new quantum state — — d wave molecular superfluid. Therefore, this work also laid the foundation for the future study of d-wave molecular superfluids.
The above work was supported by the Chinese Academy of Sciences, the Ministry of Science and Technology, the Natural Science Foundation and Anhui Province.