原子冷却技术的发展

新的物理现象的发现往往得益于新实验技术的发明,制冷技术的进步推动了包括凝聚态物理学和原子物理学等现代科学多个领域的重要发现,并促进了超导强磁铁、冷冻电镜等需要极低温度条件的新技术的发展.近年来,随着激光冷却技术的发明和不断发展,人们得以在极端低温下开展统计力学和量子力学相关的实验研究,迄今,人们已经实现了玻色-爱因斯坦凝聚态这种新奇的物态,并掌握了在单原子尺度开展量子调控研究的能力.同时,由于描述量子多体系统的希尔伯特空间的维度随系统粒子数呈指数增长,即便使用经典超级计算机处理此类问题也仍面临巨大困难,这使得基于超冷原子、离子、超导等体系的量子模拟研究成为热点.人们通过前所未有的调控能力制造人工量子系统,再直接调控并观测其量子相变过程,这为研究强关联量子系统提供了一条崭新的途径.在获得极限低温的道路上,基于热力学定律的传统制冷技术能够达到的温度极限在mK量级,但激光冷却技术却另辟蹊径,巧妙地运用光与原子的相互作用,将原子的温度降低到nK量级,这大大推动了基于超冷原子的量子模拟研究的发展.尽管激光冷却技术获得的超冷原子的温度是传统制冷技术远不能及的,但由于中性原子间相互作用强度很弱,转换成温度一般在nK级别,这意味着要观测超冷原子强关联体系中的量子多体行为,就需要进一步降低原子体系温度以减小热涨落带来的影响,这也是当前超冷原子量子模拟研究中最关键的问题之一.在本文中,我们对原子冷却技术的发展进行了回顾,总结了20世纪70年代至今超冷原子技术的突破性进展,并从调控体系的熵的角度分析并展望了超冷原子低温技术未来发展方向.

Progress on Atom Cooling techniques

Discovery of new physical phenomena often benefits from invention of novel experimental techniques. The development of cooling techniques has been advancing many areas in physics including condensed matter atomic physics, and so on. It has given birth to superconducting magnets, cryoelectron microscopy, and important discoveries and technologies which are restricted in low-temperature environment. The past decades witnessed a prodigious advance of laser cooling technology, based on which scientists have been able to carry experimental studies on quantum mechanics and statistical mechanics at extremely low temperatures. For example the novel matter of Bose-Einstein condensation has been realized, and coherent control of many-body quantum systems can be performed at single-atom level. Furthermore, the dimension of the many-body Hilbert space eases exponentially with the number of particles in a quantum system, which results in a formidable task for numerically modeling the system consisting a large number of particles with classical supercomputers. Quantum simulations based on ultracold atoms, superconductors, and etc. open an avenue for efficiently solving the above hard problems, becoming hot research topics gradually. Nowadays, we can create artificial quantum systems with unprecedented capabilities of coherent control, thereafter drive and observe quantum phase transitions, which provides a new way to study strongly correlated quantum systems. With traditional cooling techniques based on thermodynamics, the temperature limit that can be reached is at the level of millikelvin. In contrast, laser cooling techniques are based on the interaction between light and atoms, with which the temperature of ultracold atoms has reached the order of nanokelvin. This progress has greatly promoted the researches of quantum simulation with ultracold atoms. Although the temperature of nanokelvin is far below that achieved with traditional cooling techniques, it is necessary to further cool the atoms for strengthen the quantum effect as the interacting energy between neutral atoms is comparable with the thermal energy of atoms at nanokelvin. Therefore, an even lower temperature would suppress thermal fluctuations while enhance quantum fluctuations. Deep cooling becomes one of the most critical topics in quantum simulation with ultracold atoms. In this paper, we review atom cooling techniques developed since the 1970s, and conclude with an outlook about the future of ultracold atom physics from the perspective of engineering entropy of quantum systems