Department of Physics, Harvard University, 17 Oxford Street, Cambridge, Massachusetts 02138, USA.
Nature. 2011 Apr 21;472(7343):307-12. doi: 10.1038/nature09994. Epub 2011 Apr 13.
Understanding exotic forms of magnetism in quantum mechanical systems is a central goal of modern condensed matter physics, with implications for systems ranging from high-temperature superconductors to spintronic devices. Simulating magnetic materials in the vicinity of a quantum phase transition is computationally intractable on classical computers, owing to the extreme complexity arising from quantum entanglement between the constituent magnetic spins. Here we use a degenerate Bose gas of rubidium atoms confined in an optical lattice to simulate a chain of interacting quantum Ising spins as they undergo a phase transition. Strong spin interactions are achieved through a site-occupation to pseudo-spin mapping. As we vary a magnetic field, quantum fluctuations drive a phase transition from a paramagnetic phase into an antiferromagnetic phase. In the paramagnetic phase, the interaction between the spins is overwhelmed by the applied field, which aligns the spins. In the antiferromagnetic phase, the interaction dominates and produces staggered magnetic ordering. Magnetic domain formation is observed through both in situ site-resolved imaging and noise correlation measurements. By demonstrating a route to quantum magnetism in an optical lattice, this work should facilitate further investigations of magnetic models using ultracold atoms, thereby improving our understanding of real magnetic materials.
理解量子力学系统中的奇异形式的磁性是现代凝聚态物理的一个核心目标，其应用范围涵盖了从高温超导体到自旋电子器件等各种系统。由于组成磁自旋之间的量子纠缠所带来的极端复杂性，在经典计算机上模拟量子相变附近的磁性材料在计算上是难以处理的。在这里，我们使用铷原子的简并玻色气体在光晶格中进行限制，以模拟经历相变的相互作用量子伊辛自旋链。通过占据位点到赝自旋的映射来实现强自旋相互作用。随着磁场的变化，量子涨落驱动从顺磁相到反铁磁相的相变。在顺磁相中，自旋之间的相互作用被外加磁场所压倒，该磁场使自旋对齐。在反铁磁相中，相互作用占主导地位，并产生交错的磁有序。通过原位局域成像和噪声相关测量观察到磁畴的形成。通过在光晶格中实现量子磁性的途径，这项工作应该有助于使用超冷原子进一步研究磁性模型，从而提高我们对真实磁性材料的理解。
