New techniques for nanokelvin condensed matter physics

对越来越冷的温度的追求导致了许多非凡的发现。最难液化的气体是氦，但这最终在20世纪初实现了。这一突破导致观察到了有趣的超流性现象(一种液体在没有摩擦的情况下流动，就像在超导体中流动的电流没有阻力一样)，现在用氦制冷具有基本的技术重要性，例如对于大型国际公司，如牛津仪器公司。在20世纪末，发展了新的技术，利用激光冷却原子(尽管这看起来可能有悖常理)，以及将原子限制在非常好的真空区域的磁陷阱。这项新技术允许在较低温度下制备稀碱金属原子蒸气。令人惊讶的是，这种金属原子没有聚集在一起(形成分子)，蒸发进一步将云层冷却到几十纳米开尔文的极低温度。正如著名物理学家爱因斯坦和玻色预言的那样，这使得第一次实验观察到了弱相互作用稀薄气体中的玻色-爱因斯坦凝聚(BEC)。这一非常有趣的量子现象与之前关于超流氦的工作有关，但液体比超冷原子气体更难理解。使用为天文学和显微镜开发的最先进的相机可以拍摄到非常详细的冷原子图像，这种直接看到量子流体中正在发生的事情的能力使人们在理解这些系统方面取得了非常迅速的进展，并提供了丰富的新知识。这一点在BEC上的第一个实验中就很明显，在这个实验中，从普通气体到量子体系的相变被观察到密度和形状发生了戏剧性的变化。我们已经建立了一种新的设备，在这种设备中，原子通过磁场移动时所经历的势能图景是通过应用射频辐射以一种精确可控的方式定制的。(所施加的辐射改变了原子在空间中特定位置的量子状态，从而改变了它们感受到的势能。)事实证明，这对于二维系统和创建有趣的几何图形特别有用，比如环形陷阱(在环路周围有量子相干)。这种方法的一个很大的优点是，与用激光捕获原子时产生干涉条纹相比，这种方法的电势非常平滑且没有缺陷。我们将这种新的方法与时间平均相结合，以允许在陷阱中有更大范围的潜力和更长的寿命。我们将继续开发和测试捕获原子的新方案，例如创建双环(原子在两个同心圆上)，并在存在相互作用共振增强的磁场中捕获原子(Fano-Feshbach共振)。我们将把这项技术应用于强关联系统的直接量子模拟，并探索这种物质波干涉术在精密测量设备中的应用。在将原子气体冷却到纳开尔文温度方面的这一进展，现在使我们能够实现理查德·费曼的说法：“因此，我相信，用合适的量子机器类别，你可以模拟任何量子系统，包括物理世界。”这经常引用在量子信息处理的背景下，但它更直接地应用于可控量子系统的创建，在这些系统中，我们设计量子哈密顿量，使其看起来与感兴趣的物理系统的哈密顿量相同。这是我们直接量子模拟工作的基本原理。在这种情况下，量子力学被用来设计量子机，即一种可控制地创建多体量子态的设备，就像汽车力学用于制造车辆一样。

The quest for colder and colder temperatures has led to many remarkable discoveries. The most difficult gas to liquify is helium but this was finally achieved at the beginning of the 20th century. That breakthrough led the observation of the intriguing phenomena of superfluidity (a liquid that flows without friction just like the current in a superconductor flows without resistance) and nowadays refrigeration with helium is of fundamental technological importance, e.g. for large international companies such as Oxford Instruments. At end of the the 20th century new techniques were developed that used laser light to cool atoms (although this may seem counterintuitive) and magnetic traps that confine the atoms in a region of very good vacuum. This new technology allowed dilute vapours of alkali metal atoms to be prepared a low temperatures. Amazingly this atoms of metal do no clump together (to form molecules) and evaporation cools the cloud further to extremely low temperatures of tens of nanokelvin. This allowed the first experimental observation of Bose-Einstein condensation (BEC) in a weakly interacting dilute gas, as predicted by the famous physicists Einstein and Bose. This extremely interesting quantum phenomenon has links with previous work on superfluid helium but the liquid is more complicated to understand than the ultra-cold atomic gases. Wonderfully detailed images of the cold atoms can be taken using state-of-the-art cameras developed for astronomy and microscopy and this ability to see directly what is going on in quantum fluids has allowed very rapid progress in understanding these systems and provided a wealth of new knowledge. This was evident in the very first experiment on BEC where the phase transition from an ordinary gas to the quantum regime was observed as a dramatic change in the density and shape.We have built a novel apparatus in which in the the potential energy landscape that atoms experience as they move through the magnetic field is tailored in a precisely controllable way by the application of radio-frequency radiation. (The applied radiation changes the quantum state of the atoms at particular positions in space hence changing the potential they feel.) This has proven to be particularly useful for two-dimensional systems and for creating interesting geometries such as ring traps (with quantum coherence around the loop). A great advantage of this approach is that the potential is very smooth and free from defects, as compared to trapping atoms with laser light where interference fringes arise. We have combined this new approach with time-averaging to allow an even greater range of potentials and long lifetimes in the traps. We shall continue to develop and test new schemes for trapping atoms, such as the create of double rings (atoms on two concentric circles) and trapping atoms at magnetic fields where there is resonant enhancement of the interactions (Fano-Feshbach resonance). We shall apply this technology to the direct quantum simulation of strongly correlated systems, and explore applications such a matter-wave interferometry for precision measuring devices. This progress in cooling atomic gases to nanokelvin temperatures now allows us to fulfill the statement of Richard Feynman,``I therefore believe it's true that with a suitable class of quantum machines you could imitate any quantum system, including the physical world''. This is often quoted in the context of quantum information processing but it applies more directly to the creation of controllable quantum systems in which we engineer the quantum Hamiltonian so that it looks the same as that of the physical system of interest. This is the underlying principle of our work on Direct Quantum Simulation. In this context quantum mechanics is used to design a quantum machine, i.e. an apparatus to controllably create many-body quantum states, in the same way that automotive mechanics is used in the creation of vehicles.