Superfluid Dynamics of Quantum Ferrofluids

在世界各地最先进的实验室里，原子气体正在被冷却到绝对零度以上不到百万分之一度的温度。在这种极端寒冷的情况下，量子力学占据了上风；原子失去了各自的身份，被涂抹成一股巨大的物质波。这种量子气体有一系列奇怪的行为，从它承受波状干扰的能力到它的化身超流体，一种没有运动阻力的流体。量子气体远非仅仅是科学上的好奇。它代表了一个干净而纯粹的多粒子量子系统的范例，给了我们对量子世界的丰富洞察力。原子物理技术使实验者能够精确地调整其物理性质，并在时间和空间上操纵它。由于这些方面的原因，量子气体正被用作“仿真器”来重现和理解复杂的物理现象，从超导体、湍流到黑洞和大爆炸。量子气体还拥有令人兴奋的技术前景。它们对受到干扰的特殊敏感性正在推动它们作为超精密传感器的发展，例如重力传感器，因此它们被吹捧为石油和矿产勘探的重大进步。同时，它们史无前例的量子控制使这些气体成为执行量子门操作的候选气体，这是备受赞誉的量子计算机的基础。最近对量子气体的实验创造了一种“量子铁磁流体”。作为一种超流体和铁磁流体，这种新的状态位于我们最奇怪的两种流体的交界处。磁流体是一种分散有微小磁性铁颗粒的液体。就像条形磁铁一样，粒子在长距离内相互作用，喜欢在北极和南极相邻的情况下躺着，并在外加磁场中排列成一条直线。这会导致流体中的特殊图案和不稳定性，但更重要的是，可以通过磁场控制流动和物理性质，就像医学、信息显示和密封剂中的磁流体技术所利用的那样。磁流体的量子兄弟，量子磁流体，是由磁原子的超冷量子气体形成的。人们正在对这种气体进行研究，以探索其新的性质和潜在的开发潜力。它的磁性将量子气体的上述能力扩展到新的领域，例如，提供量子磁学的试验台，模拟具有远程相互作用的系统，以及对磁场的敏感性，可以在新一代磁性传感器中利用，潜在的应用从地质勘探到军事探测。同时，原子之间的长程磁相互作用对量子计算特别有吸引力，因为它允许计算操作在远距离执行。量子铁磁流体中超流的基本性质仍然是未知的，揭开它是这个项目的核心目标。由于超流性是系统输运性质的基础，我们将揭示量子铁磁流体如何运动、流动、旋转和旋转，以及对搅拌的响应。这对于我们从总体上理解超流性是很有意义的，更具体地说，对于未来对量子铁磁流体的操纵和开发有很大的实际意义。传统磁流体的独特行为及其通过磁场进行的良好控制暗示了丰富的新超流体行为和控制超流体状态的新维度。量子铁磁流体可以反过来提供对传统铁磁流体的洞察；作为超流体，由于没有粘度，量子铁磁流体体现了磁流体的简化版本，由此可以重新解决磁流体中的突出问题。

In state-of-the-art laboratories worldwide, gases of atoms are being cooled down to temperatures less than a millionth of a degree above absolute zero. At this extreme coldness, quantum mechanics takes over; the atoms lose their individual identities and become smeared out into a giant wave of matter. This quantum gas hosts a range of bizarre behaviours, from its capacity to undergo wave-like interference to its embodiment of a superfluid, a fluid with no resistance to motion. The quantum gas is far from just a scientific curiosity. It represents a clean and pure exemplar of a many-particle quantum system, giving rich insight into the quantum world. Atomic physics techniques empower experimentalists to precisely tune its physical properties and manipulate it in time and space. Due to these facets, quantum gases are being exploited as "emulators" to recreate and understand complicated physical phenomena, from superconductors and turbulence to black holes and the Big Bang. The quantum gas also holds exciting technological prospects. Their exceptional sensitivity to being disturbed is driving their development as ultra-precise sensors, e.g. of gravity, for which they are touted to lead to major advancement in oil and mineral exploration. Meanwhile, their unprecedented quantum control makes these gases candidates for performing quantum gate operations, the basis of the much-lauded quantum computer.Recent experiments in quantum gases have created a "quantum ferrofluid". Being both a superfluid and a ferrofluid, this novel state lies at the interface of two of our most bizarre fluids. Ferrofluids are liquids dispersed with tiny magnetic iron particles. Just like bar magnets, the particles interact over long-range, prefer to lie with north and south poles being adjacent, and become aligned in an imposed magnetic field. This leads to peculiar patterns and instabilities in the fluid, but, more importantly, enables the flow and physical properties to be controlled via magnetic fields, as exploited in ferrofluid technologies in medicine, information display and sealants. The quantum sibling of the ferrofluid, the quantum ferrofluid, has been formed from an ultracold quantum gas of magnetic atoms. This gas is being hotly researched to probe its novel properties and potential exploitation. Its magnetic nature extends the above-mentioned capabilities of the quantum gas into new territories, e.g., providing a testbed of quantum magnetism, emulation of systems with long-range interactions, and a sensitivity to magnetic fields which can be exploited in a new generation of magnetic sensors, with potential applications from geological exploration to military detection. Meanwhile, the long-range magnetic interaction between atoms is particularly attractive for quantum computation since it allows the computational operations to be performed at a distance. The fundamental nature of superfluidity in the quantum ferrofluid remains uncharted, and uncovering it is the core aim of this project. With superfluidity underpinning the transport properties of the system, we will reveal how the quantum ferrofluid moves and flows, swirls and gyrates, and responds to agitation. This is of fundamental interest to our understanding of superfluidity in general, but, more specifically, is of great practical benefit for future manipulation and exploitation of the quantum ferrofluid. The distinctive behaviour of conventional ferrofluids and their virtuous control via magnetic fields is suggestive of a rich plethora of novel superfluid behaviour and a new dimension of control over the superfluid state. The quantum ferrofluid may in turn provide insight into the conventional ferrofluid; being superfluid, with an absence of viscosity, the quantum ferrofluid embodies a simplified version of the ferrofluid from which outstanding problems in ferrofluids can be tackled afresh.