Quasiparticle Imaging and Superfluid Flow Experiments at Ultralow Temperatures

我们的团队开创了将超流3 He冷却到超低温的新技术，在超低温下，热准粒子激发是高度弹道的，具有接近公里长度尺度的固有平均自由路径。超流3 He显示了广泛的奇异自旋，轨道和质量超流行为，提供了一个理想的系统来研究和更好地理解量子系统。我们已经开发出产生弹道准粒子束的技术，并将它们从超流体中形成的各种障碍物中散射出来。超流体结构如涡旋、纹理和相边界具有用于Andreev反射（超流体和超导体特有的激发的一种近乎完美的逆反射形式）的大横截面，因此容易被激发光束探测。我们的目标是进一步发展的技术，建立一个准粒子相机，直接图像超流体结构提供时间和空间分辨率，以研究其动力学。我们将首先应用该技术的图像的激发束，这是由一个振动线发射超过某个临界速度。激发是由金属丝分裂“成对原子”超流凝聚体时产生的。量子涡旋（线缺陷的超流体与一个明确的循环流）也产生以上的临界速度。这两个过程似乎密切相关，但其机制尚不清楚。这些图像将使我们能够同时观察到成对断裂的光束和涡旋，这将提供有关生成过程的详细信息。在更高的速度振幅下，振动线会产生量子湍流，一种复杂的涡旋线缠结。尽管湍流对广泛的科学和技术具有压倒性的重要性，但人们对湍流的一般认识却很少。量子湍流在概念上比经典湍流简单得多，而且更适合计算机模拟。因此，对超流湍流的研究最终可能会提供对湍流的更好理解。在量子湍流中，有几个有趣的问题有待解决，例如在低温下没有粘性力的情况下，它是如何发展和衰变的。我们的目标是使用新的成像技术直接成像量子湍流，提供详细的动力学信息，直接与理论进行比较。我们还将开发一种新的技术，允许非常精确地控制超流体中物体的运动，以研究各种与流动相关的性质。特别是，它将允许在宽的频率范围内，包括近均匀（零频率）流的对破坏机制的全面研究。这很有趣，因为该机制被认为涉及束缚表面态的布居（包括最近受到大量理论关注的所谓马约拉纳态），并且可能与量子霍金辐射类似。流动装置也将允许我们测量极低的速度。这将使我们能够探索可能的超固体行为在固体4 He在低温下通过测量缓慢运动的导线通过固体。超固体是一种非常奇特的现象，一些理论预测，量子固体可以表现出无摩擦流动。有广泛的猜测认为，在低温下已经观察到4 He的超固体性，但观察结果也可能是由具有超流体核心的固体中的缺陷来解释的。这个问题仍然很有争议，并引起了很大的兴趣。目前的设备将是非常敏感的缺陷和超固体流动，从而提供关键信息，以解决这个问题。我们还将探索新的途径，研究奇异的超流相，自旋超流和高频/小长度尺度现象。开发的新技术将是非常通用的，具有广泛的未来应用。

Our group has pioneered novel techniques to cool superfluid 3He to ultra-low temperatures where thermal quasiparticle excitations are highly ballistic, having intrinsic mean-free-paths which approach kilometre length scales. Superfluid 3He displays a wide range of exotic spin, orbital and mass superfluid behaviour, providing an ideal system to study and to gain a better understanding of quantum systems in general. We have developed techniques to generate beams of ballistic quasiparticles and scatter them from various obstacles formed in the superfluid. Superfluid structures such as vortices, textures and phase boundaries have a large cross-section for Andreev reflection (a form of near-perfect retro-reflection of excitations unique to superfluids and superconductors), so are readily probed by excitation beams. We aim to further develop the techniques to build a quasiparticle camera to directly image superfluid structures providing time and spatial resolution to study their dynamics.We will first apply the technique to image the beam of excitations which is emitted by a vibrating wire above some critical velocity. The excitations are created by the wire as it breaks apart the `paired-atom' superfluid condensate. Quantum vortices (line defects in the superfluid with a well-defined circulating flow) are also produced above this critical velocity. The two processes appear to be closely linked, but the mechanism is not understood. The images will allow us to simultaneously observe the pair-breaking beam and vortices, which will provide detailed information about the generation processes.At higher velocity amplitudes, vibrating wires produce quantum turbulence, a complex tangle of vortex lines. Despite its overwhelming importance to a wide range of science and technology, turbulence in general is poorly understood. Quantum turbulence is conceptually much simpler than classical turbulence and is far more amenable to computer simulation. The study of superfluid turbulence may thus eventually provide a better understanding of turbulence in general. There are several interesting unanswered questions to be addressed in quantum turbulence, such as how it develops and decays in the absence of viscous forces at low temperatures. We aim to use the new imaging techniques to directly image quantum turbulence to provide detailed dynamical information for direct comparison with theory.We will also develop a new technique to allow very precise controlled motion of an object in the superfluid to study various flow related properties. In particular, it will allow a comprehensive study of the pair-breaking mechanism over a wide frequency range, including near-uniform (zero frequency) flow. This is interesting since the mechanism is thought to involve the population of bound surface states (including so-called Majorana states which have received a great deal of recent theoretical interest) and there may be analogies with quantum Hawking radiation.The flow device will also allow us to measure extremely low velocities. This will enable us to explore possible supersolid behaviour in solid 4He at low temperatures by measuring the slow motion of a wire through the solid. Supersolidity is a very exotic phenomenon, predicted by some theories, where a quantum solid can exhibit frictionless flow. There is widespread speculation that supersolidity has been observed in 4He at low temperatures, but the observations might also be explained by defects in the solid with superfluid cores. This topic remains highly controversial and has received a great deal of interest. The current device will be very sensitive to both defects and supersolid flow, thus providing key information to resolve this issue.We will also investigate new avenues for research in exotic superfluid phases, spin superfluidity and high frequency/small length scale phenomena. The new techniques developed will be very versatile with a wide range of future applications.