Levitated Quantum Diamonds (LQD)

量子力学很好地描述了原子和分子，但更大的东西呢？Erwin Schrödinger指出，猫从来没有被证明存在于量子叠加中，但最近的实验正在推动哪些物体被证明可以做到这一点。最强的测试需要长时间的叠加中的大质量，叠加距离很大。叠加距离是叠加的两个分量之间的距离。已经用超导体、超流体和振动悬臂进行了开创性的实验，但到目前为止创造的最宏观的叠加态是著名的2000个原子组成的分子的两缝实验的变体。我们的项目有一个雄心勃勃的目标，即测试由超过100万倍的原子组成的悬浮纳米钻石是否可以表现出这种量子行为。这一实验前沿最令人兴奋的事情是，它可能在10-15年内导致对量子引力的测试。爱因斯坦的广义相对论解释了引力，是让GPS工作所必需的，但我们不知道如何将它与量子力学相结合来解释量子物体产生的引力效应。成功地将这两个最基本的物理学理论结合起来，将产生一个追求了100年的量子引力理论。已经提出了弦理论和圈量子引力等量子引力理论，但缺乏经验证据。史蒂芬·霍金和罗杰·彭罗斯等物理学家对黑洞进行了研究，证明了黑洞是约束量子引力理论的肥沃游乐场，但在黑洞上进行实验是不现实的。我们和其他人的一项新提议展示了一种方法，可以通过在桌面上进行实验室实验来测试量子引力的一个关键方面。这个想法是创造两只纳米钻石薛定谔猫，并观察它们是如何在引力作用下相互作用的。这个项目之所以有可能，要归功于量子科技界已经展示的进步。事实上，这项研究最终将导致一种新的更灵敏的传感器，将用于检测加速度、旋转、倾斜、重力和磁场。在已经发表了如何测试宏观量子力学和量子重力的描述之后，我们现在将改变我们的初步实验，开始交付这些建议。为了达到大的叠加距离和长的持续时间，我们将使用包含单个氮空位中心(NVC)的金刚石纳米颗粒(直径约800 nm)。这遵循了我们的建议，即提供了一条清晰的路线来实现超过1000纳米的叠加距离，尽管我们最初的实验只会达到0.1 PM我们和其他人使用光学陷阱以及Paul陷阱和磁性陷阱将纳米钻石悬浮在真空中。我们发现，在真空条件下，光学陷阱中的俘获光束对金刚石的加热是一个严重的障碍。为了解决这一问题，我们(与合作伙伴奥利弗·威廉姆斯)开发了大量高纯度纳米钻石，现在已改用磁捕捉器，因为这进一步减少了悬浮钻石的加热。磁捕获器还提供将自旋与运动相耦合所需的不均匀磁场。其核心思想是将NVC电子自旋置于自旋叠加中，因为不均匀的磁场随后在钻石上提供力的叠加，从而导致空间叠加。为了证明这一点，我们将反转自旋，重新组合用于物质波干涉测量的叠加分量，并重复干涉测量，作为相对于重力的实验倾斜的函数，以搜索干涉条纹。

Atoms and molecules are very well described by quantum mechanics, but what about much larger things? Erwin Schrödinger pointed out that cats have never been shown to exist in a quantum superposition, but recent experiments are pushing back the boundaries of which objects have been shown to do this. The strongest tests require a large mass in a superposition for a long time with a large superposition distance. The superposition distance is the distance between the two components of the superposition. Pioneering experiments have been done with superconductors, superfluids and vibrating cantilevers but the most macroscopic superposition state created so far is a variant of the famous two-slit experiment for molecules made of 2000 atoms. Our project has the ambitious goal of testing whether levitated nanodiamonds made up of more than a million times more atoms can display this quantum behaviour. The most exciting thing about this experimental frontier is that it could, in 10-15 years, lead to a test of quantum gravity. Einstein's general relativity explains gravity and is needed to make GPS work, but we don't know how to combine it with quantum mechanics to explain the gravitational effects produced by a quantum object. Successfully combining these two most fundamental theories of physics would produce a theory of quantum gravity, which has been sought for 100 years. Theories of quantum gravity such as string theory and loop quantum gravity have been proposed, but suffer from a lack of empirical evidence. Physicists such as Stephen Hawking and Roger Penrose worked on black holes and showed they are fertile playgrounds to constrain theories of quantum gravity, but black holes are not practical to experiment on. A new proposal from us and others shows a way to test one key aspect of quantum gravity with a lab experiment on a table-top. The idea is to create two of the nanodiamond Schrödinger cats and see how they interact gravitationally. This project is only possible thanks to the advances already demonstrated by the quantum technology community, and indeed this research will, in time, lead to a new class of more sensitive sensors that would be used to detect acceleration, rotation, tilt, gravity and magnetic fields.Having already published our descriptions for how to test macroscopic quantum mechanics and quantum gravity, we will now transform our preliminary experiments to begin the delivery of these proposals. To reach large superposition distances and long durations we will use diamond nanoparticles (around 800 nm across) containing a single nitrogen vacancy centre (NVC). This follows our proposals which provide a clear route to achieve a superposition distance of over 1000 nm, although our initial experiments will only reach 0.1 pm. Nanodiamonds have been levitated in vacuum using optical traps by us and others, as well as in Paul traps and magnetic traps. We showed that the heating of the diamond by the trapping beam in an optical trap in vacuum is a serious obstacle. To get around this we developed (with collaborator Oliver Williams) large quantities of high-purity nanodiamonds, and have now switched to using a magnetic trap as this further minimises the heating of the levitated diamond. A magnetic trap also provides the inhomogeneous magnetic field which is required to couple the spin to the motion. The core idea is to put the NVC electron spin into a spin superposition because the inhomogeneous magnetic field then provides a superposition of forces on the diamond leading to a spatial superposition. To evidence this, we will then flip the spin to recombine the superposition components for matter-wave interferometry and repeat the interferometry as a function of experimental tilt with respect to gravity to search for interference fringes.