Quantum Sensing for Antimatter Gravity

为什么宇宙表面上没有反物质？根据我们目前对物理学的最好理解(由粒子物理的标准模型(SM)表示)，我们不应该存在；我们的太阳系、银河系，甚至整个可观测的宇宙在这个范例中没有意义，因为它们似乎只由物质组成，几乎没有任何反物质被发现。这很神秘，因为SM背后的基本对称性意味着，当大爆炸后能量转化为物质时，也应该产生等量的反物质。这是因为物质和反物质是高度对称的，就像我们用来描述它们的基本量子场理论一样；打破这种对称性会对现代物理学最珍视的一些性质造成极大的破坏。尽管如此，我们面临着一场危机，我们目前的理解必须有所改变。SM的巨大成功有一个具有讽刺意味的副作用，即它并没有告诉我们下一步该去哪里，尽管我们有许多迹象表明它本身并不完整。例如，暗物质的存在，强CP问题，层级问题，等等(还有更多)。同样，物质主宰的宇宙的形成应该是不可能的，但这似乎已经发生了。这种观察似乎有坚实的基础：一个仔细平衡的宇宙，其中包含等量的物质和反物质似乎不太可能，因为我们看不到星系规模的湮灭事件。因此，物质和反物质之间的精确对称性似乎必须以某种方式被打破。然而，没有明确的迹象表明，这将如何以一种与SM兼容的方式发生。虽然可能很难用一般的方式与SM调和，但某种反重力效应可能会为我们看似不合理的存在提供一种解释。反物质引力量子传感(QSAG)计划是一项测量正电子(Ps)在地球磁场中引力相互作用的尝试。PS是一个由物质和反物质(一个电子和一个正电子)组成的类氢原子系统。我们的目标是测量它是否以与其他物质相同的方式落下；这本质上是对弱等价原理的测试，该原理以前从未在含有反物质的系统中直接测试过。实验的基础是一个高灵敏的量子传感协议，使用干涉效应来探测高激发态Ps的里德堡态的量子叠加上的引力。这种形式的(反)物质波干涉计量学还没有被证明用于Ps，但最近已经被开发用于里德堡He原子[J E.Palmer和S.D.Hogan Electric Rydberg原子干涉计量学，Phys Rev Lett122,250404(2019年)]。由于ps原子的短寿命，使得对ps原子的实验变得更加困难，因此将它们激发成不会湮灭的里德堡态是非常有利的。因此，QSAG实验将能够利用光学和微波辐射进行干涉测量，这将使我们能够测试Ps中的反重力效应，并在这样做时可能解释我们是如何能够存在的。

Why is the Universe apparently devoid of antimatter? According to our best current understanding of physics (as expressed by the Standard Model (SM) of particle physics), we should not exist; our solar system, the Milky Way Galaxy, and indeed the entire observable Universe do not make sense in this paradigm because they seem to be composed only of matter, with hardly any antimatter to be found. This is mysterious because the fundamental symmetries underlying the SM mean that, when energy was converted to matter after the Big Bang, an equal amount of antimatter should also have been created. This arises because matter and antimatter are deeply symmetrical, as are the fundamental quantum field theories that we use to describe them; breaking this symmetry does extreme damage to some of the most cherished properties of modern physics. Nevertheless, we have a crisis and something in our current understanding has to change. The great success of the SM has the ironic side effect that it doesn't tell us where to go next, even though we have many indications that it is itself incomplete. Examples are the existence of Dark Matter, the strong CP problem, the hierarchy problem, and so (there are many more). Similarly, the formation of a matter-dominated Universe should be impossible, and yet this is what seems to have happened. The observation seems to be on solid ground: a carefully balanced Universe containing both matter and antimatter in equal quantities does not seem very likely, as we don't see galaxy-scale annihilation events. Therefore it appears that the exact symmetry between matter and antimatter must somehow be broken. However, there is no clear indication of how this could happen in a way that is compatible with the SM. While it may be difficult to reconcile with the SM in a general way, some sort of antigravity effect may offer an explanation for our seemingly unreasonable existence. The Quantum Sensing for Antimatter Gravity (QSAG) project is represents an attempt to measure the gravitational interaction of positronium (Ps) in the field of the Earth. Ps is a hydrogen-like atomic system composed of both matter and antimatter (an electron and a positron). Our aim is to measure if it falls in the same way as other matter; this is essentially a test of the weak equivalence principle, which has not previously been tested directly for systems containing antimatter. The basis of the experiment is a highly sensitive quantum sensing protocol using interference effects to probe the force of gravity on a quantum superposition of highly excited Rydberg states of Ps. This form of (anti)matter wave interferometry has not been demonstrated for Ps, but has recently been developed for Rydberg He atoms [J E. Palmer and S. D. Hogan Electric Rydberg atom interferometry, Phys Rev Lett 122, 250404 (2019)]. Experiments with Ps atoms are made more difficult by the short Ps lifetime and so it is highly advantageous to excite them into Rydberg states that do not annihilate. Thus the QSAG experiment will be able to employ optical and microwave radiation to perform interferometric measurements that will allow us to test for antigravity effects in Ps, and in so doing possibly explain how it is that we are able to exist.