What happens when quantum mechanics and relativity meet?
What happens when quantum mechanics and relativity meet?
当量子力学与相对论相遇时会发生什么？
Almost a hundred years ago, physicists theorized out what free fall should do to a quantum wave. If the solution they came up with is wrong, quantum mechanics and Einstein’s theory of gravity flatly contradict each other. But testing it has been impossible because nobody has managed to build an interferometer that could perform the necessary measurement. 大约一百年前，物理学家们推导出了自由落体对量子波的影响。如果他们得出的结论是错误的，那么量子力学和爱因斯坦的引力理论将产生直接矛盾。但验证这一理论一直是不可能的，因为此前没有人能制造出能够进行必要测量的干涉仪。
Now, a team led by Ron Folman, a physicist at Ben-Gurion University of the Negev, with collaborators in Germany, the UK, and the US, including Nobel laureate Roger Penrose, has done it. They built a new interferometer that gives a single atom two possible paths at once: one that involves a free fall, and another where it is held perfectly still. Both paths end at the same place at the same moment, allowing the team to measure what the fall does to a wave-like property of the atom. 现在，由内盖夫本-古里安大学物理学家罗恩·福尔曼（Ron Folman）领导的团队，与来自德国、英国和美国的合作者（包括诺贝尔奖得主罗杰·彭罗斯）一起完成了这一壮举。他们制造了一种新型干涉仪，能让单个原子同时拥有两条可能的路径：一条是自由落体，另一条则保持完全静止。两条路径在同一时刻汇合于同一点，使研究团队能够测量自由落体对原子的波动特性产生了什么影响。
Long time coming
漫长的等待
Ever since Galileo, physicists have known how to describe a falling object—where it is, how fast it goes, or how quickly it accelerates. Quantum mechanics, though, insists that every object is also a wave. “Every particle, doesn’t matter if it’s a car or a spaceship or an atom, is a wave,” Folman says. “Everything that is a wave, like sea waves or sound waves, goes up and down. And if you’re up or down, this is measured by something called a phase. A phase just tells you if you are at the top of the wave or at the bottom.” 自伽利略时代以来，物理学家们就知道如何描述一个下落的物体——它在哪里、速度有多快或加速度是多少。然而，量子力学坚持认为每个物体也都是波。“每一个粒子，无论是汽车、宇宙飞船还是原子，都是波，”福尔曼说。“任何波，比如海浪或声波，都有起伏。而这种起伏是通过一种叫做‘相位’的东西来衡量的。相位只是告诉你，你处于波峰还是波谷。”
The wave nature of an atom shows itself only when the atom is barely moving, which only happens when it is cooled down to nearly absolute zero. For many years after theorists had first looked at this problem, this sort of temperature wasn’t an option—cooling atoms down to such temperatures only became possible in the late 1990s. “But this was just the start of the journey of this experiment,” Folman says. 原子的波动性只有在原子几乎静止时才会显现，而这种情况只有在原子被冷却到接近绝对零度时才会发生。在理论家首次研究这一问题后的许多年里，这种温度条件是无法实现的——直到20世纪90年代末，将原子冷却到如此低温才成为可能。“但这仅仅是这项实验旅程的开始，”福尔曼说。
The second obstacle is that a phase cannot be measured on its own—it appears only when you perform a comparison. “It can only be a relative measure done by splitting a single particle into two trajectories, and then bringing the two trajectories together,” Folman says. He told Ars that this is the same logic behind the double-slit experiment, where particles fired through two narrow slits toward a background screen form an interference pattern that reveals their phase. 第二个障碍是相位无法单独测量——它只有在进行比较时才会显现。“它只能通过将单个粒子分裂成两条轨迹，然后再将这两条轨迹汇合来进行相对测量，”福尔曼说。他告诉《Ars》，这与双缝实验背后的逻辑相同，即粒子穿过两条狭缝射向背景屏幕，形成干涉图样，从而揭示它们的相位。
Measuring the same thing in free fall adds another layer of complexity. “Drop a stone and after one second it is 5 meters below you,” Folman says. The question asked by the team was how to bring a wave packet that has fallen back into contact with the one that stayed behind—the same atom on the other path—so that the two can interfere. Finding the solution took his team years, even though in hindsight it sounds rather obvious. 在自由落体状态下测量同样的东西增加了另一层复杂性。“扔下一块石头，一秒钟后它就在你下方5米处，”福尔曼说。团队提出的问题是：如何让下落的波包与留在原处的波包（即另一条路径上的同一个原子）重新接触，以便两者能够发生干涉。尽管事后看来这似乎显而易见，但他的团队花了数年时间才找到解决方案。
The cannon and the parachute
大炮与降落伞
Grabbing a falling particle and somehow bringing it back to where it started was out of the question. “Even if you could do it with some tweezer or some trap, this very violent action would create so much noise that we wouldn’t have known what is fake news and what is really the effect that we want to measure,” Folman joked. So the team figured the right way to do it would be to put an atom into a superposition of paths, where one path sees the atom thrown upward and has gravity return it to its original location. The other path isn’t really a path at all; the atom just stays where it started. “You have to shoot one with a cannon, into a ballistic motion like artillery,” Folman says. 抓住一个下落的粒子并以某种方式将其带回起点是不可能的。“即使你能用某种镊子或陷阱做到这一点，这种剧烈的动作也会产生巨大的噪声，以至于我们无法分辨什么是‘假新闻’，什么是我们真正想要测量的效应，”福尔曼开玩笑说。因此，团队认为正确的方法是将原子置于路径叠加态中：一条路径让原子向上抛出，利用重力使其回到原始位置；另一条路径实际上根本不是路径，原子只是停留在原地。“你必须用大炮发射其中一个，让它像炮弹一样进行弹道运动，”福尔曼说。
But that was just the start of the problems. The challenge, obviously, is designing a cannon that may or may not affect an atom (technically, it only affects one wave packet of the atom without affecting the other one). Then there is the issue of keeping the system in a quantum state. Interference only occurs when there is no way, even in principle, to tell which path the particle took—every measurement destroys the quantum superposition and forces the particle to behave like a localized classical object. Unfortunately, a particle returning from a ballistic flight is moving fast, while its stationary twin is not. That difference in speed is itself information about its path, so no interference pattern would appear. Atom interferometers have existed for 30 years, but none could accomplish what Folman and his colleagues wanted to do. So, they built a new one called the Quantum Galileo Interferometer (QGI). 但这仅仅是问题的开始。显而易见，挑战在于设计一种可能影响（或不影响）原子的大炮（从技术上讲，它只影响原子的一个波包而不影响另一个）。此外，还有保持系统处于量子态的问题。只有在原则上无法分辨粒子走了哪条路径时，干涉才会发生——任何测量都会破坏量子叠加态，并迫使粒子表现得像一个局域化的经典物体。不幸的是，从弹道飞行中返回的粒子运动速度很快，而其静止的“孪生兄弟”则不然。这种速度差异本身就是关于其路径的信息，因此不会出现干涉图样。原子干涉仪已经存在了30年，但没有一个能完成福尔曼及其同事想要做的事情。因此，他们制造了一个名为“量子伽利略干涉仪”（QGI）的新装置。
Atoms under a chip
芯片下的原子
The QGI is rather tiny and relies on microwave and magnetic pulses. It uses about 20,000 rubidium atoms maintained as a Bose-Einstein condensate, held by currents in microscopic wires on a chip that hangs upside-down roughly 113 micrometers above them. A microwave pulse puts every atom into superposition of two states, where one state feels magnetic forces, and the other is blind to them. A magnetic pulse then plays the role of the cannon, kicking any magnetically sensitive atoms upward. This converts the superposition of states into a superposition of trajectories. Immediately afterward, a second microwave pulse flips the atom into the opposite state, so the superposition that’s flying upward changes into a magnetically blind one and starts falling under gravity, with no other forces influencing it. At the same time, its stationary twin becomes sensitive to magnetism, but the field is tuned so its upward force exactly cancels the atom’s weight, effectively making it hover motionless with respect to the Earth. As one of the trajectories falls back, a second magnetic pulse acts as a parachute, killing its speed just as it returns to where it started. In Folman’s experiment, at their farthest apart, the two trajectories were about 7.5 micrometers apart, and the longest flight lasted two-thousandths of a second. At the end of each run, the atoms that landed in one of the interferometer’s two exits were sorted by the phase accumulated by the falling path. QGI非常微小，依赖于微波和磁脉冲。它使用了大约20,000个处于玻色-爱因斯坦凝聚态的铷原子，这些原子由悬挂在它们上方约113微米处的芯片上的微型导线电流所固定。微波脉冲使每个原子进入两种状态的叠加态，其中一种状态会受到磁力影响，而另一种则对磁力“视而不见”。随后，磁脉冲扮演了大炮的角色，将任何对磁敏感的原子向上踢出。这便将状态的叠加转化为了轨迹的叠加。紧接着，第二个微波脉冲将原子翻转到相反的状态，使得向上飞行的叠加态转变为对磁力不敏感的状态，并在重力作用下开始下落，不受其他任何力的影响。与此同时，其静止的“孪生兄弟”变得对磁力敏感，但磁场经过调节，使其向上的力正好抵消了原子的重量，从而使其相对于地球保持静止悬浮。当其中一条轨迹下落返回时，第二个磁脉冲充当了降落伞，在其回到起点时抵消了它的速度。在福尔曼的实验中，两条轨迹相距最远时约为7.5微米，最长的飞行持续了千分之二秒。在每次实验结束时，落入干涉仪两个出口之一的原子会根据下落路径所积累的相位进行分类。