// ARS TECHNICA — MONDO
What happens when quantum mechanics and relativity meet?
Experiment put atoms in a superposition of trajectories to find out.
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.
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.
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.”
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.
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.
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.
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. 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 return