Quantum experiment finally settles a century-old row between Einstein and Bohr

The thought experiment that was at the center of a debate between famous physicists Albert Einstein and Niels Bohr in 1927 has finally become a reality. The study's findings shed light on one of the great mysteries of quantum physics: is light actually a wave, a particle, or a complex mixture of the two?

Einstein and Bohr's argument concerns double-slit experimentwhich dates back a hundred years ago to physicist Thomas Young in 1801. Young used this test to prove that light is a wave, while Einstein argued that light is actually a particle. Meanwhile, Bohr's work in quantum physics boldly suggested that in some sense it could be both. Einstein didn't like this controversial idea and came up with a modified version of Young's experiment to counter it.

Now, Chao-Yan Lu of the University of Science and Technology of China and his colleagues conducted an experiment implementing Einstein's idea, using the best tools of modern experimental physics to show that quantum objects are as unusual in their dual wave and particle nature as physicists in the 1920s suspected. “Seeing quantum mechanics in action at this fundamental level is absolutely breathtaking,” says Lu.

In the classic double-slit experiment, researchers shine light onto a pair of narrow, parallel, horizontally oriented slits placed in front of a screen. If light were a particle, the screen should have shown a spot of light behind each slit, but Young and the scores of researchers who followed him instead saw an “interference pattern” of alternating dark and light fringes. This indicated that the light was more like a wave that passed through the slits and was captured by the screen. ripples collide with each other. Remarkably, the interference pattern persists even when the light intensity is reduced to a single light particle or photon. Does this mean that a photon, which is exactly like a particle, somehow interferes with itself as if it were also a wave?

Bohr argued for the concept of “complementarity”, where it is impossible to see the particle nature of a photon when it exhibits wave-like behavior, and vice versa. In the debate over whether this was indeed the case, Einstein imagined that an extra slit would be placed in front of an ordinary pair, which would be equipped with springs, so that it could bounce back when a photon entered it. Based on the movement of the springs, physicists were able to determine whether the photon passed through the top or bottom slit. According to Einstein, this would mean being able to simultaneously describe the particle behavior of a photon (passing through a specific slit, like a tiny ball) and its wave behavior, as evidenced by the interference pattern, which would contradict complementarity.

Lu says his team wanted to build this device at the “ultimate quantum limit,” so they created single photon not into a gap, but into an atom, which could bounce off in the same way. Additionally, hitting an atom puts the photon into a quantum state equivalent to a combination of left and right motion away from the atom, which also creates an interference pattern when it hits the detector. To use an atom in this way, researchers used lasers and electromagnetic forces that made it incredibly cold, allowing for extremely precise control of its quantum properties. This was crucial for testing Bohr's answer to Einstein: he argued that Heisenberg Uncertainty Principlewhich says that if the change in the momentum of the slit due to recoil was very well known, then its position would become very unclear and, on the contrary, could destroy the interference pattern.

“Bohr's counterargument was brilliant. But the thought experiment remained theoretical for almost a century,” Lu says.

By tuning the lasers, Lu and his colleagues were able to control the uncertainty of the atom's momentum in the form of a gap. In doing so, they discovered that Bohr's statement was correct, and they could erase the interference pattern by changing the fuzziness of its momentum. Amazingly, the researchers also used this tuning ability to access a more intermediate mode in which they could measure some recoil information as well as see a blurred version of the interference pattern. Here, the photon effectively exhibited both wave and particle properties, Lu says.

“The real interest lies in [this] between them,” says Wolfgang Ketterle at the Massachusetts Institute of Technology. Earlier this yearhe and his colleagues performed a version of Einstein's experiment. They used ultracold atoms controlled by lasers to implement a version of Einstein's experiment in which a pair of slits can move. While Lu and his colleagues used a single atom to scatter light in two directions, here two atoms scattered light in one direction, and the effect of a photon hitting each atom could be detected by changes in their quantum states. Ketterle says this is a conceptually different way of exploring wave-particle duality that more clearly describes the photon's actions because the information about “which direction” is stored in one of the two individual atoms, but it's a slight departure from Einstein's original idea.

He and his colleagues also experimented with suddenly turning off lasers (equivalent to removing springs from moving slits) and then shooting photons at atoms. Bohr's conclusion still stands: the exchange of momentum between the atoms and the photon, as well as the uncertainty principle, can still “blur” the fringes of the interference pattern. This springless version of Einstein's idea had not been tested before, Ketterle said. “In atomic physics, with cold atoms and lasers, we have real opportunities to demonstrate quantum mechanics with clarity that has not been possible before.”

Philip Treutlein from the University of Basel in Switzerland say these two experiments convincingly demonstrate some of the fundamentals of quantum mechanics. “With our current understanding, we know the answer to how quantum mechanics works on the microscopic scale. But it always makes a difference if you see it for real, so to speak, if someone actually does this experiment.” Lu and his team's experiment is conceptually consistent with the drawings that remain in the historical record of the debate between Bohr and Einstein, and behaves exactly as quantum mechanics predicts, he said.

Lu still has a lot to study, such as classifying the quantum state of the gap in even more detail, as well as increasing its mass. But the experiment also has enormous educational value. “Above all, I hope it captures the pure beauty of quantum mechanics,” he says. “If a few more young people watch the interference pattern appear or disappear in real time and say, ‘Wow, this is really how nature works,’ then the experiment has already been a success.”