One-atom experiment settles Einstein’s challenge in Bohr’s favour

In the 1920s, Albert Einstein proposed a twist on the double-slit experiment. Rather than a wall with two fixed slits, he imagined a very light slit that would recoil when a single photon passed through it. If one could measure that recoil, one could in principle tell which path the photon took. At the same time, one could also look for an interference pattern, which is a sign that light behaves like a wave.

Einstein hoped this would show a conflict inside quantum theory itself. Niels Bohr however argued that the plan would fail. For almost a century, the argument stayed on paper because nobody had a way to test the idea.

A new study by University of Science and Technology of China researchers has now realised a way, by replacing the movable slit with a single atom. The findings were published in Physical Review Letters on December 2.

A focused laser beam held the atom in place. The researchers cooled the atom close to its ground state, where its random motion was as small as quantum physics allowed. In this state, the uncertainty in the atom’s momentum is similar to the momentum of a single photon. The idea was to check whether the atom’s recoil could be used to say which way a photon went and how that information affected the interference pattern.

In the study team’s setup, a photon scattered off the trapped atom in a way that defined two possible paths, which were then recombined to form bright and dark fringes on a detector. At the same time, the photon gave the atom a small ‘kick’ upwards or downwards, depending on the path. Thus, knowing more about the atom’s recoil told the team more about which path the photon took.

By tweaking the beam’s strength, the team could tune the atom’s momentum uncertainty. When the uncertainty was large, the two recoil states overlapped and it became impossible to tell which path the photon took, yet the interference pattern was sharp. When the uncertainty was small, the recoil was distinct but the interference became less visible.

Thus, the measurements followed the predictions of quantum theory and vindicated Bohr’s critique. They also provide a platform to explore the gradual transition from quantum to classical behaviour in systems where light and matter are strongly linked, with possible applications in future quantum technologies.