Nearly 100 Years After Debating Bohr On Quantum Mechanics, New Experiment Proves Einstein Wrong – Again

Quantum mechanics is weird. When you think you have reached the bottom of its weirdness, you always discover a new subbasement with even weirder stuff. The number one hater of this weirdness was none other than Albert Einstein; he believed that reality had to be deterministic. His famous quip, “God does not play dice with the universe,” is about that very idea. He got it wrong, though, and that’s just been proven once again.

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In 1927, the fifth Solvay Conference was a revolutionary moment for modern physics. Among the many topics discussed, there was the principle of complementarity, which underpins both Heisenberg's uncertainty principle and the wave-particle duality. Complementarity states that in quantum mechanics, certain pairs of complementary properties exist and cannot be measured simultaneously.

Danish physicist Niels Bohr regarded this as a cornerstone of quantum mechanics, while Einstein thought it was wrong. For this reason, the German scientist suggested a Gedankenexperiment (thought experiment) to establish if this was indeed correct. Together, the two reimagined the double slit experiment, with a movable slit tuned to the momentum of the particle. A new experiment brings this to light in the most fascinating way.

The double slit experiment proved first that light is made of waves, and then proved that electrons are also waves. With Einstein proving that photons are particles of light, this experiment has cemented the reality of particle-wave duality. In the Einstein-Bohr interferometer, before getting to the double-slit, the particle would pass through a single slit – that’s the one that is momentum-sensitive.

The first slit would push the particle in a certain direction, related to the particle's momentum. Einstein argued that then the double slit part would still work – producing the diffraction fringes – and thus violating complementarity by showing both particle and wave behavior. Bohr argued that due to the uncertainty principle, the diffraction fringes would end up washed out.

Jian-Wei Pan of the University of Science and Technology of China (USTC) and his collaborators have created a new version of this interferometer in real life. They used optical tweezers to trap a rubidium atom mid-air, just like a tractor beam made of light. The atom was entangled with the momentum of a photon before sending the photon through the double slit. Unsurprisingly, the setup behaves as Bohr had postulated – sorry, Albert!

Complementarity, and even this interferometer, have been tested before, but the new setup using optical tweezers has intriguing new applications. The system is tunable, so the team was able to make the fringes more or less blurry in line with the theory. It can also be used in more complex quantum mechanical problems, such as entanglement and the loss of it (decoherence), which are open subjects, especially when it comes to quantum computing.

The study is published in the journal Physical Review Letters.