Nobel Prize in Physics 2022 – For answering a question from very early quantum physics

It has never happened before that the Nobel Prize in Physics is awarded for an experiment that relates to a fundamental problem in physics from more than 90 years ago and that ultimately succeeded in solving it. The prize for 2022 is such a unique case: In addition to Anton Zeilinger, who already developed applications of this fundamental insight in physics in the 1990s, John Clauser and Alain Aspect received the Nobel Prize for ultimately answering the fundamental question of quantum physics that had been open until then: Does the phenomenon exist in nature that particles that are very far away from each other can nevertheless be in direct contact? Equivalent to this question is whether there are hidden variables in quantum physics. This question was answered by Alain Aspect in 1982 (in his doctoral thesis).

In order to explain this question and its fundamental importance for physics, as well as the foundation of an application that will presumably shape the future just as fundamentally, to non-physicists, let us first consider its historical foundation. The quantum theory developed in 1925 by (alongside Erwin Schrödinger) Werner Heisenberg contained a characteristic that is significant for the nano world: certain variables of a particle, such as its location and momentum, can no longer be determined with arbitrary precision at the same time (physicists speak of the “Heisenberg uncertainty principle”). For many physicists, this was a fundamentally new and for some – like Albert Einstein – unacceptable property of quantum particles. For the latter, there should be variables in the quantum world that we cannot (yet or fundamentally) measure, so-called “hidden variables” that ultimately “give the particles back their classical properties”. But in 1932, the great, but then still quite young, mathematician John von Neumann announced a proof that such hidden variables cannot exist. With this, the problem seemed to be solved once and for all – even if Einstein still resisted it throughout his life. Schrödinger, not a supporter of the quantum interpretation represented by the majority, either, introduced a principle in 1935 as a consequence of the absence of hidden variables that quantum particles can thus also be coupled (“entangled”) over arbitrarily large distances – and even with macroscopic variables, something that is not possible in our everyday world. Thus he wrote:

“It [the Ψ-function of the measured object] has, according to the inevitable law for the total Ψ-function, become entangled with that of the measuring instrument (…)”.

But von Neumann’s seemingly irrefutable proof had one big problem: it was simply wrong. For over thirty years, it never occurred to anyone to contradict the great John von Neumann. The most important physicists of the 20th century, from Bohr to Heisenberg to Pauli, from Dirac to von Weizsäcker to Feynman, accepted the proof as valid without contradiction. Even Schrödinger and Einstein, who after all had a strong interest in questioning every argument that supported this quantum interpretation (the so-called “Copenhagen interpretation”), did not think of casting doubt on von Neumann’s proof. Only one person recognized the error in his mathematical derivation right at the beginning, also in 1935: Grete Hermann. Surprisingly, her clear (and not at all difficult to understand) rebuttal had no consequences for quantum physics for a long time. It was as if she had never published her work.

It was not until the 1960s and 1970s that the discussion about hidden variables in the quantum world was taken up again. In the meantime, enormous technological progress had been made in the application of quantum physics – from the laser in the CD player to the modern computer, but not much progress had been made in answering the question of how the phenomena of the quantum world could be explained in detail.

It was the Northern Irish physicist John Bell who brought the inadequacy of von Neumann’s proof to light a second time. He discovered its error in 1964. The physics community had to acknowledge that hidden variables might exist in the quantum world after all. A part of Bell’s paper explained exactly – partly even in similar formulation – what Grete Hermann had long since found out more than thirty years earlier. But Bell went one step further in his publication. He succeeded in stating a mathematical criterion in the form of an inequality that names the circumstances under which hidden variables can occur in a quantum theory. If someone could prove that Bell’s inequality does not apply, it would simultaneously be proven that there are no hidden variables. The sensational thing about Bell’s inequality was that it could only be verified experimentally. Theoretically, it was not possible.

In the following years, physicists tried to create and carry out an experiment whose result violated Bell’s inequality. However, a successful experiment only succeeded towards the end of the 20th century. It was in 1982 when Alain Aspect finally succeeded in showing unambiguously a violation of Bell’s inequality experimentally. This initiated a new upswing for basic research in quantum physics 50 years after Grete Hermann’s refutation. Among other things, the way was now clear for a deeper understanding of the entanglement of spatially separated particles, as Schrödinger had already introduced in 1935. The new theoretical insights in turn spurred the technological application of quantum theory. For example, the experimental proof of the existence of entangled particles led to the vision of quantum computers articulated by Richard Feynman, also in the early 1980s, and whose first experimental steps were implemented by Anton Zeilinger in the 1990s. At last, the Nobel Prize in Physics has been awarded for this.