How can we test whether quantum theory is false?

Two researchers from the Austrian Academy of Sciences have proposed a new type of experimental tests of quantum physics. Generalizing similar methods that rule out a classical description of the world (known as “contextuality”), their proposal applies even in situations with very limited control and no alternative theory to test quantum physics against.

Even our best physical theories might only be approximately true, and we should submit them to precise experimental testing. But how do we test a flexible and general theory like quantum physics, if we have no alternative theory to test it against? Even if we find unexpected statistical results in an experiment, how do we know that quantum physics is broken, and that we have not simply made a mistake in modelling our setup within quantum theory?

To solve such problems, Dr. Markus Müller and Dr. Andrew Garner from the Institute for Quantum Optics and Quantum Information in Vienna have studied a closely related foundational theoretical question: if we assume that quantum physics is fundamentally true, which kinds of effective statistical data can we expect to see in the laboratory? Their study, recently published in the scientific journal Physical Review X, gives a precise mathematical answer to this question, and uses it to propose a novel type of experimental test of quantum theory.

Their analysis is motivated by the conceptually related question of how to distinguish quantum physics from classical physics. Over the last few decades, quantum physicists have developed a thorough understanding of this: if experimental data admits only contextual classical models, then such models are implausible, and the physics can be said to be genuinely quantum. This notion of contextuality is necessary for quantum technological advantages such as computational speed-ups or more efficient ways to do metrology. Building on these insights, Garner and Müller have formulated a similar criterion for when an underlying quantum model is implausible.

“We would really like to see this in action”, says Dr. Garner. The authors have recently begun discussing concrete experimental implementations and think that large quantum systems such as Bose-Einstein condensates might be a suitable playground to test their proposal. In addition to the physical insights, the mathematical theory behind the proposal turns out to be interesting too. “We see unexpected relations to structures like Jordan algebras, proposed by the pioneers of quantum physics already in the 1930s, and to the study of violations of Bell inequalities”, says Dr. Müller. While they both believe that quantum physics will most likely survive all experimental tests for the foreseeable future, they are fascinated by the prospect of pushing it to the extreme. “It is like a new lens that we can use examine the quantum world”, says Dr. Garner, “and we will only know what we see when we look through it.”