Usurping Quantum Theory

Theoretical physicist Miguel Navascués has always had bold ambitions. "I wanted to build a time machine," he says when asked why he first got into physics. However, he quickly realised others were less concerned with such grand ideas. "I couldn’t understand why you’d go to study physics if you didn’t want to build one too." In fact, Navascués admits that his first years learning about the subject were boring, to the point that he even considered quitting. But then he came across quantum physics. "It was so different, a completely new way of understanding reality," he says. "Something clicked in my brain—I fell in love."

Fast-forward to today and quantum physics still dominates Navascués’s working life at the Institute for Quantum Optics and Quantum Information in Vienna, Austria. Except now he’s trying to destroy the very theory that inspired him, looking for concepts to replace quantum physics. It may sound crazy, at first, but it fits with the history of physics, he notes, in which theories, like kings, are often usurped. "Every theory proposed so far to describe the universe has been refuted," Navascués says. It happened with Aristotelian physics, the geocentric model of the solar system, and the Newtonian theory of gravity, to name but a few. "One day, somebody will conduct an experiment that will not have an explanation in terms of quantum theory," he adds.

Navascués hopes he’ll live to see such an experiment, but is doubtful of his chances. "I feel that I was born in the wrong time," he says, bringing his need for the time machine of his youth into sharper focus. But he’s not waiting around for a TARDIS or DeLorean; instead, he’s already trying to build the usurping theory mathematically before we see how quantum theory breaks down under experiment. Recently awarded an FQXi grant of over $130,000, Navascués is developing what he calls "almost quantum theory."

The arena that Navascués is exploring is that of quantum correlations, something he explains using two oft-used characters, Alice and Bob. Each has a quantum lab for measuring particle properties, such as its speed, or whether it moves left or right. A correlation manifests as Alice and Bob’s results being linked. Perhaps every time Alice’s particle moves to the right so does Bob’s. "Ultimately, there must have been some connection between Alice and Bob before they conducted the experiment," Navascués says. "Otherwise there shouldn’t be any correlation between the labs."

Roll of the Dice

These correlations don’t necessarily have to be quantum. Say Alice and Bob choose which result to announce based on dice that you had rolled before hand. By distributing the result of your roll to both parties, you correlate their results, and there’s nothing particularly mysterious about that. But the quantum world is different and a bit stranger. Imagine creating two quantum particles in the same place and passing one to Alice and the other to Bob to experiment on and produce a measurement. Those particles might exhibit the properties of entanglement: change the quantum state of Alice’s particle and Bob’s instantly changes too. Some of those correlations cannot be explained using everyday "classical" physics.

The set of quantum correlations is actually greater than the set of classical correlations, with the latter being a particular sub-set of the former. And just as quantum theory usurped classical theory, Navascués hopes to topple quantum theory by finding an alternative that, in turn, has a greater set of correlations, but that still contains those of quantum theory within it. Three years ago he made a breakthrough. "We call it the ’almost quantum’ set," he says. "It contains correlations and outcomes not possible within quantum theory." They cannot be explained as the equivalent of creating two quantum particles in the same place and passing one to each experimenter, which is how entanglement is usually understood to be created. In recent work, Navascués and colleagues have identified a way to distinguish conventional quantum correlations from almost-quantum correlations (Belén Sainz et al., Phys. Rev. Lett. 120, 200402 (2018).)

Eventually, an experiment might be performed that catches such an almost-quantum violation in action. Then quantum theory will fall just like all its predecessors. "Alice and Bob would conclude that quantum mechanics is false," Navascués says. The way in which it breaks could point the way to the much-sought-after ’Theory of Everything,’ a model of quantum gravity that combines the microscopic world of sub-atomic particles with the macroscopic universe full of stars, planets and black holes.

But quantum theory has been around for over a century, so, if Navascués is right, why haven’t we seen a trace of behaviour beyond it yet? "Perhaps we don’t know how to make the proper measurements or we don’t know how to prepare the particles in the right way," he says. Most of the experiments people have done in this area rely on photons—particles of light. "Quantum mechanics is a theory that was built for photons," notes Navascués. "Perhaps if you start conducting weird measurements, with more exotic particles, you might get results beyond what quantum theory predicts."

"I find his work very intriguing, and I have been fascinated by it since hearing it for the first time a few years ago," says Gerardo Adesso, a mathematical physicist at the University of Nottingham, UK. It "leaves room for a little more beyond quantum theory. This little more is still far from being formalised and understood, both mathematically and physically, but some surprising connections with certain approaches to quantum gravity make Miguel’s approach even more plausible." Adesso adds that what’s needed is an experiment that might falsify or validate Navascués’s ideas, but that might take fifty years.

"What Miguel and colleagues did was thought-provoking," says Marco Piani, from the University of Strathclyde. But he adds that the approach’s weakness is that, so far, it only deals with correlations. It may be easier to infer the underlying theory if Navascués was able to include other quantum properties in the analysis. "Nonetheless, it is clear that we can learn a lot in the process of trying, and the work by Miguel and colleagues is certainly an important contribution."