A different picture of quantum surrealism

With its ideas of particles zipping in and out of existence, quantum mechanics is probably the kookiest-sounding theory in science. And our understanding of it is little helped by the mysterious “probability fields” most physicists say dictate the zipping.

But a more intuitive picture may lie beneath. As new research demonstrates, beneath the shroud of probability, particles can in fact be viewed as behaving like billiard balls rolling along a table – although in surreal fashion.

The result helps resurrect an 80-year-old picture of quantum mechanics, and provides one of the most stirring demonstrations yet of an effect Einstein called “spooky action at a distance”.

The work, reported in Science Advances, is a new version of the most famous experiment in quantum mechanics, in which particles of light, called photons, are fired at two slits before being detected on a screen.

Hog-tied by Heisenberg’s uncertainty principle, for decades physicists thought they could never know which slit a particular photon went through – any attempted measurement stops it in its tracks.

But in 2011, physicist Aephraim Steinberg at the University of Toronto achieved the seemingly impossible by tracking the trajectories of photons using a series of “weak” measurements, gentle enough not to disturb their position.

This method showed trajectories that looked similar to classical ones – like those of balls flying through the air.

Although it was a seemingly outstanding result, some physicists were not convinced, highlighting the experiment’s inability to deal with “entanglement” (where two particles, in this case photons, are intimately connected so that measurement on one instantly affects the other, no matter how far away it is).

The interpretation treats quantum objects just like classical particles, but imagines them riding like a surfer on top of a so-called pilot wave.

The critics pointed out that doing the same experiment with two entangled photons would lead to a contradiction – such as the photon’s trajectory being measured as going through the top slit, but the photon itself hitting the bottom of the detector (as if it came from the bottom slit). They coined the term “surreal trajectories” to describe this result.

Now Steinberg’s team has achieved the experiment for entangled photons, and shown how the surreal behaviour is caused by the “spooky” influence of the other particle.

The team first entangled two photons, then sent one of the pair through the regular two-slit apparatus, and the other through an apparatus that monitored polarisation – the plane the light waves are travelling in.

Weirdly, the choice made by the experimenters in how to measure the polarisation determined which slit the first photon went through – as if interfering with one particle caused the other to change direction instantaneously.

This kind of bizarre phenomenon is exactly what Einstein had in mind when he dubbed it “spooky action”. Physicists have seen evidence of it before, but never in such a direct fashion.

The results bolster a non-standard interpretation of quantum mechanics, which throws out the notion of abstract probability fields altogether.

First put forward by Louis de Broglie in 1927, the interpretation treats quantum objects just like classical particles, but imagines them riding like a surfer on top of a so-called pilot wave.

The wave is still probabilistic, but the particle does take a real trajectory from source to target.

The new work does not disprove the standard “probabilistic” view of quantum mechanics, but it does highlight that the pilot-wave interpretation is perfectly valid too. That is “something that’s not recognised by a large part of the physics community”, says Howard Wiseman, a physicist at Griffith University who proposed the experiment.

It may be easier to visualise real trajectories, rather than abstract wave function collapses.

“I would phrase it in terms of having different pictures,” says Steinberg. “Different pictures can be useful. They can help shape better intuitions.”