Pilot-wave theory survives a new test on quantum tunneling

The de Broglie-Bohm theory, often pictured as a "quantum surfer", has withstood a fresh challenge. A recent analysis argues that a tunneling experiment published in Nature, which seemed to contradict the theory, is in fact consistent with it once light leaks in the device are included.

A third way at the 1927 Solvay congress

Almost a century ago, the fifth Solvay congress in Brussels opposed two views of quantum mechanics. The Copenhagen school of Niels Bohr, Werner Heisenberg and Max Born held that nature is intrinsically random and cannot be described with classical ideas of space, time and causality. Albert Einstein defended Newtonian determinism, summed up in his line "God does not play dice".

Louis de Broglie offered a third path: his pilot-wave theory. Particles remain point-like objects with real, continuous trajectories, guided by a quantum wave, like a surfer carried by a swell. In the double-slit experiment, the particle goes through one slit only, while the wave passes through both and steers it. Over many repetitions, slightly different initial conditions produce the interference fringes statistically.

The idea was criticized, and de Broglie himself abandoned it. It resurfaced in 1952 through the American David Bohm, hence the name de Broglie-Bohm, or dBB, theory. It is now regarded as a coherent alternative that also dissolves paradoxes such as Schrödinger's cat.

The tunneling objection

Last year, a team led by Jan Klaers in the Netherlands published a Nature paper on quantum tunneling, in which a particle crosses a barrier that classical physics would make impassable, thanks to an evanescent wave seeping through it. Performed with photons, the experiment seemed to put dBB in trouble, because the theory predicted a zero velocity for photons inside the barrier.

The new analysis takes into account light leaking out of the experimental device. With those leaks included, the trajectories do cross the barrier, which reconciles the theory with the observations.

The result does not settle the interpretation debate, which remains open among physicists and philosophers. But it shows that the pilot-wave picture keeps passing experimental tests a century after it was first proposed.