Quantum Tunneling Is Not Superluminal

Figuring out how long it takes an atom to engage in quantum tunneling is very experimentally challenging, because the distances are short, the time elapsed is tiny and hard to measure, and the things observed are on the wave-particle boundary. But a couple of scientists came up with an experimental design that addresses the challenges. 

The possibility of superluminal travel had seemed particularly plausible because the correct propagator path integral used to determine the probability of a photon moving from points A to point B using quantum mechanics includes both superluminal and subluminal paths in the calculation, which is pretty much the only time in all of physics that calls for physicists to even contemplate that massless particles move at anything other than the speed of light, in all of physics. It is also a good place to look because quantum tunneling is one of the most famous circumstances where key conservation laws, that are ironclad in classical physics, apply to end states of an interaction but not to the intermediate states. 

But they found that quantum tunneling does not appear to happen at faster than the speed of light, as some theorists had suggested might be possible in this situation. Their experiment showed no signs of superluminal or instantaneous movement of particles.

What time does a clock tell after quantum tunneling? 

Predictions and indirect measurements range from superluminal or instantaneous tunneling to finite durations, depending on the specific experiment and the precise definition of the elapsed time. Proposals and implementations use the atomic motion to define this delay, although the inherent quantum nature of atoms implies a delocalization and is in sharp contrast to classical trajectories. 

Here, we rely on an operational approach: We prepare atoms in a coherent superposition of internal states and study the time read-off via a Ramsey sequence after the tunneling process without the notion of classical trajectories or velocities. Our operational framework (i) unifies definitions of tunneling delay within one approach, (ii) connects the time to a frequency standard given by a conventional atomic clock that can be boosted by differential light shifts, and (iii) highlights that there exists no superluminal or instantaneous tunneling.

Patrik Schach and Enno Giese, "A unified theory of tunneling times promoted by Ramsey clocks" 10(16) Science Advances (April 19, 2024).