Using ultra cold atoms:
The time it takes for an atom to quantum-mechanically tunnel through an energy barrier has been measured by Aephraim Steinberg of the University of Toronto and colleagues. The team observed ultracold atoms tunnelling through a laser beam, and their experiment provides important clues in a long-standing mystery in quantum physics.
Quantum tunnelling involves a particle passing through an energy barrier despite lacking the energy required to overcome the barrier, as required by classical physics. The phenomenon is not fully understood theoretically, yet it underpins practical technologies ranging from scanning tunnelling microscopy to flash memories.
There has been a long controversy about the length of time taken to cross the barrier – a process that cannot be described as a classical trajectory. This problem arises because quantum mechanics itself provides no prescription for it, explains Karen Hatsagortsyan of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. “Many definitions have been invented, but they describe the tunnelling process from different points of view”, he says, “and the relationship between them is not simple and straightforward.” …
Steinberg adds that the technique could reveal something about the trajectory within the barrier itself. “We hope in the future to restrict our effective magnetic field to regions even smaller than the barrier”, he says, “so that when we look at the final spin, we’re measuring not how much time the atom spent somewhere ill-defined in the barrier, but in one particular region.” According to one theoretical description, he says, it looks as though a particle “appears on the far side without ever crossing the middle. This is what we’d like to test.”
Philip Ball, “Quantum-tunnelling time is measured using ultracold atoms” at Phys.org
One wonders if they will end up with several measurements because the particle can’t decide. 😉
Tunnelling is one of the most characteristic phenomena of quantum physics, underlying processes such as photosynthesis and nuclear fusion, as well as devices ranging from superconducting quantum interference device (SQUID) magnetometers to superconducting qubits for quantum computers. The question of how long a particle takes to tunnel through a barrier, however, has remained contentious since the first attempts to calculate it1. It is now well understood that the group delay2—the arrival time of the peak of the transmitted wavepacket at the far side of the barrier—can be smaller than the barrier thickness divided by the speed of light, without violating causality. This has been confirmed by many experiments3,4,5,6, and a recent work even claims that tunnelling may take no time at all7. There have also been efforts to identify a different timescale that would better describe how long a given particle spends in the barrier region8,9,10. Here we directly measure such a time by studying Bose-condensed 87Rb atoms tunnelling through a 1.3-micrometre-thick optical barrier. By localizing a pseudo-magnetic field inside the barrier, we use the spin precession of the atoms as a clock to measure the time that they require to cross the classically forbidden region. We study the dependence of the traversal time on the incident energy, finding a value of 0.61(7) milliseconds at the lowest energy for which tunnelling is observable. This experiment lays the groundwork for addressing fundamental questions about history in quantum mechanics: for instance, what we can learn about where a particle was at earlier times by observing where it is now11,12,13.
Ramón Ramos, David Spierings, Isabelle Racicot & Aephraim M. Steinberg , “ Measurement of the time spent by a tunnelling atom within the barrier region” at Nature | Vol 583 | 23 July 2020 |
For a limited time (we suspect), the article is free here.