How long quantum tunneling takes to happen?

The difficulty in answering this question lies in the definition of time itself.

Quantum tunneling is a phenomenon in which a particle passes through an energy barrier despite lacking the energy to do so. Although it remains obscure how quantum tunneling works, scientists have known its uses.

The question of how long a particle takes to tunnel through a barrier, however, has remained contentious since the first attempts to calculate it.

The difficulty in answering this question lies in the definition of time itself and how it applies to quantum tunneling.

In this new study, scientists at the University of Toronto have demonstrated a way to measure how long quantum tunneling takes to happen. Through several experiments, they found the result when attempting to estimate how long quantum tunneling takes under certain circumstances.

Scientists took a simplified approach to measure how long it takes for one type of particle to pass through a particular kind of energy barrier. The “clock” in their experiments was the spin of the rubidium atoms used—since the duration of their turn is a known quantity, they can be used as clocks by measuring how much spin occurs while they are subjected to tests—such as passing through a laser beam.

Thus, all scientists had to do was note the current state of spin for the atom before it entered the beam and then measure it again when it exited.

Execution of the plan included trapping a cloud of rubidium atoms utilizing a laser beam and afterward using the same laser beam to move the atoms into the path of another laser beam—and estimating their spin on either side of the second bar. To make it simpler to quantify the spin of the atoms, the analysts first ultra-cooled the cloud before sending them through the energy barrier. Measurement of the change in spin demonstrated the tunneling took roughly 0.62 milliseconds.

Aephraim Steinberg, a professor in the department of physics and co-director of CIFAR’s quantum information science program and senior investigator of the study said, “This whole idea of probing the history of a quantum particle is one that’s been central to my research, and it’s come up over and over again in discussions at CIFAR program meetings.”

“This had been kind of a niche topic in quantum optics. But I think as the technology evolves and as we’re building these larger-scale agglomerations of qubits – the basic unit of quantum information – and trying to learn how to characterize and control each of them, this has renewed practical importance.”

In the future, scientists are planning to learn more about the trajectory of the atoms as they move through the barrier.

Journal Reference:
  1. Ramón Ramos et al. Measurement of the time spent by a tunneling atom within the barrier region, Nature (2020). DOI: 10.1038/s41586-020-2490-7

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