Tunneling in action
By Margaret Harris
How long does an electron take to tunnel out of an atom exposed to a strong laser field?
Given the somewhat esoteric nature of the question, you might assume that the answer would lie firmly in the realm of theory. But Ursula Keller, whose talk opened this year’s International Conference on Quantum, Atomic, Molecular and Plasma Physics (otherwise known as QuAMP, is an experimentalist, and she and her group at ETH Zurich have made some interesting progress towards pinning down just how long this fundamental quantum-mechanical process takes.
Using a technique called attosecond angular streaking, Keller’s team found an upper bound for the tunneling time of 34 attoseconds. That’s quick — in fact, Keller claims it is the fastest process ever measured, although some might quibble with that distinction. I’m afraid I only grasped her group’s methodology in small chunks — that’s the trouble with talks sometimes — but you can read more in a paper published in Nature Physics last year.
One development that isn’t addressed in the paper, but which Keller touched on in her talk, is just how controversial their result has been among theorists. The idea that tunneling takes a tiny but finite time makes some intuitive sense, but this is quantum mechanics — intuitive sense doesn’t always come into it. Indeed, some theorists have predicted that the electron’s escape literally takes no time at all, while others suggest that tunneling isn’t even the right way to look at the process.
The arguments on this have become so heated, Keller says — half-jokingly I think — that a few of the people involved aren’t on speaking terms anymore. One thing is clear: the debate on electron tunneling is sure to carry on much longer than the process itself.