The tunnelling of a particle through a potential barrier is a key feature of quantum mechanics that goes to the core of wave–particle duality. The phenomenon has no counterpart in classical physics, and there are no well constructed dynamical observables that could be used to determine ‘tunnelling times’. The resulting debate1,2,3,4,5 about whether a tunnelling quantum particle spends a finite and measurable time under a potential barrier was reignited in recent years by the advent of ultrafast lasers and attosecond metrology6. Particularly important is the attosecond angular streaking (‘attoclock’) technique7, which can time the release of electrons in strong-field ionization with a precision of a few attoseconds. Initial measurements7,8,9,10 confirmed the prevailing view that tunnelling is instantaneous, but later studies11,12 involving multi-electron atoms—which cannot be accurately modelled, complicating interpretation of the ionization dynamics—claimed evidence for finite tunnelling times. By contrast, the simplicity of the hydrogen atom enables precise experimental measurements and calculations13,14,15 and makes it a convenient benchmark. Here we report attoclock and momentum-space imaging16 experiments on atomic hydrogen and compare these results with accurate simulations based on the three-dimensional time-dependent Schrödinger equation and our experimental laser pulse parameters. We find excellent agreement between measured and simulated data, confirming the conclusions of an earlier theoretical study17 of the attoclock technique in atomic hydrogen that presented a compelling argument for instantaneous tunnelling. In addition, we identify the Coulomb potential as the sole cause of the measured angle between the directions of electron emission and peak electric field: this angle had been attributed11,12 to finite tunnelling times. We put an upper limit of 1.8 attoseconds on any tunnelling delay, in agreement with recent theoretical findings18 and ruling out the interpretation of all commonly used ‘tunnelling times’19 as ‘time spent by an electron under the potential barrier’20.
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The data that support the findings of this study are available from the corresponding authors on reasonable request.
is available for this paper at https://doi.org/10.1038/s41586-019-1028-3.
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The experiments were performed at the Australian Attosecond Science Facility at Griffith University. We acknowledge the traditional custodians of the land on which this work was undertaken at Griffith University, the Yuggera people, and at ANU, the Ngunnawal people. U.S.S. and A.A.-T.-N. were supported by Griffith University International Postgraduate Research Scholarships (GUIPRS). H.X. was supported by an Australian Research Council Discovery Early Career Researcher Award (ARC DECRA: DE130101628). The work of N.D. and K.B. was supported by the United States National Science Foundation under grant no. PHY-1430245 and XSEDE allocation PHY-090031: their calculations were performed on SuperMIC at the Center for Computation and Technology at Louisiana State University. I.I. was supported by the Institute for Basic Science under grant number IBS-R012-D1.