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Although quantum tunnelling has been studied since the inception of quantum mechanics, some aspects remain controversial, particularly the duration of tunnelling events. In a recent experiment [1], we investigate the quantum-mechanical motion of particles in a system of two coupled waveguide potentials, where population transfer between the waveguides acts as a clock, enabling the measurement of particle speeds along the waveguide axis. Applying this approach to exponentially decaying states at a reflective potential step, we determine an energy–speed relationship for tunnelling particles. We find that the lower the particle energy, the higher the measured speed inside the potential step. Our findings contribute to the ongoing debate on tunnelling times and can also be interpreted as a test of Bohmian trajectories.
[1] V. Sharoglazova, M. Puplauskis, C. Mattschas, C. Toebes, J. Klaers, Nature 643, 67 (2025).