Time is not an operator in quantum mechanics, and therefore not a direct observable . As a consequence, timing information on any physical process has to be extracted indirectly by choosing other measurable observables. Following the peak of an electron wavepacket for determining photoionization time delays, for example, can be tricky and often misleading. In contrast to a light pulse, an electron wavepacket disperses even in vacuum. Since the propagation of the peak of a wavepacket follows by definition the group delay, almost any group delay can be measured during propagation in combination with an appropriate energy-dependent transmission filter. This explains why in the multi-photon or tunnel ionization regime the group delay (or the related, although not exactly equivalent, Wigner delay) gives the wrong explanation for the measured delay [2,3], whereas in the single-photon ionization regime we can show experimentally that the Wigner time delay explains the general trend correctly although it does not capture all the observed features (Fig. 1c).
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