If you know where you are, you don’t know where you’re going, and if you know where you’re going, you don’t know where you are: The Heisenberg uncertainty principle fundamentally limits how precisely position and momentum can simultaneously be known. Another formulation of the principle, linking the uncertainties of energy and time, would seem to suggest that ultrafast measurements—such as the attosecond experiments that probe electronic motion in atoms, molecules, and solids—can’t be both temporally and spectrally precise.
But that’s not quite true. The uncertainty principle does place a lower bound on the product of the time and energy uncertainties of an isolated electromagnetic pulse. An experiment’s temporal precision, however, is not limited by the duration of the pulses used in it; rather, it’s often determined by the degree of control over the relative timing of two pulses. Extreme UV (XUV) pulses with durations on the order of 100 attoseconds...
