What Can We Learn from Attosecond Pulses?

The genesis of light pulses with attosecond (10$^{-18}$ seconds) durations signifies a new frontier in time-domain physics. This achievement was recognised by the 2023 Nobel Prize in Physics awarded to Agostini, Krausz and L'Huillier. The scientific impact is obvious: the timescale necessary for probing the motion of electron(s) in the ground state is attoseconds (atomic unit of time $\approx$24 as). In the first part of the talk, I will describe the underlying attosecond dynamics that emerge when an atom interacts with an intense optical field. This will be framed in the context of the well-established semiclassical 3-step model of strong-field physics.

The second part of the talk will focus on a novel experiment that emphasizes how these pulses can be applied to access unique physics. Feynman taught us that the outcome of a quantum process is dictated by the sum over all the quantum trajectories that contribute to it. Naturally, when analysing experiments, we often refer to these individual trajectories even though they have not been measured individually. Here I will describe a fully functioning Quantum Trajectory Selector (QTS) method that allows us to directly measure the outcome of ''single'' trajectories that, summed together, make up a quantum process. Our QTS operates in the strong-field domain, where the simple semiclassical model mentioned above is ubiquitously used to describe individual quantum trajectories. In the QTS method, attosecond pulse trains are used in a reverse-engineering scheme to study strong-field physics with exquisite and selective control of the initial conditions beyond conventional experiments, thus allowing direct timing of recollision processes, both elastic and inelastic, with attosecond resolution.