This paper proposes a simple quantum picture inside high-harmonic generation (HHG): the two main electron trajectories that make the light — the “short” and “long” paths — can act like a two-level quantum system, an attosecond path qubit (APQ). HHG is driven by a strong laser and is governed by interference between different electron trajectories that recombine and emit high-frequency light. Treating the dominant short and long trajectories as the two states of a qubit turns HHG into a platform for attosecond-scale quantum interferometry.

To study how that qubit loses its quantum character, the authors build a trajectory-resolved density matrix. A density matrix is a mathematical way to describe a quantum state, including how well-defined the phase relationships (coherence) are. Using this tool they identify two distinct mechanisms that reduce coherence. The first is classical dephasing: when many experimental runs with slightly different conditions are averaged together, the interference pattern washes out. The second is a quantum “trace-out” decoherence: when parts of the system that are not observed (for example, unmeasured motion of the electron) are ignored mathematically, the remaining qubit becomes a genuinely mixed quantum state.

The paper analyzes specific examples of the two mechanisms. They look at shot-to-shot fluctuations in driving conditions and at unresolved transverse momentum (sideways motion of the electron that is not measured). The analysis shows that averaging over fluctuations (classical dephasing) suppresses the observed coherence by washing out the interference, while the trace-out channel creates a mixed quantum state even if the driving parameters are held fixed. In other words, one effect blurs the signal by averaging; the other destroys quantum purity by entangling the qubit with unobserved degrees of freedom.