Curator's Take
This article shows that the two dominant electron trajectories in high‑harmonic generation can be treated as a genuine qubit—an “attosecond path qubit”—opening a new hardware platform where quantum information is encoded in ultrafast light‑matter dynamics. By separating classical dephasing from true trace‑out decoherence, the authors provide a clear diagnostic toolbox that could help researchers preserve coherence long enough to perform elementary gate operations or interferometric protocols on attosecond timescales. If experimental mode‑selection and conditioning techniques can reliably isolate these mechanisms, HHG‑based qubits may complement solid‑state and photonic approaches for ultrafast quantum control, though the extreme sensitivity to shot‑to‑shot fluctuations remains a practical hurdle.
— Mark Eatherly
Summary
High-harmonic generation (HHG) is governed by interference between electron trajectories. We propose that the dominant short and long trajectories define an experimentally addressable two-level subsystem: an attosecond path qubit (APQ). We formulate a trajectory-resolved density matrix to identify two distinct coherence-loss mechanisms: classical dephasing from ensemble averaging and quantum decoherence arising from the trace-out of unobserved degrees of freedom. By investigating shot-to-shot fluctuations and unresolved transverse momentum, we demonstrate that while dephasing suppresses coherence through averaging, the ``trace-out'' channel produces mixed states even for fixed driving parameters. We explore how these mechanisms modify APQ purity and show that mode selection and conditioning provide operational routes to isolate them. These results establish a reduced-state framework for diagnosing coherence loss in HHG and for engineering trajectory-based quantum states in attosecond interferometry.