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Abstract:
Quantum coherent control using pulsed lasers is not only a compelling demonstration of the elegance of quantum mechanics but also an essential technology with potential applications. These include selective photochemical reactions, Floquet engineering—which seeks to realize desired Hamiltonians and quantum states by periodically driving systems externally—and fundamental techniques for quantum computing. In this talk, I will present our recent results on coherent control of solid surfaces using multiphoton photoemission spectroscopy[1].

The stage for demonstrating coherent excitation in this study is the Graphene/Ir(111) surface. This system, while structurally simple, exhibits a rich variety of physical phenomena due to the weak interaction between the graphene layer and the metallic substrate. Notably, it features ripple structures and moiré patterns that modulate the local electronic properties, providing an ideal platform for investigating coherent electron dynamics in a quasi-free-standing two-dimensional material. By employing time-resolved two-photon photoemission spectroscopy with femtosecond laser pulses, we were able to selectively excite and probe image-potential states above the surface. A distinct quantum
beat signal was observed in the transient spectra of the fourth and higher-order image-potential states, which clearly contrasts with previous studies [2] that primarily focused on lower-order states or did not resolve coherent dynamics. Quantum beats [3] arise from the interference between closely spaced quantum states and manifest as temporal oscillations in the photoemission intensity, serving as a hallmark of phase-coherent superposition. The observation of such beats at higher-order states indicates that coherence can be sustained even for electronic states further away from the surface, suggesting unexpectedly long coherence lengths and lifetimes in this system. These findings not only
deepen our understanding of the electronic structure and dynamics at graphene/metal interfaces but also offer new perspectives for utilizing such systems in quantum control and ultrafast electronic applications.

References:
[1] Y. -C.Tai, P. Amrit, C. -L.Lin, H. Ishida and R. Arafune (in preparation)
[2] D. Niesner, et al., Phys. Rev. B 85, 081402 (2012).
[3] U. Höfer et al., Science 277 1480, (1997).