Condensed matter physics seminar : Quantum Coherent Control of a Single 4f Spin on Surface

To create and control coherent states in a quantum device, a key ingredient is tunable atom-field interaction in a ‘strong coupling regime’. Due to 4f electrons, lanthanides on a surface can exhibit strong anisotropy in their couplings with magnetic field (g-factor), and with nearby spins (J) as well.[1] When located in the sub-nanometer vacuum gap of a scanning tunneling microscope (STM) – which ordinarily supports electric fields of a ~1 GV/m order [2], lanthanide spins can afford unprecedented freedom to tailor atom-field coupling. In this work, we studied electron spin quantum transitions in a single lanthanide two-level system (TLS), an Er atom on MgO/Ag(100) surface, coupled with a nearby Ti atom as a quantum sensor [1] − manifesting a spin-pair of anisotropic g and J tensors. We performed electron spin resonance (ESR) in both frequency [3] and time [4] domains on this spin-pair, revealing coherent quantum features of the Er TLS, supe-rior to a prototypical TLS, Ti on MgO/Ag [2,4,5]: (1) Rabi coupling of the Er spin, an order of magnitude stronger than a single Ti spin. (2) single-photon transition of Δm = ±2, flipping both spins together. Tuning the pair's eigenstates using a vector control of magnetic field enables to engineer g and J couplings in the spin-pair, allowing to further access unconventional transitions: (1) second-order dressed states at resonant driving condition, and (2) two-photon transition of Δm = ±2. All observations are consistently interpreted using a spin Hamiltonian of generalized g and J tensors within the framework of the Floquet formalism,[6] as results of strongly anisotropic g, J couplings and manifestations of higher-order atom-field interactions.