Anti-crossing dynamics and quantization axis control in a trapped ion using optical tweezer

Tightly focused laser beams to control and trap atoms play an important role across a range of quantum applications, such as in neutral atoms where tweezer potentials are usually the dominant trapping element. A number of suggestions have recently arisen for the use of tweezers in trapped-ion work, for locally modifying the trapping potential, allowing state-dependent modifications of motional modes for novel gate mechanisms and in long ion chains. One aspect of the use of tweezers is the presence of strong light shifts, which when strong enough, re-define the quantization axis, acting in competition with other external fields, such as magnetic fields. When the alignment of these two is not the same, the tweezer leads to a modification of the energy eigenstates and energy eigenvalues. We explore to what extent these modifications can be used to dynamically control the internal state of the atom.

We use a single trapped 40Ca+ ion to demonstrate this dynamic control with an optical tweezer that induces strong, polarization-dependent ac-Stark shifts. These shifts differentially affect the 3D5/2 Zeeman sublevels, effectively redefining the local quantization axis and enabling controlled coupling between the sublevels. By resonantly probing the 4S1/2 to 3D5/2 quadrupole transition with a narrow-linewidth 729 nm laser in the presence of the tweezer, we map the evolution of the eigenenergies and directly observe changes in the transition strengths, showing the rotation of the eigenbasis. The system exhibits characteristic Demkov-Kunike model dynamics arising from avoided crossings between the sublevels, which we control via the tweezer intensity. By tuning the passage rate, we demonstrate both adiabatic and diabatic population transfer between selected Zeeman states using only the tweezer light field. The precise and local control over the internal state dynamics could aid in qudit applications as well as in magnetic field sensing.