Optical tweezer arrays combine the scalability of optical lattices with the single-particle control capability of ion traps, making them a promising next-generation quantum platform. For fermionic atoms, however, the tight focusing by high-numerical-aperture (NA) objectives introduces substantial polarization gradients, giving rise to spatially inhomogeneous vector and tensor light shifts and thereby degrading coherence.
To address this challenge, we introduce a 4Pi focusing configuration. Taking ^87\mathrmSr as a representative fermionic species and employing the Richards–Wolf vector diffraction theory, we develop detailed optical field models for both single-objective and 4Pi dual-objective tweezers, and calculate the spatial distributions of clock-transition light shifts under the magic-wavelength condition.
Our results reveal that in the single-objective tweezer, the axial electric field component generated by high-NA focusing induces a pronounced position-dependent degree of circular polarization. This yields a vector-light-shift standard deviation of tens of mHz—approximately 40 times the natural linewidth (\approx1 mHz) of the ^87\mathrmSr clock transition—severely limiting coherence. In contrast, the 4Pi configuration, via destructive interference of the two opposing objectives, strongly suppresses the axial field component and maintains a purely linear polarization throughout the trapping volume. Consequently, the spatial standard deviation of the total light shift is reduced by three orders of magnitude to the 0.1 mHz level, far below the natural linewidth, corresponding to a fractional frequency uncertainty of 10^-19. The residual broadening is negligible for clock-transition coherence.Furthermore, the 4Pi scheme provides tighter axial confinement (ground-state width reduced to 1/3–1/4 of the single-objective value) and requires only half the incident optical power to achieve the same trap depth.
This work overcomes the key obstacle of polarization-induced inhomogeneous light shifts for fermions in optical tweezers, unlocking the inherent advantages of fermions over bosons—including Pauli-blocking-enabled single-atom occupancy without additional cooling steps and the ability to drive clock transitions without a large external magnetic field. These advances pave the way toward high-coherence fermion tweezer arrays for quantum precision measurement, scalable quantum simulation, and fault-tolerant quantum computing.