Subwavelength Spatial Control and Measurement of Cold Atoms via Optical Nonlinearity and a New Experimental Platform for Two-species Atom Tweezer Arrays

Abstract

Cold atoms trapped in optical lattices have been proven to be a versatile, well-controlled and powerful platform for simulating and studying physics, from condensed matter systems to high energy physics and cosmology. However, the diffraction limit restricts the spatial resolution of light-induced confinement and measurement. Since the length scale of confinement sets a characteristic energy scale of the system, it is desirable to circumvent this diffraction limit to provide more control of the associated energy scale. In the first half of this thesis, I describe a series of experiments that exploit the nonlinear optical response in the three-level system of 171Yb to realize subwavelength spatial control and measurement of cold atoms. First, I report the experimental realization of a conservative optical lattice for cold atoms with subwavelength spatial structure. The three-level system is coupled by two laser fields in a way that the dark state of the three-level system changes its spin composition over a narrow spatial region. The kinetic energy associated with this large gradient in the spin composition creates a lattice of narrow barriers with a width of 10 nm, which is one-fiftieth of the laser wavelength. Extending from this, I describe the realization of a wave-function density microscope with a spatial resolution of 10 nm and temporal resolution of 500 microseconds. Using the sharp spatial dependence in the spin composition of the dark state, we shelve narrow slices of the wavefunction within every unit cell of the lattice into the selected spin state, which is then selectively read out to achieve subwavelength measurement of the atomic probability density. Finally, I report the stroboscopic realization of a λ/4-spaced lattice. We stroboscopically apply shifted versions of the lattice of narrow barriers mentioned earlier, thereby creating an effective time-averaged potential with a lattice spacing of λ/4. In the past decade, significant progress has been made by extending the control of atoms to the single atom level, and single neutral atoms optically trapped in arrays have emerged as a compelling and scalable platform for quantum simulation and computing. A natural step is to extend to multi-species systems, which will help solve challenges encountered in single-species platforms. In the second half of the thesis, I motivate and describe the construction of a new dual-species tweezer array apparatus. An inherent challenge in single-species platforms is the crosstalk between atoms due to the scattered photons from nearby atoms during site-selective control and measurement. A dual-species architecture with Rb and Yb can suppress unwanted crosstalk and even allow parallel tasking on Rb and Yb separately due to the large separation in resonance frequencies. Moreover, the use of multi-species atoms provides an extra tuning knob that is useful in realizing multi-qubit gates. By manipulating the substantial difference in the intra- and inter-species van der Waals force, multi-qubit gates could be realized on Rb and Yb, giving significant speedups for some quantum algorithms and error-correction schemes.