We have imaged lithium-6 thousands of times in an optical tweezer using Λ-enhanced gray molasses cooling light. Despite being the lightest alkali metal, with a recoil temperature of 3.5 μK, we achieve an imaging survival of 0.999 50(2), which sets the new benchmark for low-loss imaging of neutral atoms in optical tweezers. Lithium is loaded directly from a magneto-optical trap into a tweezer with an enhanced loading rate of 0.7. We cool the atom to 70 μK and present a new cooling model that accurately predicts steady-state temperature and scattering rate in the tweezer. These results pave the way for ground state preparation of lithium en route to the assembly of the LiCs molecule in its ground state.
We have imaged lithium-6 thousands of times in an optical tweezer using Λ-enhanced gray molasses cooling light. Despite being the lightest alkali metal, with a recoil temperature of 3.5 μK, we achieve an imaging survival of 0.999 50(2), which sets the new benchmark for low-loss imaging of neutral atoms in optical tweezers. Lithium is loaded directly from a magneto-optical trap into a tweezer with an enhanced loading rate of 0.7. We cool the atom to 70 μK and present a new cooling model that accurately predicts steady-state temperature and scattering rate in the tweezer. These results pave the way for ground state preparation of lithium en route to the assembly of the LiCs molecule in its ground state.
This dissertation advances optical tweezer platforms for neutral-atom quantum processors by addressing three interconnected bottlenecks: imaging survival, motional ground-state preparation, and narrow-line cooling, through experimental and theoretical work on single lithium and cesium atoms.For Li, the lightest stable alkali atom, this work demonstrates a per-image survival probability of 99.950(2)% and a state-detection fidelity of 99.97(2)% across 2000 consecutive 15 ms images, a record for any trapped neutral atom at the time of publication. This was achieved by applying Λ-enhanced gray-molasses (Λ-GM) cooling continuously on the D1 line while simultaneously imaging with the same beam, and represents the first demonstration of loading lithium into a tweezer directly from a MOT. To explain this performance, the dissertation develops a unified Lindblad master-equation formalism for laser cooling in a tightly confined harmonic trap, encompassing polarization-gradient, gray-molasses, EIT, Λ-GM, and single-photon sideband cooling within a single theoretical framework, with 1D simulations that correctly predict steady-state temperature, excited-state fraction, and cooling efficiency across these techniques.For133133Cs, the work exploits the forbidden electric-quadrupole transition 6S1/2→5D5/26S1/2→5D5/2 at 685 nm (Γ ≈ 2π × 117.6 kHz) to demonstrate background-free imaging and narrow-line sideband cooling for the first time in an alkali atom. By positioning the atom at the intensity node of a Laguerre-Gaussian vortex beam, the drive is enhanced while cascade fluorescence at 852 nm is optically isolated from the excitation light, enabling both high-fidelity background-free detection and single-photon sideband cooling to near the motional ground state.Beyond the core cooling and imaging results, the dissertation includes a comprehensive experimental techniques chapter (fiber coupling, AOM design, polarimetry, laser locking, optical pumping, and a Gaussian-process optimization framework), a chapter on quadrupole transition theory and orbital-angular-momentum selection rules, and a concluding survey of the competitive quantum computing landscape across neutral-atom, superconducting, trapped-ion, photonic, spin, and topological qubit platforms. Together, this work establishes the Cs and Li cooling/imaging toolbox as a foundation for future assembly of ultracold LiCs polar molecules in optical tweezer arrays.
Gray molasses cooling of 39K atoms in optical tweezers
Robust cooling and nondestructive imaging are prerequisites for many emerging applications of neutral atoms trapped in optical tweezers, such as their use in quantum information science and analog quantum simulation. The tasks of cooling and imaging can be challenged, however, by the presence of large trap-induced shifts of their respective optical transitions. Here, we explore a system of 39K atoms trapped in a near-detuned (780 nm) optical tweezer, which leads to relatively minor differential (ground vs. excited state) Stark shifts. We demonstrate that simple and robust loading, cooling, and imaging can be achieved through a combined addressing of the D1 and D2 transitions. While imaging on the D2 transition, we can simultaneously apply Λ-enhanced gray molasses (GM) on the D1 transition, preserving low backgrounds for single-atom imaging through spectral filtering. Using D1 cooling during and after trap loading, we demonstrate enhanced loading efficiencies as well as cooling to low temperatures. These results suggest a simple and robust path for loading and cooling large arrays of potassium atoms in optical tweezers through the use of resource-efficient near-detuned optical tweezers and GM cooling.
Robust cooling and nondestructive imaging are prerequisites for many emerging applications of neutral atoms trapped in optical tweezers, such as their use in quantum information science and analog quantum simulation.The tasks of cooling and imaging can be challenged, however, by the presence of large trap-induced shifts of their respective optical transitions.Here, we explore a system of 39 K atoms trapped in a near-detuned (780 nm) optical tweezer, which leads to relatively minor differential (ground versus excited state) Stark shifts.We demonstrate that simple and robust loading, cooling, and imaging can be achieved through combined addressing of the D 1 and D 2 transitions.While imaging on the D 2 transition, we can simultaneously apply -enhanced gray molasses (GM) on the D 1 transition, preserving low backgrounds for single-atom imaging through spectral filtering.Using D 1 cooling during and after trap loading, we demonstrate enhanced loading efficiencies as well as cooling to low temperatures.These results suggest a simple and robust path for loading and cooling large arrays of potassium atoms in optical tweezers through the use of resource-efficient near-detuned optical tweezers and GM cooling.
We report the observation of sub-Doppler cooling of lithium using an irregular-tetrahedral laser beam arrangement, which is produced by a nanofabricated diffraction grating. We are able to capture 11(2)% of the lithium atoms from a grating magneto-optical trap into Λ-enhanced D 1 gray molasses. The molasses cools the captured atoms to a radial temperature of 60(9) μK and an axial temperature of 23(3) μK. In contrast to results from conventional counterpropagating beam configurations, we do not observe cooling when our optical fields are detuned from Raman resonance. An optical Bloch equation simulation of the cooling dynamics agrees with our data. Our results show that grating magneto-optical traps can serve as a robust source of cold atoms for tweezer-array and atom-chip experiments, even when the atomic species is not amenable to sub-Doppler cooling in bright optical molasses.
We present an efficient scheme to implement a gray optical molasses for sub-Doppler cooling of 6Li atoms with minimum experimental overhead. To integrate the D1 light for the gray molasses cooling into the same optical setup that is used for the D2 light for a standard magneto-optical trap (MOT), we rapidly switch the injection seeding of a slave laser between the D2 and the D1 light sources. Switching times as short as 30 $\ensuremath{\mu}\mathrm{s}$ can be achieved, inferred from monitor optical beat signals. The resulting low-intensity molasses cools a sample of $N=9\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms to about 60 $\ensuremath{\mu}\mathrm{K}$. A maximum phase-space density of $\ensuremath{\rho}=1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ is observed. On the same setup, the performance of the GM is compared to that of narrow-line cooling in an ultraviolet (UV) MOT, following the procedure in Sebastian et al. [J. Sebastian, Ch. Gross, K. Li, H. C. J. Gan, W. Li, and K. Dieckmann, Phys. Rev. A 90, 033417 (2014)]. Further, we compare the production of a degenerate Fermi gas using both methods. Loading an optical dipole trap from the gray molasses yields a quantum degenerate sample with $3.3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms, while loading from the denser UV MOT yields $2.4\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms. Where the highest atom numbers are not a priority this implementation of the gray molasses technique yields sufficiently large samples for a comparatively low technical effort.
We report observation of sub-Doppler cooling of lithium using an irregular-tetrahedral laser beam arrangement, which is produced by a nanofabricated diffraction grating. We are able to capture 11(2) % of the lithium atoms from a grating magneto-optical trap into Λ-enhanced D1 gray molasses. The molasses cools the captured atoms to a radial temperature of 60(9) μK and an axial temperature of 23(3) μK. In contrast to results from conventional counterpropagating beam configurations, we do not observe cooling when our optical fields are detuned from Raman resonance. An optical Bloch equation simulation of the cooling dynamics agrees with our data. Our results show that grating magneto-optical traps can serve as a robust source of cold atoms for tweezer-array and atom-chip experiments, even when the atomic species is not amenable to sub-Doppler cooling in bright optical molasses.