Zhenzhong Xing

Laser cooling is fundamental to quantum computing and metrology using atomic systems. Precise control often requires cooling atoms' motional degrees of freedom to the quantum ground state, imposing operation time and architectural limitations particularly in large-scale systems. Here we demonstrate how the integrated optical control of interest for scaling trapped-ion systems additionally enables laser cooling that bypasses limitations of conventional schemes. Leveraging multi-channel integrated delivery of ultraviolet to infrared wavelengths for calcium ion control including in passively phase-stable ultraviolet standing waves (SWs), we experimentally verify a long-standing prediction by Cirac et al., realizing Doppler cooling to below the conventional Doppler limit at a SW node. We also present the first realization of ground-state cooling via electromagnetically induced transparency (EIT) using a "probe" beam delivered as a SW with atoms positioned at a node, predicted to enable multi-mode sub-recoil-limit laser cooling. We demonstrate cooling of motional modes spanning an approximately 5 MHz bandwidth from the Doppler temperature to near the ground state within 150 $μ$s, reaching $\bar n \approx 0.05$ phonon number occupancies for the target mode. Direct evaluation against the comparable running-wave (RW) scheme shows the SW implementation's simultaneous advantage in cooling rate, motional mode bandwidth, and final phonon number, as previously theoretically predicted. Our results demonstrate fast cooling of multiple modes to the quantum ground state in an integrated ion trap device, and more broadly how scalable approaches to optical control can enable enhancements in fundamental atomic functionalities.

Laser cooling is fundamental to quantum computation and metrology with trapped ions, and can occupy a majority of runtime in current systems. A key limitation to cooling arises from unwanted carrier excitation, which in typically used running wave (RW) fields invariably accompanies the sideband transitions effecting cooling. We consider laser cooling in structured light profiles enabling selective sideband excitation with nulled carrier drive; motivated by integrated photonic approaches' passive phase and amplitude stability, we propose simple configurations realizable with waveguide addressing using either standing wave (SW) or first-order Hermite-Gauss (HG) modes. We quantify performance of Doppler cooling from beyond the Lamb-Dicke regime (LDR), and ground-state (GS) cooling using electromagnetically induced transparency (EIT) leveraging these field profiles. Carrier-free EIT offers significant benefits simultaneously in cooling rate, motional frequency bandwidth, and final phonon number. Carrier-free Doppler cooling's advantage is significantly compromised beyond the LDR but continues to hold, indicating such configurations are applicable for highly excited ions. Our simulations focus on level structure relevant to $^{40}$Ca$^+$, though the carrier-free configurations can be generally applied to other species. We also quantify performance limitations due to polarization and modal impurities relevant to experimental implementation. Our results indicate potential for simple structured light profiles to alleviate bottlenecks in laser cooling, and for scalable photonic devices to improve basic operation quality in trapped-ion systems.