Trapped-ion Laser Cooling Achieves sub-Doppler Limit with 5MHz Bandwidth in Phase-stable Standing Waves

Laser cooling forms the bedrock of controlling atoms for quantum technologies, and researchers continually seek methods to enhance its speed and efficiency. Zhenzhong Xing, Hamim Mahmud Rivy, and Vighnesh Natarajan, alongside colleagues from Cornell University and ETH Zurich, now demonstrate a significant advance in this field. The team achieves remarkably rapid and broadband laser cooling of trapped calcium ions by utilising a stable standing wave of light, a technique that surpasses conventional methods. This breakthrough not only confirms a long-held theoretical prediction, achieving cooling below established limits, but also offers a clear advantage in cooling rate, bandwidth, and final temperature, paving the way for more powerful and scalable quantum systems.

Rapid Multi-Mode Trapped-Ion Laser Cooling Demonstrated

Laser cooling is fundamental to precisely controlling atomic quantum systems and is crucial for scaling trapped ion technologies. This work demonstrates rapid multi-mode laser cooling of trapped ions using a phase-stable standing wave of light, achieving a cooling rate of 200kHz for a single ion. The method simultaneously addresses multiple motional modes with a bichromatic standing wave, created by interfering two laser beams tuned to the desired frequencies. This approach overcomes the limitations of single-mode cooling, enabling fast, broadband cooling of the ion’s motion and establishing a pathway towards improved ion trap performance and scalability for quantum technologies.

The team utilized multi-channel integrated delivery of ultraviolet to infrared wavelengths for calcium ion control, including passively phase-stable ultraviolet standing waves. Experiments verified a long-standing prediction, realizing Doppler cooling below the conventional Doppler limit at a standing-wave node and cooling motional modes to near their ground state within 150 microseconds, reaching exceptionally low phonon number occupancies.

Ions Cooled to Motional Ground State

This research focuses on achieving extremely high precision control of trapped ions for quantum information processing by cooling ions to their motional ground state, minimizing their movement to reduce errors in quantum operations. The work investigates the limitations and challenges in achieving this ultra-cold state and identifies sources of noise and heating that hinder performance.

Researchers achieved significant cooling of ions using both Red-Sideband and Blue-Sideband Electromagnetically Induced Transparency cooling techniques, with Blue-Sideband cooling demonstrating a higher cooling rate and lower final phonon number. However, several factors limit ultimate cooling performance, including heating due to background gas collisions and electric field fluctuations, with axial heating rates of 3. 9(3) ms -1 and R1 mode heating rates of 0. 18(1) ms -1 . Imperfect optical control and noise contribute to residual excitation of the ions’ motion.

The research identified polarization impurity of the cooling laser as a primary limitation preventing cooling to the lowest possible motional excitation. The Indium Tin Oxide coating on the trap electrodes exhibits problematic behaviors, including photo-induced heating, laser-induced charging, and electric field fluctuations, all contributing to motional heating. Detailed simulations helped understand the cooling process and identify the relative importance of different noise sources. This work represents a significant step towards building more robust and reliable quantum computers, as achieving ultra-cold ions is crucial for reducing errors in quantum operations and improving the fidelity of quantum computations. Researchers identified key sources of noise that limit the performance of trapped ion quantum computers, providing essential knowledge for developing strategies to mitigate these noise sources and improve the stability of quantum systems. The findings highlight the importance of material selection and optimization in the design of trapped ion traps.

Rapid Broadband Cooling of Calcium Ions

Researchers have successfully demonstrated rapid and broadband laser cooling of calcium ions using structured light delivered via integrated photonics. They realized cooling below the conventional Doppler limit at a standing-wave node and cooled motional modes to near their ground state within 150 microseconds, resulting in exceptionally low phonon number occupancies. This achievement utilized electromagnetically induced transparency and enabled cooling across a bandwidth of approximately 5MHz.

Experiments confirm a distinct advantage for the standing-wave implementation over traditional running-wave schemes, demonstrating improvements in cooling rate, bandwidth of cooled modes, and final phonon number. These results showcase the potential of structured light for robust ground-state laser cooling and highlight a key functionality enabled by scalable approaches to ion trap control. The researchers acknowledge limitations related to fabrication challenges, specifically delamination of narrow aluminum oxide waveguides, which necessitated adjustments to the input coupling scheme. This research establishes a significant advancement in the precision control of atomic quantum systems and provides a foundation for scaling up trapped ion quantum technologies.