Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed an innovative approach to creating some of the smallest and smoothest mirrors ever made, designed for controlling individual particles of light, or photons. This advancement has the potential to significantly impact future quantum computers, quantum networks, integrated lasers, and environmental sensing technologies.
The team, led by Marko Lončar, the Tiantsai Lin Professor of Electrical Engineering at SEAS, along with Mikhail Lukin and Kiyoul Yang, has published their findings in the journal Optica. Their method involves crafting high-performance, curved optical mirrors that can effectively trap light, an essential function for advancing quantum technologies.
Revolutionizing Optical Resonators
Optical resonators, also known as optical cavities, are critical components in various light-based devices, from lasers to precision instruments used for timekeeping and spectroscopy. These devices operate on the principle that only specific wavelengths of light can resonate within the space between two mirrors, similar to how certain sounds resonate with guitar strings.
The research team demonstrated state-of-the-art optical resonators capable of controlling light at near-infrared wavelengths, which is vital for manipulating single atoms in quantum computing applications. As the demand for quantum applications rises, the need for smaller optical cavities with minimal signal loss has become increasingly important.
The new microfabrication technique, pioneered by former graduate student Sophie Ding, addresses a significant challenge faced by physicists working with ultracold single atoms. They sought optical cavities with exceptionally smooth mirrors that would allow for strong coupling between atoms and photons, facilitating efficient interactions essential for high-fidelity quantum networking.
According to Brandon Grinkemeyer, a postdoctoral researcher in the Lukin lab, “We needed these high-quality photonic interfaces to create efficient ways to have single photons interact with single atoms, allowing for fast, high-fidelity quantum networking.”
Innovative Microfabrication Method
Traditional lithography and etching methods often fail to produce the necessary smooth surfaces for demanding quantum applications. Ding’s novel method employs thermal oxidation on a silicon wafer to create a smooth silicon surface by flattening bumps and grooves. The researchers then deposited a precisely engineered stack of transparent oxide layers, which form a dielectric mirror coating.
When a hole is etched in the back and the coating is released from the silicon wafer, it naturally buckles into a perfectly curved shape due to built-in mechanical stress, resulting in a high-quality mirror. This approach allows for precise control over the mirror’s curvature radius and the wavelengths of light it reflects, making the method both scalable and relatively straightforward.
Ding explained, “In microfabrication, we are sometimes confined by the thought that surface roughness is defined by the etch or the mask, and we try very hard to optimize them. But when we are using the properties of the materials, we can do a lot less of that and have more robust results.”
The researchers achieved a remarkable “finesse” of 0.9 million at a wavelength of 780 nanometers, indicating that light can bounce back and forth inside the cavity nearly a million times before scattering. This is particularly relevant for optical telecommunications, which typically operate at wavelengths of 1550 nanometers.
The optical cavities developed through this new method are poised to play a critical role in modular quantum computing, enabling numerous atoms to be interconnected by photons traveling through optical fibers. These cavities serve as essential interfaces for converting an atom’s quantum state into light, allowing for transmission and re-integration with another atom.
Beyond quantum computing, the versatility and scalability of this technology could lead to applications in ultra-compact lasers, spectroscopic sensors, and integrated photonics, where multiple optical resonators can be integrated directly onto chips.
The collaborative work involved contributions from researchers including G.E. Mandopoulou, R. Jiang, A.S. Zibrov, G. Huang, K. Yang, and M.D. Lukin. The project received support from the U.S. Department of Energy, the National Science Foundation, the Center for Ultracold Atoms, and the Air Force Office of Scientific Research, with fabrication conducted at the Center for Nanoscale Systems at Harvard, also supported by the National Science Foundation.
This advancement in optical technology not only underscores Harvard’s commitment to pioneering research in quantum applications but also sets the stage for future developments that could redefine the landscape of quantum computing and photonics.
