Atop Two Hawaiian Mountains, NIST Proves Ultra

Sep 14, 2023

Accurate timekeeping plays a vital role in synchronizing numerous systems around the globe, from telecommunications and power grid networks to precision sensing and scientific research. Traditionally this process is achieved by communicating with satellites that utilize atomic clocks. These clocks can tell time by reading out the resonant frequencies of atoms from certain elements such as cesium and rubidium.

The next generation of this technology, known as an optical clock, takes advantage of elements that have higher resonant frequencies, such as strontium and ytterbium, and requires laser systems for measuring. Most importantly, optical clocks offer a much higher level of timekeeping precision.

This article reviews a novel process recently developed by the National Institute of Standards and Technology (NIST) for synchronizing optical clocks without having to sacrifice fidelity.

Last year, on the mountaintops of Hawaii, a team of researchers from NIST carried out an experiment for optical time transfer in the hopes of developing a reliable method that could help large-scale free space networks interconnect existing ground-based optical clocks and future space-based optical clocks.

The researchers placed a laser module on the Mauna Loa volcano, pointed at a reflector situated on the Haleakala peak at Maui. Across a distance of about 150 kilometers, the scientists transmitted an extremely precise time signal through the air at power levels that could be compatible with future space-based missions.

The researchers suggest that this system could enable time transfer from ground-based stations to satellites that are placed 36 thousand kilometers above the Earth (in geosynchronous orbit), effectively synchronizing optical clocks with an accuracy in the femtoseconds (one quadrillionth of a second). According to NIST, this would result in a precision about 10 thousand times higher compared to the state-of-the-art approaches. Additionally, their system can operate using only the bare-minimum timing signal strength while not losing any fidelity, making it highly robust in mitigating atmospheric disturbances.

Synchronizing instruments across vast distances with this kind of precision opens up a treasure trove of new possibilities, particularly in the realm of physics, providing scientists with a path toward a deeper understanding of the fabric of the universe. For example, this method can help with testing general relativity and even give insights into the composition of dark matter. Outside of optical clocks, connecting arrays of sensors located at great distances apart can advance very long baseline interferometry (VLBI), which could be used in improving black hole imaging.

NIST's method to link satellites to optical clocks located on opposite sides of the world could redefine the SI second to an optical standard by splitting it up into even smaller chunks. This is possible thanks to an advancement known as a frequency comb.

A frequency comb is a Nobel Prize-winning discovery often described as a ruler for light that can produce very finely separated wavelengths measured to a high degree of accuracy. Using this technology, scientists can precisely energize the atoms in optical clocks and translate the terahertz oscillating frequencies into lower ones.

For their experiment, the team from NIST developed an improved version of the frequency comb, dubbed the time-programmable frequency comb. According to Laura Sinclair, a physicist at NIST’s Boulder campus and one of the authors of the paper, this method breaks the rule of frequency combs, which demands the use of a fixed-pulse spacing for precision operation—allowing the scientists to yield extremely accurate results even when a system has only a little light to work with.

Because of the time-programmable frequency comb, researchers could send the signal from Mauna Loa to Haleakala in a roundtrip of 300 kilometers using only 40 microwatts of power and just the bare-minimum signal strength needed for synchronizing devices (known as the quantum limit). In this experiment, the signal penetrated more atmospheric disturbances than it would ever encounter on a potential trip from the ground to geosynchronous orbit.

Ultimately, the NIST team's goal is to form the backbone of future sensing networks based on their recent discoveries. Improving this technology means that the researchers need to reduce the size, weight, and power consumption of this device, adapting it for use in mobile systems, the most important ones being satellites.

Because NIST is an institution with a long-standing history as the official standardization body of the United States, this technology will likely be standardized and implemented throughout many laboratories sometime in the near future.

While synchronization with femtosecond accuracy is not a first-priority upgrade for most communications networks, this technology presents new potential for sensing and measurement applications that could lead scientists and engineers to develop novel systems.