They rank among the darkest and coldest places in the solar system: Hundreds of lunar craters, many of them at the Moon's south pole, never receive direct sunlight and lie in permanent shadow. That's exactly why physicist Jun Ye and his colleagues suggest that these craters are the perfect place to build a critical component for an ultrastable laser.

On the Moon, a highly stable laser - a source of coherent light that has a nearly unwavering frequency, or color - could provide a master time signal and offer GPS-like lunar navigation, said Ye, who is affiliated with both the National Institute of Standards and Technology (NIST) and JILA, a joint institute of NIST and the University of Colorado Boulder. Multiple copies of these lunar lasers could precisely measure the distances between objects and potentially detect exotic physics phenomena such as ripples in space-time.

To construct a lunar laser, astronauts would first install a key component known as an optical silicon cavity - a block of silicon that permits only certain frequencies of light to bounce back and forth between mirrors on each end of the block. The distance between the two mirrors determines the frequencies that are allowed to resonate; for a highly stable optical cavity, that distance, and therefore those frequencies, does not vary.

The Moon is an ideal location for building an optical cavity because it's subject to relatively few vibrations compared with Earth and has a high vacuum (since its environment is devoid of air).

But the permanently shadowed craters at the lunar south pole provide an even greater advantage. Their frigid temperature of around 50 degrees above absolute zero (50 kelvins) drastically reduces the random jitter of the mirrored surfaces.

In addition, these craters have an even higher vacuum than the lunar surface, further reducing or eliminating vibrations from sound waves and stray particles that could strike the mirrors and change the distance between them.

By radiating any residual heat from the cavity system into the much colder abyss of outer space, the optical cavity could be cooled further, without the need for a cryostat or other equipment, to a temperature of 16 K. At that temperature, silicon neither expands nor contracts when exposed to tiny changes in temperature, ensuring that light entering the cavity always traverses exactly the same distance between the two mirrors.

Once the optical silicon cavity is deployed, a commercially available laser would be placed nearby - either on the rim or inside the permanently shadowed crater. A small amount of laser light directed into the optical cavity would be used to lock the laser frequency to one of the resonant frequencies allowed by the cavity, ensuring that the laser emits light of a single unchanging color.

After stabilizing the frequency of its light, the laser could act as a GPS-like signal, guiding lunar spacecraft to land safely, especially those set to touch down on dimly lit regions near the south pole. By tuning its light to the signals of atomic clocks on satellites, a high-stability lunar laser could also form the backbone of the first optical atomic clock on an extraterrestrial surface. This timekeeping signal would rival those from the most precise and accurate optical atomic clocks on Earth, which Ye and colleagues have built in Earth-bound laboratories.

A lunar laser locked to an ultrastable silicon cavity placed inside one of the Moon's permanently shadowed craters could provide the infrastructure for a lunar time scale, Earth-Moon optical communication, satellite-based space distance measurements and imaging, and a space-based optical atomic clock.

Ye and his colleagues, including researchers from JILA; NASA's Jet Propulsion Laboratory in Pasadena, California; the Physikalisch-Technische Bundesanstalt (PTB) in Germany; and Lunetronic Inc. in San Francisco, describe their proposal in a recent issue of the Proceedings of the National Academy of Sciences.

Although the idea of building a laser inside a lunar crater may seem like pie in the sky, NASA has already designated regions near the south pole's permanently shadowed craters as landing sites for the space agency's Artemis mission.

Ye, an expert on lasers and precision measurements, came up with the idea for a lunar laser after talking with colleagues about the types of instruments that the Artemis mission could carry and install on the lunar surface. Some ideas seemed impractical or involved technology not fully developed on Earth.

"I thought, 'let me throw out another crazy idea' - except it turned out to be not so crazy after all," Ye noted. After working with silicon resonant cavities for years, Ye and his colleagues at both JILA and the German national metrology institute "know exactly what the key ingredients are for building a silicon cavity," he added. "As soon as I understood what the permanently shadowed regions can offer, I felt that this would be the most ideal environment for a super-stable laser."

If astronauts were to install a network of these lunar lasers, said Ye, the instruments could measure distances between objects on the Moon with extraordinarily high precision. Such precision could enable the Moon-based lasers to act as a detector for gravitational waves, ripples in space-time that would jostle the Moon and alter ever so slightly the distance between lunar objects as they pass by.

The silicon optical cavity, small enough to fit inside an Artemis spacecraft, would be fully assembled on Earth, said study co-author Wei Zhang of NASA's Jet Propulsion Laboratory. During deployment on the Moon, the device's radiation panels would need to unfold. Astronauts would use a remote or mechanically controlled lunar rover to lower the cavity into the crater, Zhang added.

Because of poor illumination, it will be challenging to land on the Moon's polar regions, noted co-author Yiqi Ni, of Lunetronic. However, permanently shadowed regions on the Moon remain central to long-term lunar exploration because they contain water-ice and other resources needed to maintain a human presence.

Ni estimates that a silicon optical cavity could be demonstrated in low-Earth orbit within two years, deployed on the lunar surface within three to five years, and eventually installed inside a dark crater through coordinated multiagency efforts.

Paper: Jun Ye, Zoey Z. Hu, Ben Lewis, Wei Zhang, Fritz Riehle, Uwe Sterr, Yiqi Ni and Julian Struck. Lunar Silicon Cavity. Proceedings of the National Academy of Sciences. Published online May 8, 2026. DOI: 10.1073/pnas.2604438123