The moon’s south pole is home to permanently shadowed craters, and now researchers are proposing that they could be home to incredibly stable lasers that could one day serve as a lunar time standard. Although currently only a proposal, such a time standard would synchronize activity on the moon and beyond, as well as enable GPS-like navigation for spacecraft in lunar orbit and rovers on the surface.

There are many reasons the moon is an ideal location for a stable laser, says Jun Ye, a fellow at JILA (formerly known as the Joint Institute for Laboratory Astrophysics), a joint research institute between the University of Colorado Boulder and the National Institute of Standards and Technology (NIST). The moon is not tectonically active, and therefore there are fewer vibrations to jostle a laser compared to Earth. It also lacks an atmosphere that could disturb a laser’s mirrors.

Lunar craters that never receive direct sunlight offer additional advantages. Ranked among the darkest and coldest places in the solar system, the frigid temperatures of about 50 kelvin at these permanently shadowed craters significantly reduce any heat-related random jitters that a laser’s mirrors might experience. They have also trapped large caches of water ice over millions of years, making them central to plans for long-term lunar exploration.

“It is possible to build the world’s most stable laser on the moon,” Ye says.

Lunar Atomic Clock and Navigation

An exceptionally stable laser could help serve as the base for a lunar time standard, one based on what would be the first atomic clock on an extraterrestrial body. Optical atomic clocks use intersecting laser beams to entrap and monitor the quantum vibrations of atoms to keep time. Ye, whose accolades include the 2022 Breakthrough Prize in Fundamental Physics, and his colleagues suggest a lunar stable laser could support a timekeeper rivaling the most precise optical atomic clocks on Earth. The scientists detailed their proposal online on 8 May in the Proceedings of the National Academy of Sciences.

On Earth, GPS and other global navigation satellite systems (GNSSs) depend on satellite-based atomic clocks to transmit precisely timed signals, which a receiver can use to pinpoint its own location by analyzing how long it takes signals from each visible satellite to arrive. A lunar optical atomic clock could similarly lay the foundation for a GPS-like signal to help guide lunar spacecraft and rovers.

The absence of atmospheric interference on the moon would make it easier for this laser to connect with satellites around the moon and Earth and to share its extraordinarily stable frequency. With this common reference, telescopes at these nodes can work together and essentially behave as one giant telescope via a technique known as long baseline interferometry for deep space imaging and gravitational wave detection. This stable master laser could also serve as the backbone for virtually un-hackable quantum networks in space.

An ultrastable silicon cavity laser placed inside one of the moon’s permanently shadowed craters could provide the infrastructure for satellite-based space distance measurements and imaging and a space-based optical atomic clock.J. Ye/NIST; NASA Visualization Studio

Building a Silicon Cavity Laser

To construct a stable laser in a permanently shadowed lunar crater, Ye envisions astronauts first installing a key component known as a silicon cavity, a block of silicon that only allows certain frequencies of light to reflect back and forth between mirrors at each end of the block. The distance between the mirrors determines which frequencies are permitted to resonate. With a highly stable cavity, that distance and those frequencies vary extraordinarily little.

By radiating any residual heat from the cavity into the much greater cold of outer space, it can be cooled further to 16 K without the need for any other equipment, the researchers note. At that temperature, silicon neither expands nor contracts when exposed to tiny changes in temperature. This means the distance between the cavity’s mirrors will remain stable.

“For silicon cavities in our laboratories, we need to have a cryostat to lower the temperature, a vacuum pump to remove air from the empty cavity and maintain high vacuum, and a vibration isolation platform,” Ye says. “All of these conditions can be satisfied in a passive manner in the permanently shadowed region on the moon, without using any active machinery. This was particularly satisfying to me.”

Once this silicon cavity is in place, astronauts would place a high-power laser nearby, either on the crater’s rim or within it. This laser would direct some of its light at the cavity, and the stable beam the laser got back from the cavity would in turn help stabilize all the other beams the laser might emit.

In terms of coherence—the property of lasers that allows light waves from them to flow in perfect unison—a lunar cavity could enable a laser with a coherence exceeding a minute. This is more than 10 times more coherent than the best current terrestrial laser, which is located in Ye’s lab.

The concept of building a laser in a lunar crater may seem like a moonshot. However, NASA has already chosen regions near the lunar south pole’s craters as landing sites for its Artemis program.

An Artemis spacecraft could carry a fully assembled silicon cavity from Earth to the moon, said Wei Zhang, an optical engineer at NASA’s Jet Propulsion Laboratory, in a statement. Astronauts would use a remote or mechanically controlled lunar rover to lower the cavity into a crater.

A major question “is the feasibility of this plan,” Ye says. “Perhaps we will start with housing silicon cavities on near-Earth-orbit satellites first.”