At the bottom of lunar craters, where sunlight never reaches, an eternal cold reigns — about minus 240 degrees Celsius. In these icy pockets, known as permanently shadowed regions, plans are underway to install a mirror resonator made of monolithic silicon. This device would passively trap light, making a laser 'tick' with incredible precision.
The idea is simple: the cold and stillness of the lunar shadow eliminate the thermal jitter of atoms that blurs signals on Earth. Estimates suggest that the stability of such a laser could reach 10⁻¹⁸: that's like a clock losing just one second over thirty billion years. Light in the resonator would maintain its wave coherence for over a minute, ten times longer than the best Earth-based setups.
An ultra-stable beam would open the door to quantum technologies in space: from optical interferometry with baselines spanning thousands of kilometers to secure quantum communication links between satellites. It would also allow testing general relativity with precision unattainable on Earth, where the atmosphere and tides interfere with measurements.
The Moon, forever showing us one face, hides in its shadows not only ice but also the possibility of building the most accurate clock in the Solar System. And then, the cold that seemed like an obstacle becomes an ally.
The temperature in the Moon's permanently shadowed regions is so stable that over billions of years, it has varied less than the temperature in an ordinary room changes in a minute.
In Arthur C. Clarke's novel 'A Fall of Moondust,' lunar bases use extreme conditions for technology, but even he didn't imagine that shadow would become the perfect place for quantum clocks.