Earth's magnetic field is not constant: it trembles from the solar wind, from distant thunderstorms, and from the subway running nearby. To notice these tremors, you need an instrument that doesn't tremble itself. Physicists have built such an instrument from ytterbium atoms cooled to nearly absolute zero.

In a normal gas, atoms dart around chaotically, and their signal smears out within milliseconds. Here, ytterbium atoms are fixed in the cells of an optical lattice, like eggs in a cardboard carton. Two isotopes: ytterbium-171 with spin 1/2 and ytterbium-173 with spin 5/2. Spin is a tiny magnet inside the atom. Together, they measure the magnetic field, and the difference in their responses allows subtracting common noise. The result is a co-magnetometer: two sensors in one.

The main challenge is that the light holding the atoms also disrupts their spins. The light field slightly rotates each spin, like a stream of water turning a light boat. To avoid this, the light polarization was made strictly linear, and for ytterbium-173 they used a Schrödinger's cat state — a superposition of two opposite spin orientations where the shifts cancel each other out. As a result, both isotopes could be probed simultaneously with high precision.

What does this achieve? Magnetic noise suppression by more than 30,000 times. The ratio of nuclear magnetic moments is measured with a precision of 4 parts per million. Such sensitivity allows searching for ultra-weak interactions that do not fit into the Standard Model: for example, fields from hypothetical dark matter particles.

Now physicists have a platform where quantum states live for a minute, not an instant. This is a step toward sensors that can detect what was previously indistinguishable against thermal noise.