The two states of a strontium atom are like twins running down the same track, but one is slightly higher than the other. Usually, the light used to manipulate atoms pushes one twin harder and the other more weakly. Here, they chose a wavelength of 813 nanometers—'magic'—where both get the same kick.
In the device, strontium atoms were launched simultaneously in two states: the ground state and an excited state. Both passed through a sequence of light pulses that split and then recombined their paths. The phase difference was measured and came out to zero within 30 milliradians. This means that in free fall, both states accelerate identically to within $10^{-5}$.
The same device measured the force that light exerts on the excited state and found a wavelength of 478.95 nanometers at which this force vanishes. Such a tool could become the basis for ultra-sensitive force sensors and new tests of gravity using quantum clocks.
The magic wavelength of 813 nm is chosen so that light acts equally on both states of strontium, even though their energies differ by an optical quantum.