Tiny magnets inside atoms, nuclear spins, can 'freeze' for long minutes in a cold crystal. This hinders the quick tuning of quantum sensors: everything takes too long to reach the desired state. Physicists have found a way to nudge them with light.
In crystals of lead titanate and a similar material at a temperature of 10 Kelvin (minus 263 degrees Celsius), a laser with a wavelength of 405 nanometers (violet) knocks out electrons and creates defects — paramagnetic centers. These centers, like tiny magnets, start interacting with the nuclear spins of lead-207 and speed up their relaxation. The time T1—the time it takes for a nuclear magnet to lose its orientation — is roughly halved: from 17 to 7 seconds at one frequency, and from 1550 to 850 seconds at another.
Controlling the relaxation rate of nuclei opens the way to faster polarization of samples. This is important for precise NMR measurements, where the signal needs to be accumulated many times. Moreover, such a technique could assist in experiments searching for axions — hypothetical dark matter particles that might weakly affect nuclear spins.
Light turns out to be a convenient control knob for a world that usually follows its own, very slow laws of cold.
The T1 time for lead nuclei at 10 Kelvin can reach half an hour — enough time to enjoy a cup of tea while the nucleus 'remembers' which way to look.