An ordinary antenna picks up radio waves, but there are fields it barely feels. We're talking about slow electric fields: those that change once a second or less. For them, physicists built a sensor from a beam of rubidium atoms. The beam flies in a vacuum, and the atoms in it behave like tiny compass needles, except they respond not to a magnet but to an electric field.
The atoms in the beam are first irradiated with a laser. One of the electrons jumps to a very distant orbit — the atom becomes "inflated," like a balloon. This state is called Rydberg. An external electric field slightly shifts the energy levels of this inflated atom. The shift is tiny, but it can be detected: if the field is strong enough, the atom loses an electron and turns into an ion. The sensor counts these ions — the more there are, the stronger the field.
The device detects fields with frequencies from 1 hertz — slower than a wave of the hand. Sensitivity above 20 hertz is less than 1 millivolt per meter. For comparison: the field between your palms from rubbing against clothing can be thousands of times stronger. The sensor isn't afraid of strong fields: it operates over a range that spans 50 decibels — meaning the field can vary by a factor of one hundred thousand.
Such an antenna could be useful where you need to sense very weak and slow fields. For example, for communication underwater or underground, where ordinary antennas are useless. Or for finding hidden electronics by their weak electric field.
Rydberg atoms are so large that a single one can be bigger than many viruses. If the nucleus were the size of a pea, the electron on a Rydberg orbit would fly at the distance of a football field.