The violins in an orchestra play a single note, but if each one starts at its own moment, you get noise instead of music. The same goes for the axion, a particle that dark matter may be made of. It flies through the instrument and, in a magnetic field, turns into a faint radio wave. But a single such wave is so quiet that no receiver would ever hear it.
The DALI instrument gathers this whisper into a loud signal. Inside it sit parallel ceramic plates, and behind them a conducting mirror. Each plate gives rise to its own wave, and if the distances are chosen correctly, all the waves add up into one, like the voices of a choir. That is the first amplifier. The second is a resonator: the wave races back and forth between the plates and the mirror, reflecting many times, and grows stronger with every reflection.
The key discovery: these two amplifiers do not get in each other's way. Tuning the plates affects only how much energy goes into the wave in a single pass, while the number of reflections stays the same. That is why the instrument can be tested with an ordinary radio source: if it works with an external signal, it will work just as reliably with an axion. It is as if a conductor knew that the arrangement of the music stands has no effect on the tempo, so the two can be adjusted separately.
The DALI prototype uses ceramic plates of yttrium-stabilized zirconium oxide only 1 mm thick, and the quality factor grows linearly with the number of layers up to 20, regardless of whether that number is even or odd.