The receiver doesn't catch radio waves but submillimeter light — the radiation emitted by cold clouds of gas and dust in distant galaxies. Inside, there's an array of tiny resonators, each tuned to its own frequency, like the strings of a grand piano tuned to notes. Such a device is called an integral field spectrometer: it captures a whole sky map at once, dispersing light into colors. The problem was that these arrays lost a large fraction of the light, as if half the piano strings sounded into the void.

The authors built a filter bank based on directional filters. This means each resonator doesn't just absorb its frequency but directs it straight into a detector, like a funnel collecting water into a bottle. To measure the efficiency, they used a cryogenic thermal load and a continuous terahertz source, carefully accounting for all losses in the optical path. On average, 75% of the light reached the detectors in the range from 125 to 220 GHz. For comparison, earlier versions of such devices often lost more than half.

Now that it's clear directional filters work, we can assemble large arrays of such resonators. They will become the heart of integral field modules for submillimeter telescopes. These instruments will allow us to build detailed maps of molecular clouds where stars are born and study the chemistry of the early universe. The piano can finally play at full volume.