Spectrometer on a Chip: Catching Light from Distant Galaxies express
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.
Original abstract
as written by the authors arXiv:2603.06334
Integrated superconducting spectrometers are developing to the point that they are enabling integral field units, providing large area spectral mapping capabilities for astronomy in the sub-millimeter band. However, these integral field units are only worthwhile if they have a high efficiency, but to date the efficiency of on-chip filterbanks has been quite poor. Here we demonstrate a filterbank with high efficiency by using directional filters. Using a cryogenic thermal load and a noise measurement in combination with a continuous-wave terahertz source to obtain the spectral response of the filters, we are able to accurately measure the filterbank efficiency, accounting for all quasi-optical elements within our setup. We experimentally obtain an average peak coupling efficiency to the detectors of 75% in a filterbank that sparsely samples between 125 GHz to 220 GHz using filters with a mean loaded quality factor of 19.6. Our results demonstrate that a filterbank with a high efficiency is achievable using directional filters, giving a clear route towards efficient integral field units.
The submillimeter range is light with wavelengths shorter than a millimeter but longer than infrared. The human eye can't see it, but superconducting detectors cooled to temperatures lower than in outer space sense it perfectly.
In Carl Sagan's novel "Contact," the characters pick up a signal from space at frequencies close to submillimeter. Efficient chip-based spectrometers could make such searches more realistic.