Colibrì will bear in its heart one of the coldest human-made devices: an array of superconducting transition-edge sensors (TES). Such ultracold sensors work at a temperature of about 0.1 K, which is 3,000 times colder than the human body temperature! Working at such low temperatures is crucial to achieve a high-resolution in energy and timing, and that is what Colibrì is all about. Indeed, using TESs will enable Colibrì to measure X-ray photons energy with a precision of about 1 eV, and its time of arrival to within a millionth of a second.
A TES is made of a superconducting metal film functioning near its transition temperature (typically 0.1 K, as mentioned above). While electrons manoeuvre freely in a superconducting metal, they encounter some significant resistance when the metal switches to its normal phase. The transition from superconductor to normal metal occurs within about 1 mK change in the temperature but results in a much larger change in resistance.
An array of TES made by researchers at NIST, Colorado. In the back a penny coin for scale. 
A section of a TES: the superconductor (a film of molybdenum Mo) is in thermal contact with the absorber (copper Cu and bismuth Bi)
When an X-ray photon hits a TES, the provided energy provokes the needed rise in temperature for the transition to happen, electrons move more slowly in the TES, the measured electric current drops, and the X-ray photon is detected. Indeed, measuring how much the current diminishes and for how long enables us to determine the energy of that photon. Finally, after the absorption of a photon, a TES needs to be cooled down to its initial temperature by using a cooling bath, so as to be able to detect the next photon.
Colibrì would be able to detect over 250,000 X-rays each second, and using such accurate devices as TESs would enable Colibrì to perform unprecedented precision X-ray timing and spectroscopy over the widest range of photon energies (100 eV to 15 keV).
Image credit banner: NASA/SOFIA/Lynette Cook
Image credit TES: Dan Schmidt/NIST, Hays-Wehle et al/NIST