  Ultra-fast single-photon detectors with high current density and operating
temperature can benefit space and ground applications, including quantum
optical communication systems, lightweight cryogenics for space crafts, and
medical use. Here we demonstrate magnesium diboride (MgB$_2$) thin-film
superconducting microwires capable of single-photon detection at 1.55 $\mu$m
optical wavelength. We used helium ions to alter the properties of MgB$_2$,
resulting in microwire-based detectors exhibiting single-photon sensitivity
across a broad temperature range of up to 20 K, and detection efficiency
saturation for 1 $\mu$m wide microwires at 3.7 K. Linearity of detection rate
vs incident power was preserved up to at least ~100 Mcps. Despite the large
active area of up to 400$\times$400 $\mu$m$^2$, the reset time was found to be
as low as $\sim1$ ns. Our research provides new possibilities for breaking the
operating temperature limit and maximum single-pixel count rate, expanding the
detector area, and raises inquiries about the fundamental mechanisms of
single-photon detection in high-critical-temperature superconductors.
