  Shortly after their inception, superconducting nanowire single-photon
detectors (SNSPDs) became the leading quantum light detection technology. With
the capability of detecting single-photons with near-unity efficiency, high
time resolution, low dark count rate, and fast recovery time, SNSPDs outperform
conventional single-photon detection techniques. However, detecting lower
energy single-photons (<0.8 eV) with high efficiency and low timing jitter has
remained a challenge. To achieve unity internal efficiency at mid-infrared
wavelengths, previous works used amorphous superconducting materials with low
energy gaps at the expense of reduced time resolution (close to a nanosecond),
and by operating them in complex mK dilution refrigerators. In this work, we
provide an alternative approach with SNSPDs fabricated from 5-9.5 nm thick
NbTiN superconducting films and devices operated in conventional
Gifford-McMahon (GM) cryocoolers. By optimizing the superconducting film
deposition process, film thickness and nanowire design, our fiber-coupled
devices achieved > 70% system detection efficiency (SDE) at 2 {\mu}m and sub-15
ps timing jitter. Furthermore, detectors from the same batch demonstrated unity
internal detection efficiency at 3 {\mu}m and 80% internal efficiency at 4
{\mu}m, paving the road for an efficient mid-infrared single-photon detection
technology with unparalleled time resolution and without mK cooling
requirements.
