  Improving the temporal resolution of single photon detectors has an impact on
many applications, such as increased data rates and transmission distances for
both classical and quantum optical communication systems, higher spatial
resolution in laser ranging and observation of shorter-lived fluorophores in
biomedical imaging. In recent years, superconducting nanowire single-photon
detectors (SNSPDs) have emerged as the highest efficiency time-resolving
single-photon counting detectors available in the near infrared. As the
detection mechanism in SNSPDs occurs on picosecond time scales, SNSPDs have
been demonstrated with exquisite temporal resolution below 15 ps. We reduce
this value to 2.7$\pm$0.2 ps at 400 nm and 4.6$\pm$0.2 ps at 1550 nm, using a
specialized niobium nitride (NbN) SNSPD. The observed photon-energy dependence
of the temporal resolution and detection latency suggests that intrinsic
effects make a significant contribution.
