  To better understand the origins of the timing resolution, also known as
jitter, of superconducting nanowire single-photon detectors (SNSPDs), we have
performed timing characterizations of a niobium nitride SNSPD with a dual-ended
readout. By simultaneously measuring both readout pulses along with an optical
timing reference signal, we are able to quantify each independent contribution
to the total measured jitter. In particular, we are able to determine values
for the jitter due to the stochastic nature of hotspot formation and the jitter
due to the variation of the photon detection location along the length of the
nanowire. We compare the results of this analysis for measurements at
temperatures of 1.5 K and 4.5 K.
