 This paper presents a new time domain diffuse correlation spectroscopy, TDDCS, system based on a superconducting nanowire single photon detector, SNSPD. The SNSPD has a high quantum efficiency, a narrow timing response, and a negligible dark count noise, making it ideal for use in TDDCS systems. This system was compared to a typical single photon avalanche diode, SPAD, detector and was found to be faster and more efficient than the SPAD. Additionally, the SNSPD enabled the authors to perform in vivo measurements of pulsatile blood flow index, BFI, on the adult human forehead. This article was authored by Veronica Parfencheva, Lorenzo Colombo, Pranav Lankar, and others.