  Propagation and relaxation of nonequilibrium quasiparticles in
superconductors are of key importance for functioning of numerous nanoscale
devices, enabling operation of some of them, and limiting the performance of
the others. The quasiparticles heated above lattice temperature may relax
locally via phonon or photon emission channels, or diffuse over appreciable
distances in a nanostructure altering the functionality of their remote
components. Tracing quasiparticles experimentally in real-time domain has
remained the challenging task owing to their rapid dynamics. With electronic
nanothermometry, based on probing of the temperature-dependent switching
current of a superconducting nanobridge, we monitor heat pulse carried by a
flux of nonequilibrium quasiparticles as it passes by our detector with a
noise-equivalent temperature of $10\,$mK /$ \sqrt N$, where $N$ is the number
of pulses probing the bridge (typically $N=10000$), and temporal resolution of
a single nanosecond. The measurement provides the picture of quasiparticle
diffusion in a superconducting aluminum strip and direct determination of the
diffusion constant $D$ equal to $100$ cm$^2$/s with no energy dependence
visible.
