  We investigate experimentally the capacitive coupling between a two-electron
singlet-triplet spin qubit and flying electrons propagating in quantum Hall
edge channels. After calibration of the spin qubit detector, we assess its
charge sensibility and demonstrate experimentally the detection of less than
five flying electrons with average measurement. This experiment demonstrates
that the spin qubit is an ultrasensitive and fast charge detector with the
perspective of a future single shot-detection of a single flying electron. This
work opens the route toward quantum electron optics experiments at the single
electron level in semiconductor circuits.
