  Emission from the relativistic jet located at hundreds of kpc from the core
of the superluminal quasar PKS 0637-752 was detected at 3.6 and 5.8 microns
with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. The
unprecedented sensitivity and arcsecond resolution of IRAC allows us to explore
the mid-infrared emission from kiloparsec-scale quasar jets for the first time.
The mid-infrared flux from the jet knots, when combined with radio and optical
fluxes, confirms a synchrotron origin of the radio-to-optical emission and
constrains very well the high energy end of the nonthermal electron
distribution. Assuming the X-rays are produced in the relativistically moving
knots via inverse Compton scattering of cosmic microwave background (CMB)
radiation, the infrared observation puts constraints on the matter content of
the quasar extended jet. Specifically, pure electron-positoron pair jet models
are unfavorable based on the lack of an infrared bump associated with ``bulk
Comptonization'' of CMB photons by an ultrarelativistic jet.
