  We present an X-ray timing and spectral analysis of the Be/X-ray binary SAX
J2103.5+4545 at a time when the Be star's circumstellar disk had disappeared
and thus the main reservoir of material available for accretion had
extinguished. In this very low optical state, pulsed X-ray emission was
detected at a level of L_X~10^{33} erg/s. This is the lowest luminosity at
which pulsations have ever been detected in an accreting pulsar. The derived
spin period is 351.13 s, consistent with previous observations. The source
continues its overall long-term spin-up, which reduced the spin period by 7.5 s
since its discovery in 1997. The X-ray emission is consistent with a purely
thermal spectrum, represented by a blackbody with kT=1 keV. We discuss possible
scenarios to explain the observed quiescent luminosity and conclude that the
most likely mechanism is direct emission resulting from the cooling of the
polar caps, heated either during the most recent outburst or via intermittent
accretion in quiescence.
