  We present a charge integration photon detector (CIPD) that enables the
efficient measurement of photon number states at the telecom-fiber wavelengths
with a quantum efficiency of 80% and a resolution less than 0.5 electrons at 1
Hz sampling. The CIPD consists of an InGaAs PIN photodiode and a GaAs JFET in a
charge integration amplifier, which is cooled to 4.2 K to reduce thermal noise
and leakage current. The charge integration amplifier exhibits a low noise
level of 470 nV/Hz1/2. The dark count is as low as 500 electrons/hour.
