  The Wilkinson Microwave Anisotropy Probe (WMAP) has mapped the full sky in
Stokes I, Q, and U parameters at frequencies 23, 33, 41, 61, and 94 GHz. We
detect correlations between the temperature and polarization maps significant
at more than 10 standard deviations. The correlations are present in all WMAP
frequency bands with similar amplitude from 23 to 94 GHz, and are consistent
with a superposition of a CMB signal with a weak foreground. The fitted CMB
component is robust against different data combinations and fitting techniques.
On small angular scales theta < 5 deg, the WMAP data show the
temperature-polarization correlation expected from adiabatic perturbations in
the temperature power spectrum. The data for l > 20 agree well with the signal
predicted solely from the temperature power spectra, with no additional free
parameters. We detect excess power on large angular scales (theta > 10 deg)
compared to predictions based on the temperature power spectra alone. The
excess power is well described by reionization at redshift 11 < z_r < 30 at 95%
confidence, depending on the ionization history. A model-independent fit to
reionization optical depth yields results consistent with the best-fit
LambdaCDM model, with best fit value tau = 0.17 +- 0.04 at 68% confidence,
including systematic and foreground uncertainties. This value is larger than
expected given the detection of a Gunn-Peterson trough in the absorption
spectra of distant quasars, and implies that the universe has a complex
ionization history: WMAP has detected the signal from an early epoch of
reionization.
