  We demonstrate an ultrabright narrow-band two-photon source at the 1.5 -\mu m
telecom wavelength for long-distance quantum communication. By utilizing a
bow-tie cavity, we obtain a cavity enhancement factor of $4.06\times 10^4$. Our
measurement of the second-order correlation function $G^{(2)} ({\tau})$ reveals
that the linewidth of $2.4$ MHz has been hitherto unachieved in the 1.5 -\mu m
telecom band. This two-photon source is useful for obtaining a high absorption
probability close to unity by quantum memories set inside quantum repeater
nodes. Furthermore, to the best of our knowledge, the observed spectral
brightness of $3.94\times 10^5$ pairs/(s$\cdot$MHz$\cdot$mW) is also the
highest reported over all wavelengths.
