  We demonstrate a new class of frequency-entangled states generated via
spontaneous parametric down-conversion under extended phase matching
conditions. Biphoton entanglement with coincident signal and idler frequencies
is observed over a broad bandwidth in periodically poled KTiOPO$_4$. We
demonstrate high visibility in Hong-Ou-Mandel interferometric measurements
under pulsed pumping without spectral filtering, which indicates excellent
frequency indistinguishability between the down-converted photons. The
coincident-frequency entanglement source is useful for quantum information
processing and quantum measurement applications.
