Changchen Chen, Jane E. Heyes, Kyung-Han Hong, Murphy Yuezhen Niu, Adriana E. Lita, Thomas Gerrits, Sae Woo Nam, Jeffrey H. Shapiro, and Franco N. C. Wong
We use pulsed spontaneous parametric down-conversion in KTiOPO 4 4, with a Gaussian phase-matching function and a transform-limited Gaussian pump, to achieve near-unity spectral purity in heralded single photons at telecommunication wavelength. Theory shows that these phase-matching and pump conditions are sufficient to ensure that a biphoton state with a circularly symmetric joint spectral intensity profile is transform limited and factorable. We verify the heralded-state spectral purity in a four-fold coincidence measurement by performing Hong-Ou-Mandel interference between two independently generated heralded photons. With a mild spectral filter we obtain an interference visibility of 98.4±1.1%98.4±1.1% which corresponds to a heralded-state purity of 99.2%. Our heralded photon source is potentially an essential resource for measurement-based quantum information processing and quantum network applications.