Generation and characterization of spectrally factorable biphotons

Abstract

Spectrally unentangled biphotons with high single-spatiotemporal-mode purity are highly desirable for many quantum information processing tasks. We generate biphotons with an inferred heralded-state spectral purity of 99%, the highest to date without any spectral filtering, by pulsed spontaneous parametric down-conversion (SPDC) in a custom-fabricated periodically-poled KTiOPO₄ (PPKTP) crystal under extended Gaussian phase-matching conditions. The high purity achieved is made possible by the Gaussian phase-matching function of our custom PPKTP crystal. Without applying spectral filtering and using a standard PPKTP crystal, the highest previously reported purity is 93%. We characterize the joint spectral density of the generated biphoton by converting the spectral content to temporal information via dispersion through a 42-km SMF28 fiber. To characterize the JSD at high spectral resolution and more eciently, we employ a commercially available dispersion compensation module (DCM) with a dispersion equivalent to 100km of standard optical fiber and with an insertion loss of only 2.8 dB. Compared with the typical method of using two temperature-stabilized equal-length fibers that incurs an insertion loss of 20 dB per fiber, the DCM approach achieves high spectral resolution in a much shorter measurement time. We also verify the indistinguishability of the SPDC signal and idler photons via Hong-Ou-Mandel interferometric measurements. The near perfect interference visibility confirms that they are indeed indistinguishable.