Ultrabright source of polarization-entangled photons from cavity-enhanced downconversion

Abstract

This thesis describes the design, construction, and testing of a new source of entanglement. The goal is to produce pairs of photons which are not only polarization-entangled, but also have a high brightness within a narrow bandwidth. This novel source is more suitable than previous SPDC sources for transferring entanglement to future qubit storage such as a trapped rubidium memory. The narrow bandwidth is imposed by modifying the spectrum of the photon pairs by performing the down-conversion inside a cavity. The collinear downconversion geometry inside the linear cavity is achieved by using a quasi-phased-matched periodically-poled potassium titanyl phosphate (PPKTP) crystal. The single-pass free-space photon-pairs produced were demonstrated to be polarization-entangled by measuring the Hong-Ou-Mandel interference dip and measuring a violation of Bell's inequality of 2.711 ± 0.010 (which was greater than the classical limit of 2). The cavity-enhanced downconversion was observed with a brightness of 0.7 pairs/s per mW of pump per MHz of bandwidth in the Gaussian mode collected (a generation rate of 110 pairs/s/mW/MHz is inferred). The interference dip from the pairs was measured to have a visibility of 75% when near the ideal equal-FSR operating point, where the pairs are in the biphoton triplet state. When detuned to have unequal FSR the output pairs show the an interference dip behavior consisting of a combination of triplet and singlet states that depends on the time separation of the pair as it leaves the cavity. The observed results corroborate detailed predictions of a Gaussian-state model of cavity-enhanced downconversion.