Multi-photon entanglement from parametric downconversion

Several optics experiments at the few-photon level are presented. First, we demonstrate the production of entangled states of up to 100 photons and methods to analyze their entanglement with single photon detectors. We then develop a method to generate path entangled states of light from the output of a parametric downconverter without postselection. We observe the bunching of composite states of photons, and show that they can exhibit bosonic or fermionic behavior depending on their internal phase. We demonstrate a novel way to produce entangled photon pairs that provides unmatched quantum interference visibility when using an ultrafast pump source. We then theoretically analyze the multi-photon state produced with such a source. By using the Schmidt decomposition of the state, we obtain intuitive expressions for four-fold correlation functions and also the non-perturbative solution to the downconversion interaction Hamiltonian. Finally, we present two measurements of the output of a Mach-Zehnder interferometer using a novel multiphoton detector. We show that the individual photon absorption probabilities vary on a subwavelength scale. We also show, using a Bayesian analysis of the detection results, that the phase sensitivity of an interferometer is in fact independent of phase angle, in contrast to the widely used standard theory.